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Rodolfo Carrera, Editor
WEEK OF FEBRUARY 12, 2024 [No. 1562]
Superfluid second sound observed:
researchers at MIT in Cambridge, MA have captured direct images of the
predicted second sound to be produced by
pure heat propagation as T-waves in a strongly interacting
6Li quantum gas superfluid held at nK in a box potential
independent of its constituent particles. They realized spatially
resolved thermography of the superfluid transition in the strongly
interacting fermionic gas by using RF spectroscopy to map out local
temperature changes with sub-nK resolution. Above the superfluid
transition, heat propagated diffusively, but below the transition,
wave-like propagation characteristic of second sound was observed. The
superfluid phase transition was directly observed as the sudden change
from thermal diffusion to second-sound propagation and is accompanied by
a peak in the second-sound diffusivity. This method yields the full heat
and density response of the strongly interacting fermionic gas with all
the defining properties of Landau’s two-fluid hydrodynamics. In the
Tisza two-fluid model for superfluidity (1938), a mixture of viscous
fluid and friction-free superfluid allows for density waves and for
temperature waves (named by Landau as second sound). In ultracold gases
so far only faint reflection of density ripples had been observed. Using
RF spectroscopy the researchers here found that the 6Li
fermions resonate at different RF frequencies depending on their
temperature. When the cloud is at warmer temperatures, and carries more
normal liquid, it resonates at a higher frequency. Regions in the cloud
that are colder resonate at a lower frequency. Applying the higher
resonant RF, prompted any normal, hot fermions in the liquid to ring in
response. Focusing on the resonating fermions the researchers tracked
them over time to create movies that revealed heat's pure wave motion.
They imaged the system as they cooled it through the critical
temperature, directly showing the transition from heat equilibration to
heat oscillation.
For more information:
Science, February 8 (2024) page 629; Phys.org, February 8 (2024).
Chiral magnetization orders observed in spin ice:
researchers at University of Augsburg in Augsburg have used the
anomalous Hall effect (AHE) to uncover hidden symmetry in spin ice. They
have used electrical measurements at low temperatures to identify
discrete degeneracies in a metallic kagome ice compound, determining
chiral orders with similar magnetization but opposite sense of rotation.
The researchers here show that a time-reversal-like degeneracy appears
in the metallic kagome spin ice HoAgGe when magnetic fields are applied
parallel to the kagome plane. They find vanishing hysteresis in the
field dependence of the magnetization at low temperature, but finite
hysteresis in the field-dependent AHE. This suggests the emergence of
states with nearly the same energy and net magnetization but different
AHE signals and longitudinal magnetoresistance. By analyzing the
experimental data and a minimal tight-binding model, they identify a
time-reversal-like operation connecting these near-degenerate states,
which is related to the non-trivial distortion of the kagome lattice. In
magnetic crystals, despite the explicit breaking of time-reversal
symmetry, two equilibrium states related by time reversal are always
energetically degenerate. In ferromagnets, this time-reversal degeneracy
is reflected in the hysteresis of the magnetic field dependence of the
magnetization and, if metallic, of the AHE. Under time-reversal, both
these quantities change signs but not their magnitude. Unexpectedly
here, two such states with equal magnetization, produce distinctly
different AHE signals. The experiments here used the magnetic metal
HoAgGe, with a triangular configuration of atomic electron spins of Ho
atoms. Since it is impossible to simultaneously fulfill all the pairwise
interactions on each triangle, a magnetically frustrated state emerges.
It features several energetically degenerate configurations per triangle
(Kagome spin ice with spins located at the edges of corner-shared
triangles) and magnetic moments having complex chiral pattern. They are
created with an applied magnetic field at low temperatures and feature
fractionalized magnetization plateaus at values 1/3 and 2/3. The
researchers analyzed the AHE at low temperatures. Unexpectedly,
different values of the AHE were found for the two patterns of 1/3
magnetization. Modeling of the data revealed an underlying unique hidden
symmetry. The combination of a 180° rotation and a distortion reversal
is required for transforming one pattern into the other one. Conduction
electrons scattering off the two different patterns result in different
curvatures of their wave function phase, and this leads to a difference
in the AHE signals, even if the two patterns have similar energy and
magnetization.
For more information:
Phys.org, February 7 (2024); Nat. Phys.,
January 10 (2024) page 442.
WEEK OF FEBRUARY 5, 2024 [No. 1561]
Cooperative Lamb shifts due to atom gas dipole-dipole interactions measured:
researchers at JILA in Boulder, CO have
used a optical atomic 3D lattice, to measure spatially varying frequency
shifts produced by resonant atomic dipole interactions within a
high-density ultra-cold 87Sr quantum gas. The interactions
produce spatially dependent cooperative Lamb shifts when
spectroscopically interrogating the mHz-wide optical clock transition in
87Sr. They show that the ensemble-averaged shifts can be
suppressed below the level of evaluated systematic uncertainties for
optical atomic clocks. They demonstrate that excitation of the atomic
dipoles near a Bragg angle can enhance these effects by nearly an order
of magnitude compared with nonresonant geometries. These mHz-level
frequency shifts (theoretically determined in 2004 to limit clock
accuracy) were spatially studied and compared with calculated values
using imaging spectroscopy techniques developed in this experiment.
These shifts, normally neglected, arise from collective interference
between the atoms behaving as dipoles when they are prepared in a
superposition of the two clock states. Because the spatial ordering of
the atoms within the cubic lattice influences the dipolar coupling,
researchers could amplify or diminish the dipole interactions by
manipulating the angle of the clock laser relative to the lattice.
Operating at the Bragg angle the researchers expected strong
constructive interference and observed a correspondingly larger
frequency shift. With stronger dipole-dipole interactions occurring
within the lattice, they found that these interactions create local
energy shifts throughout the clock system. Left uncontrolled, the shifts
can affect the accuracy of atomic clocks as atoms are added to the
lattice. The researchers observed that the cooperative Lamb shifts were
not uniform across the lattice, but varied depending on each atom's
specific location. They observed that there was a close connection
between the cooperative Lamb shifts and the propagation direction of the
clock probe laser within the lattice. This relationship allowed them to
find a specific angle where a zero crossing (a quantum state that
experiences zero collective Lamb shift with equal superposition of
ground state and excited state) was observed and the sign of the
frequency shift transitioned from positive to negative. Playing around
with the connection between the laser propagation angle with respect to
the cubic lattice and the cooperative Lamb shifts allowed the
researchers to control and minimize the frequency shifts to fine-tune
the clock further to be more robust against them.
For more information:
Science, January 25 (2024) page 384; Phys.org,
January 27 (2024).
Electron-nuclear spin interaction provides long-lived time
crystal:
researchers at Technische Universität Dortmund in
Dortmund have made and imaged a robust continuous time crystal in an
electron–nuclear spin system of a semiconductor tailored by tuning the
material composition. Continuous, time-independent external driving of
the sample produces periodic auto-oscillations with a coherence time
exceeding hours. Varying the experimental parameters reveals wide ranges
in which the time crystal remains stable. At the edges of these ranges,
the researchers find chaotic behavior with a lifted periodicity
corresponding to the melting of the crystal. The periodic dynamics of
the nuclear spin polarization of the time crystal lived millions of
times longer than could be shown in previous experiments. According to
Wilczek's postulate (2012) in a time crystal one of the physical
properties would have to spontaneously begin to change periodically in
time, even though the system does not experience corresponding periodic
interference. Previous potential time crystal demonstrations were
subjected to a temporal excitation with a specific periodicity. A BEC
crystal that behaves periodically in time (for ms) with time-independent
excitation, was demonstrated in 2022. The researchers here used an
InGaAs crystal, in which the nuclear spins act as a reservoir for the
time crystal. The crystal is continuously illuminated so that a nuclear
spin polarization forms through interaction with electron spins. This
nuclear spin polarization spontaneously generates oscillations as in a
time crystal. The status of the experiments at the present time provide
a time crystal lifetime > 2/3 h (time duration for time translational
invariance is x 107 longer than has been demonstrated to date),
and it expected to be increased. It is possible to vary the crystal's period
over wide ranges by systematically changing the experimental conditions.
It is also possible to move into areas where the crystal melts by losing
its periodicity (chaotic behavior that can be maintained over long
periods of time and was analyzed theoretically here).
For more information:
Phys.org, February 1 (2024); Nat. Phys.,
January 24 (2024) page 631.
WEEK OF JANUARY 29, 2024 [No. 1560]
Distribution of the strong force inside a proton
analyzed:
researchers at the Jefferson Lab in Newport News, VA
have analyzed the gravitational form factors of the proton, and thus,
revealed a snapshot of the distribution of the strong force inside the
proton including details of the shear stress the force may exert on the
quark particles that make up the proton. The decades-old data used here
came from experiments conducted with the Continuous Electron Beam
Accelerator Facility (CEBAF). The high energy electron beam interacting
with the proton in a target of LH applies a force greater than the four
tons needed to pull out a quark/antiquark pair to the proton. The
experimental program studied deeply virtual Compton scattering (DVCS),
with an electron exchanging a virtual photon with the proton so the
proton remains the same and recoils, an energetic photon is produced,
and the electron scatters away). At the time the data were taken, the
researchers were not aware that beyond the 3D imaging intended with the
data, they were also collecting the data needed for accessing the
mechanical properties of the proton. It turns out that the DVCS process
can be connected to gravity interaction and that, by now the
relationship between the measurement of DVCS to the gravitational form
factor has been established. The researchers here used that data to
extract the proton pressure (2018), and now they have extracted the
normal force and the shear force. A detailed description of the
connections between the DVCS process and the gravitational interaction
is provided in this report plus some initial experimental results. The
researchers plan to use the same technique to determine the proton's
mechanical size as well.
For more information:
Phys.org, January 23 (2024); Rev. Mod. Phys.,
December 22 (2023) page 041002.
Dynamical phases of superconductivity observed in a cavity
QED simulator:
researchers at JILA in Boulder, CO have
simulated superconductivity under excited conditions using an
atom-optical cavity quantum system with laser-cooled (10 µK) levitated
88Sr atoms. Their system encodes the presence or absence of a
Cooper pair in a long-lived electronic transition in the 88Sr
atoms coupled to the optical cavity and represent interactions between
electrons as photon-mediated interactions through the cavity. To fully
explore the phase diagram, they manipulate the ratio between the
single-particle dispersion and the interactions after a quench and
perform real-time tracking of the subsequent dynamics of the
superconducting order parameter using nondestructive measurements. They
observe regimes in which the order parameter decays to zero (phase I),
assumes a non-equilibrium steady-state value (phase II) or exhibits
persistent oscillations (phase III). In this simulator, the presence or
absence of a Cooper pair was encoded in a two-level system and
photon-mediated interactions between electrons were realized between the
atoms within the cavity. Three distinct dynamical phases for a
superconductor to experience when it evolves in out-of-equilibrium are
predicted here. In Phase I, the strength of superconductivity decays
rapidly to zero. Phase II represents a steady state in which
superconductivity is preserved. In phase III the strength of
superconductivity has persistent oscillations with no damping. In the
phase III regime, instead of suppressing the oscillations, many- body
interactions can lead to a self-generated periodic drive to the system
and stabilize the oscillations. In their quantum simulator, the atoms
emulated Cooper pairs and experienced a collective interaction that
parallels the attraction experienced by electrons in BCS
superconductors. An atom in the excited state simulates the presence of
a Cooper pair, and the ground state represents the absence of one. They
prepared the cavity atoms in a highly collective superposition state
between ground and excited states. Then, they induce a quench by turning
on a laser beam that gives all the atoms different energies. By changing
the nature of this quench, the researchers could see different dynamical
phases. They devised a trick to observe the elusive Phase III, which
involved splitting the cloud of atoms in half. Using two clouds of atoms
with separate control over energy shifts turn out to be key to achieve
Phase III. In superconductors, energy levels of electrons can be split
into two sectors, largely occupied or barely occupied, separated by the
Fermi level. In the setup here with a spin system there is no intrinsic
Fermi level, so the researchers take that into account by using two
atomic clouds: one cloud simulates the states below the Fermi level,
while another cloud simulates the other quantum states. To measure the
dynamics of the superconductor within the cavity, the researchers
tracked the light leaking from the optical cavity in real time. Their
data found distinct points where the simulated superconductor
transitioned between phases, eventually reaching Phase III.
For more information:
Nature, January 24 (2024) page 679; Phys.org, January
24 (2024).
WEEK OF JANUARY 22, 2024 [No. 1559]
Microscopic topological-skin-effect based semiconducting device built:
researchers at IFW Dresden in Dresden have created a 2D AlGaAs
semiconductor device with size 100 µm where high robustness and
sensitivity are ensured by a quantum topological skin effect that
shields the functionality of the device from external perturbations. The
researchers here directly observe one of the characteristic signatures
associated with non-Hermitian topology in the quantum regime of a
condensed-matter system. Rather than relying on gain and loss, they
build on the quantum transport properties of a Hermitian topological
phase: the quantum Hall phase. Its unidirectional edge modes provide a
link to non-Hermitian topology that can be accessed in conventional
multi-terminal conductance measurements. Non-Hermiticity is often
associated with gain and loss, which requires precise tailoring to
produce the signatures of non-trivial topology. The researchers here
instead of gain and loss, use the non-reciprocity of quantum Hall edge
states to directly observe non-Hermitian topology in a multi-terminal
quantum Hall ring. Their transport measurements evidence a robust,
non-Hermitian skin effect, characterized by currents and voltages
showing an exponential profile that persists across Hall plateau
transitions away from the regime of maximum non-reciprocity. Introducing
gain and loss in a quantum device is easily achieved, for instance, by
not shielding it sufficiently well from its local environment. However,
customizing these two processes such as to reach a topological phase is
not, and no quantum, condensed-matter devices showing non-Hermitian
topology have been reported to date. The existing experimental
observations have used ultracold atoms and optical systems and
metamaterials through non-quantum mechanical effects. Their operating
principle is based on the fact that Kirchoff’s laws, Newton’s laws and
Maxwell’s equations can be used to mimic the Schrödinger equation
describing the dynamics of quantum particles. The non-Hermitian topology
in this multi-terminal quantum Hall device results from the setting of
contacts on the AlGaAs material. The material impurities or temperature
changes can disrupt the flow of electrons in semiconductor devices,
leading to instability. In the device built here, electron flow, usually
susceptible to interference, is safeguarded by a topological quantum
phenomenon. The topological skin effect makes all of the currents
between the different contacts on the quantum semiconductor to be
unaffected by either impurities or other external perturbations. This
quantum phenomenon was initially demonstrated at a macroscopic level
three years ago in an artificial metamaterial. In the quantum device
here, the I-V relationship is protected by the topological skin effect
because the electrons are confined to the edge. Even with impurities in
the semiconductor material, the current flow remains stable. The
contacts can detect even the slightest fluctuations in either I or V.
The design used here sets materials and contacts on a 2D AlGaAs
semiconductor device, inducing the topological effect under ultra-cold
conditions and a strong magnetic field. The contacts were set in such a
way that the electrical resistance could be measured at the contact
edges, directly revealing the topological effect.
For more information:
Phys.org, January 18 (2024); Nat. Phys., January 18 (2024) page 395.
Long-lived valleys in bilayer graphene QDs observed:
researchers at ETH Zurich in Zurich have found evidence of long-lived
valley states in bilayer graphene (BLG) quantum dots (QD). They measured
the characteristic relaxation times of the spin and valley states in
gate-defined BLG-QD devices. Different valley states were distinguished
from each other with fidelity > 99%. The relaxation time between
valley triplets and singlets is > 500 ms (more than one order of
magnitude longer than for spin states). In the BLG double QD used in
this work, electrons have both an intrinsic angular momentum (spin) and
a pseudo-spin (valley, by the spins rotating in opposite directions).
BLG is semimetal with a tunable band gap by application of an electric
field perpendicularly to the plane of the sheets. For BLG to host a QD,
a band gap needs to be established. Here spin-orbit coupling and
hyperfine interaction are both weak. The hexagonal symmetry observed in
real space is also present in momentum space. In momentum space, free
electrons are found in the local minima and maxima of the energy
landscape, valleys where the conduction and valence bands meet. In BLG,
the hexagonal symmetry dictates the existence of two degenerate energy
valleys with same electron energy. These pseudo-spins corresponding to
opposite electron momentum values have relaxation time > 0.5 s. The
spin relaxation time measured in the BLG double QD is < 25 ms, in
good agreement with spin relaxation times measured in semiconductor QDs.
The relaxation time is shown to increase with higher energy detuning,
which does not match observations in other systems. Varying the
inter-dot coupling leaves the valley relaxation time unaffected. In
addition, the symmetry of the hexagonal Bravais lattice of BLG gives
rise to a valley degeneracy, which behaves analogously to spins. This
unique valley degeneracy in BLG with electrically tunable valley
g-factor provides an additional degree of freedom to realize and
manipulate qubits with the prospect of realizing highly robust qubits
with valley states. Whereas charge qubits couple to electric fields and
spin qubits to magnetic fields, valley qubits consist of two degenerate
states with the same charge distribution and the same spin
configuration, but differ in their locations in reciprocal space.
Theories have proposed various intervalley scattering mechanisms,
requiring a short-range event on the scale of the lattice period. Hence,
for sufficiently low atomic defect density, the valley lifetimes are
expected to be limited not by intrinsic mixing mechanisms such as
phonon-mediated spin–valley coupling but by the finite size of the dot
ultimately breaking translational invariance. So far, however, only very
short valley lifetimes have been observed, not in graphene but in
optically addressed valley qubits in other 2D materials. In recent
experiments, the researchers here found spin-relaxation times
T1 of up to 50 ms measured with the single-shot Elzerman
readout technique in a single quantum dot, comparable with values from
other semiconductor quantum dot systems, such as in III–V, Si-based and
Ge-based heterostructures. The researchers here demonstrate single-shot
readout with both spin and valley Pauli blockade in gate-defined BLG
double quantum dots and the associated measurement of characteristic
spin and valley relaxation times T1 between spin- or
valley-triplet and singlet states. Using a property unique to BLG, they
can select between spin- or valley-blockade regimes by choosing
appropriate perpendicular magnetic fields. The spin-T1 time
is measured to be up to 60 ms at B⊥ = 700 mT, corroborating
their recent findings in single quantum dots. By increasing the interdot
tunnel coupling, the spin-T1 time is reduced. They observe
very long valley-T1 times, longer than 500 ms at
B⊥ = 250 mT. Unlike in the relaxation of spin states,
intervalley relaxation times are found to be robust against variation of
the interdot tunnel coupling strength. This valley lifetime is
comparable with the state-of-the-art spin singlet-triplet T1
measured in Si/SiGe and Si/SiO2 and an order of magnitude
longer than their T1 reported at low magnetic field.
For more information:
Phys.org, January 17 (2024); Nat. Phys., January 17 (2024) page 428.
WEEK OF JANUARY 15, 2024 [No. 1558]
Anomalous 2D insulator-superconducting phase transition observed:
researchers at Princeton University in Princeton, NJ have discovered
an unconventional abrupt change in quantum fluctuations in the
transition from a superconductor to a resistive state in monolayer
WTe2. They identified an anomalous superconducting quantum
critical point in the gate-tuned excitonic quantum spin Hall insulator
WTe2 when extending Nernst experiments down to mK. The vortex
Nernst effect that they observe reveals singular superconducting
fluctuations in the resistive normal state induced by magnetic fields or
temperature, even well above the transition. Near the doping-induced
quantum critical point, the Nernst signal driven by quantum fluctuations
is large in the mK regime, an indication of the proliferation of
vortices. Unexpectedly, the Nernst signal abruptly disappears when the
doping falls below the critical value. The main way fluctuations destroy
2D superconductivity is by the spontaneous emergence of quantum vortices
(microscopic strand of magnetic field trapped inside a swirling electron
current). When the sample is raised above a certain temperature,
vortices spontaneously appear in pairs (vortices and anti-vortices) and
their rapid motion destroys the superconducting state. Superconductivity
in ultrathin films does exist below a certain critical temperature
(temperature-driven BKT transition). The researchers began with a bulk
crystal of the layered semi-metal WTe2, and converting it
into a 2D material by increasingly exfoliating the material down to a
single, atom-thin layer. At this level of thinness, the material behaves
as a very strong insulator. They controlled the observed switching
behavior between insulating and superconducting phases by building a
device that functions just like an on and off switch. Then they cooled
the sample to ~ 50 mK and switched the material from insulator to
superconductor by introducing some electrons to the material with a low
gate voltage. They controlled the superconductor properties by adjusting
the density of electrons in the material with the gate voltage. At a
critical electron density, the quantum vortices rapidly proliferate and
destroy the superconductivity through a quantum phase transition. To
detect the presence of the quantum vortices, they established a small
temperature gradient on the sample so the vortices drifted to the cooler
side and generated a detectable nV signal (Josephson effect). They
further verified the Josephson effect by reversing the magnetic field
and getting the detected voltage reversed. The direct detection of the
vortex current provides an experimental tool to measure quantum
fluctuations in the sample. From the measurement of these quantum
fluctuations, they discovered the robustness of the vortices persisting
to much higher temperatures and magnetic fields than expected in the
resistive phase of the material. The vortex signal abruptly disappeared
when the electron density was tuned just below the critical value at the
quantum phase transition. At this critical value of electron density
quantum fluctuations drive the phase transition. The researchers
expected the strong fluctuations to persist below the critical electron
density on the non-superconducting side, just like the strong
fluctuations persist well above the BKT transition temperature. Instead
the vortex signals suddenly vanish when the critical electron density
value is crossed. The researchers found that neither the Ginzburg-Landau
theory or the BKT theory explain the observed phenomena.
For more information:
Phys.org, January 12 (2024); Nat. Phys., January 5 (2024) page 269.
RT quantum coherence of entangled excitons shown:
a group lead by researchers at Kyushu University in Fukuoka has
achieved quantum coherence at RT of entangled quintets by embedding a
chromophore in a metal-organic framework (MOF) to suppress the molecular
motion sufficiently to maintain a well-defined state of the quintet
state for more than 100 ns without being affected by surrounding
disturbances. Singlet fission (ST) can generate an exchange-coupled
quintet triplet pair state 5TT, which could lead to the
realization of quantum computing and quantum sensing using entangled
multiple qubits at RT. The suppressed motion of the chromophores in
ordered domains within the MOF leads to controlled fluctuation of the
exchange interaction as needed for 5TT generation without
causing 5TT decoherence. Quantum coherence of 5TT
has been observed at cryogenic temperatures to date. Qubit
initialization by photoexcitation uses chromophores to excite electrons
with desirable electron spins at RT through SF although it causes qubits
to lose quantum superposition and entanglement so quantum coherence
needs LN. To suppress the molecular motion and have RT quantum
coherence, the researchers introduced a chromophore based on pentacene
in a UiO-type MOF hat can densely accumulate chromophores. The nanopores
inside the crystal enable the chromophore to rotate at a very
constrained angle. The MOF structure facilitated enough motion in the
pentacene units to allow the electrons to transition from the triplet
state to a quintet state, while sufficiently suppressing motion at RT to
maintain quantum coherence of the quintet multiexciton state. After
photoexciting the electrons with microwave pulses, the researchers
observed quantum coherence for > 100 ns at RT. Among the
photogenerated molecular qubits, SF has the unique ability to generate
high-spin quintet state. SF is a process in which two triplet excitons
(T1) are generated from one photoexcited singlet. A triplet
pair with singlet multiplicity 1TT (two T1
strongly coupled by an exchange interaction), arises from one
photo-excited singlet (S1), and the spin evolution of the
triplet pair leads to the formation of a four-spin entangled quintet
multiexciton 5TT. 5TT can be generated and
detected by light through the inverse process of SF (triplet-triplet
annihilation) and then, can be addressed in a single spin level. The
researchers report here RT observation of quantum coherence in
SF-derived 5TT by the suppressed molecular motion in a MOF
nanoporous crystalline material composed of metal ions and organic
ligands. By integrating chromophores into the ligand, the distance and
angle between chromophores can be precisely regulated in MOFs. Unlike
dense molecular crystals, MOFs have nanoscale voids in the crystals,
which allow the chromophores to move, and this movement can be
controlled by the topology of the network and the local molecular
density around the chromophores. The researchers prepared a UiO-type
pentacene-based MOF (Pn-MOF), synthesized by combining diamagnetic Zr
ions with a dicarboxylate ligand containing pentacene chromophore
exhibiting exothermic SF. The noninterpenetrated UiO-type structure can
prevent π-stacking between pentacene planes, which gives the pentacene
units enough motion to make the conversion from 1TT to
5TT . The dense integration in the Pn-MOF makes the pentacene
motion sufficiently suppressive that the quantum coherence of
5TT generated by microwave irradiation was observed for >
100 ns at RT. The decay of the echo signal from the quintet observed by
pulsed electron paramagnetic resonance (EPR) was longer than the decay
of the quintet-derived signal in continuous-wave time-resolved EPR
(CW-TREPR) measurements, suggesting that two components with different
degree of order exist and the highly crystalline domains, present as
minor components in the disordered MOF structure, exhibit long quantum
coherence. It has been unclear until now what kind of molecular motion
is required to obtain effective quintet generation and noise suppressed
qubits for quintet multiple excitons generated by photo-induced SF.
Conformational motion is necessary for the spin conversion from
1TT to 5TT. However, if the change in orientation
angle of the chromophore relative to the magnetic field is too large,
then exciton hopping cause dissociation and recombination between
excitons and 5TT quantum decoherence occurs.
For more information:
Phys.org, January 11 (2024); Science Advances, January 3 (2024).
WEEK OF JANUARY 8, 2024 [No. 1557]
Semiconducting graphene epitaxially grown on SiC:
researchers at the Georgia Institute of Technology in Atlanta, GA and
Tianjin University in Tianjin have fabricated a functional semiconductor
made from 2D semimetal graphene. In the past two decades, attempts to
modify the bandgap either by quantum confinement or by chemical
functionalization failed to produce viable semiconducting graphene. The
researchers here demonstrate that semiconducting epigraphene (SEG) on
single-crystal SiC substrates has a band gap of 0.6 eV and RT mobilities
> 5,000 cm2 V−1 s−1, which is ten
times larger than that of Si and 20 times larger than that of the other
2D semiconductors. When Si evaporates from SiC crystal surfaces, the
C-rich surface crystallizes to produce graphene multilayers. The first
graphitic layer to form on the Si-terminated face of SiC is an
insulating epigraphene layer that is partially covalently bonded to the
SiC surface. Spectroscopic measurements of this buffer layer
demonstrated semiconducting signatures, but the mobilities of this layer
were limited because of disorder. The researchers here demonstrate a
quasi-equilibrium annealing method that produces SEG (well-ordered
buffer layer) on macroscopic atomically flat terraces. The SEG lattice
is aligned with the SiC substrate. It is chemically, mechanically and
thermally robust and can be patterned and seamlessly connected to
semimetallic epigraphene using conventional semiconductor fabrication
techniques. By inducing a band gap in the intrinsic non-bandgap graphene
materials, the researchers produced a robust graphene semiconductor with
ten times the mobility of Si and unique properties not available in Si
including handling large currents and resisting heating. They achieved a
breakthrough on this work when they figured out how to grow epitaxial
graphene on SiC wafers using special furnaces. When properly made, the
epitaxial graphene chemically bonded to the SiC and started to show
semiconducting properties. To prove that their platform could function
as a viable semiconductor, the researchers needed to measure its
electronic properties without damaging it. They doped graphene with
electron donors which worked out without damaging the material or its
properties.
For more information:
Nature, January 3 (2024) page 60; Phys.org, January 3 (2024).
Tetraneutrons searched in 235U thermal fission:
researchers at the Tokyo Institute of Technology in Tokyo have
explored the possible emission rate of particle-stable tetraneutron via
thermal neutron-induced fission of 235U by performing γ-ray
spectroscopy in a 88SrCO3 sample irradiated in a
nuclear research reactor core. It was posited that stable
88Sr would produce 91Sr by a tetraneutron-induced
(4n,n) reaction and that observation of γ rays followed by β decay of
91Sr would indicate particle-stable tetraneutron emission.
However, the γ-ray spectrum of irradiated 88SrCO3
sample did not show any photopeak for 91Sr. The researchers
concluded that the emission rate of particle-stable tetraneutrons, if
they exist, is estimated to be < 8×10−7 per fission at the 95%
confidence level (using best guesses for the cross sections of the
reactions induced by hypothetical particle-stable tetraneutrons). The
existence of a tetraneutron comprising four neutrons has long been
debated. The researchers here motivated by a recent observations of
particle-stable tetraneutrons in bound and resonant states. However,
theoretical studies indicate that tetraneutrons will not exist in a
bound state if the interactions between neutrons are governed by the
common understanding of two or three-body nuclear forces. The
researchers here posited that the bound tetraneutron could be a ternary
particle in 235U fission (in addition to the binary fission
in thermal fission process for 235U there is ternary fission
with 0.2% probability). They used the instrumental neutron activation
analysis method. 88SrCO3 was chosen as the target sample and
it was irradiated for 2 h at a thermal power of 5 MW in a nuclear
research reactor. The 88Sr nuclei were posited to convert
into 91Sr with a Q value of 20 MeV minus the binding energy
of the tetraneutron. Since 91Sr is unstable, its radioactive
decay followed by the release of γ-rays would indicate the emission of
particle-stable tetraneutrons. The γ-ray spectroscopy results for the
irradiated 88Sr sample, however, did not show any photopeak
corresponding to the formation of 91Sr. The researchers plan
to increase the sample purity and the experimental sensitivity to
continue the search for tetraneutrons.
For more information:
Phys.org, January 4 (2023); Phys. Rev., C., November 22 (2023) page 054004.
WEEK OF JANUARY 1, 2024 [No. 1556]
Atmospheric neutrino oscillations measurements:
an
international group lead by researchers at Harvard University in
Cambridge, MA has found evidence indicating that additional measurements
of neutrinos generated in Earth's atmosphere when cosmic rays collide
with atmospheric atoms, could be used to reveal how the three types of
neutrino masses are ordered. They analyze the current shared systematic
uncertainties arising from the common flux and neutrino-water
interactions in several neutrino observatories. Then, they implement the
systematic uncertainties of each experiment and develop the atmospheric
neutrino simulations for Super-Kamiokande, with and without
neutron-tagging capabilities, IceCube Upgrade, ORCA, and
Hyper-Kamiokande detectors. They review the synergies and features of
these experiments to examine the potential of a joint analysis of these
atmospheric neutrino data in resolving the θ23 octant at 99% confidence
level, and determining the neutrino mass ordering above 5σ. They assess
the capability to constrain θ13 and the CP-violating phase (δCP) in the
leptonic sector independently from reactor and accelerator neutrino
data. A combination of the atmospheric neutrino measurements will
enhance the sensitivity to a greater extent than the simple sum of
individual experiment results reaching more than 3σ for some values of
δCP. The researchers have conducted an analysis of the expected
sensitivities of current and near- future water/ice-Cherenkov
atmospheric neutrino experiments as they may relate to the three types
of neutrino oscillations. By analyzing a large sample of such collisions
and the data they have revealed thus far, the research team found that
information is amassing at a rate that by 2030 a combined analysis of
these four leading atmospheric-neutrino experiments could determine the
neutrino-mass-ordering and set tighter constraints on CP violation.
Through an analysis of 24 years of data, Super-Kamiokande recently
showed that atmospheric-neutrino measurements are sensitive to mass
ordering. This sensitivity owes to the fact that the oscillation
probability for neutrinos propagating through Earth is modified by
matter effects, which would lead to different behavior for different
mass ordering. To assess what further information can be gleaned from
atmospheric-neutrinos experiments, the researchers here investigate the
sensitivity achievable through a combined analysis of data from
Super-Kamiokande and from three near-future atmospheric-neutrino
Cherenkov radiation experiments (Hyper-Kamiokande, KM3Net-ORCA, and an
upgrade to IceCube with 258-KT , 7 MT , 1 BT water detectors
respectively). Each of the detectors can provide unique contributions to
a global analysis. Due to their larger detector volume and
photon-detection scheme, IceCube Upgrade and KM3Net-ORCA will be
particularly sensitive to high-energy atmospheric neutrinos. The
expected statistics of their detections should allow precise
measurements of mixing angles and mass differences. With its higher
density of photon sensors, Hyper-Kamiokande will be more sensitive to
lower-energy neutrinos, which should enhance the sensitivity to CP
violation. With a LAr time-projection-chamber technique, DUNE (set to
start taking data with far detectors in 2029) will be better in probing
low-energy atmospheric-neutrino interactions The CP-violation phase is
the parameter that will benefit the most from the combined analysis. The
global analysis could yield a determination of neutrino mass ordering
with 5σ statistical confidence by 2030.
For more information:
Phys.org, December 27 (2023); Physics, December 20 (2023); Phys. Rev. X,
December 20 (2023) page 041055.
Condensate-like nuclear cluster structure observed:
an international group lead by researchers at Peking University in
Beijing and IKEN in Wako has observed the 02+ state of the neutron-rich
nucleus 8He, a cluster structure with two strongly correlated
neutron pairs (02+ state has dineutron clusters with spin 0, energy
state 2, and parity +). In its ground state, the 8He nucleus
includes a 4He nucleus and four neutrons. If, before decaying
(100's ms lifetime) it transitioned into its first 0+ excited
state, the four neutrons form two pairs (dineutron clusters). The
condensate-like bosonic cluster state made of a 4He nucleus
plus two dineutron clusters has been theoretically predicted in the
8He nucleus ground state but it has not been experimentally
observed until now. The researchers here observed it in nuclear
scattering experiments at the RIKEN Nishina Center. Together with
theoretical calculations, the results here provide evidence that the
four valence neutrons in the 02+ excited state of 8He form
two strongly correlated dineutron clusters and an exotic condensate-like
cluster structure. In the experiment the researchers fired a
high-intensity beam of 8He nuclei at PE and C targets. Some
collisions excited the nuclei into the condensate state, which quickly
decayed into a 6He nucleus and a neutron pair. The
6He nuclei were directed through dipole magnets to drift
detectors and plastic scintillators for characterization. The neutrons
struck a plastic scintillator whose layered construction made it
possible to identify which neutrons were correlated in a dineutron
cluster and which were not. The correlated neutron pairs and the
scattering count rate’s dependence on energy, angle, and type of target
were consistent with theoretical predictions. The researchers showed
that this observed state is characterized by a spin parity of 0+, a
large isoscalar monopole transition strength, and the emission of a
strongly correlated neutron pair, in line with theoretical
predictions.
For more information:
hys.org, December 28 (2023); Physics, December 13 (2023); Phys. Rev. Let.,
December 13 (2023) page 242501.
WEEK OF DECEMBER 25, 2023 [No. 1555]
QCCD trapped-ion quantum processor scaled:
researchers at Quantinuum in Broomfield, CO have shown a QCCD
trapped-ion quantum processor (Quantinuum System Model H2) in which ions
move around a racetrack-like structure with the loops made by rf
electrodes and ions stored in the curved regions, sorted in the top
straight region, and entangled in the bottom straight region. The
researchers benchmarked the performance of the system at three
progressively higher levels. They measured the errors of individual
gates and other primitive operations that are the building blocks of the
quantum processor. Then, they looked at more complex circuits, where
errors can accumulate in nontrivial ways. From these measurements, they
extracted effective gate fidelities that were consistent with the
individual gate benchmarking. Finally, they ran example algorithms that
closely matched potential near-term use cases. Their results suggest
that increasing circuit depth or time to a nontrivial degree is viable
given the gate fidelities. The researchers here benchmark a QCCD
trapped-ion quantum computer based on a linear trap with periodic
boundary conditions, which resembles a race track. The system, initially
operated with 32 qubits, incorporates several technologies for future
scalability including electrode broadcasting, multilayer rf routing, and
magneto-optical trap loading while maintaining and exceeding, the gate
fidelities of previous QCCD systems. They benchmark the performance of
primitive operations, including an average state preparation and
measurement error of 1.6(1)×10-3, an average single-qubit
gate infidelity of 2.5(3)×10-5, and an average two-qubit gate
infidelity of 1.84(5)×10-3. The system-level performance of
the quantum processor is assessed with mirror benchmarking, linear
cross-entropy benchmarking, a quantum volume measurement of QV =
2Â16, and the creation of 32-qubit entanglement in a GHz
state. They tested application benchmarks, including Hamiltonian
simulation, QAOA, error correction on a repetition code, and dynamics
simulations using qubit reuse. The researchers have increased here the
number of qubits (from 20 to 32) without increasing the per-qubit error
rate, and have put the system through its paces with full
component-level testing, a suite of industry-standard benchmark tests,
and a set of diverse applications. The rf electrodes are routed under
the top surface of the device, leading to increased adaptability for the
electrode geometry. A set of dc voltages are applied to multiple
electrodes in parallel, reducing the number of individual control
voltages that need to be sent into the vacuum chamber housing the
device. The ions are loaded into the device from a cloud of cold neutral
atoms in a magneto-optical trap (instead of from a warm vapor as is
conventional), enabling faster ion loading and reducing the time taken
to initialize an experiment. The researchers perform a fully automated
calibration of their system and keep track of characterized qubit
phases. They also implemented midcomputation measurements and real-time
feedback (essential for future demonstrations of fault tolerance). They
characterized each possible component of a quantum algorithm:
single-qubit operations, two-qubit operations, state preparation and
measurement, and ion transport. With this information, the researchers
fully cataloged all error sources, finding that the reliability of their
system is limited by errors associated with two-qubit operations and
with state preparation and measurement. They also performed system-level
benchmark tests. Although single-operation characterization gives a good
first guess of how a machine will perform, full system operation might
be worse (ex: crosstalk). The inferred error rates from component-level
testing matched quite well those from system-level benchmark tests. They
implemented a set of algorithms, each of which verified a separate
capability of the device. Building a truly 2D architecture comes with
new challenges such as achieving low-error ion transport through
junctions and scaling up the necessary electrical control signals. In
the system only 1%–2% of the computation time was spent doing quantum
operations; the rest was spent shuttling the ions and cooling them. This
could be increased by increasing the number of ions in each chain.
For more information:
Physics, December 18 (2023); Phys. Rev. X, November 18 (2023) page 041052.
Chiral phonon magnetic moment connected to electron band topology:
an international group of researchers at Rice
University in Houston, TX have used a tabletop pulsed-laser spectrometer
to examine the behavior of materials that are cooled near 0K and
subjected to a high pulse of magnetic energy. They have studied the
magnetic response of transverse optical phonons in a set of pseudobinary
alloy Pb1−xSnxTe films. The material is a
topological crystalline insulator (TCI) for x > 0.32 has a
ferroelectric transition at an x-dependent critical temperature, and has
exhibited chiral phonons in strong magnetic fields for x = 0. The
researchers report results of terahertz time-domain spectroscopy
(THz-TDS) experiments. Polarization-dependent THz magneto spectroscopy
measurements revealed Zeeman splittings and diamagnetic shifts,
demonstrating a large phonon magnetic moment. Films in the topological
phase exhibited phonon magnetic moment values that were larger than
those in the topologically trivial samples by two orders of magnitude.
The sign of the effective phonon g-factor was opposite in the two
phases, a signature of the topological transition. The recently
demonstrated chiral modes of lattice motion carry angular momentum so
directly couple to magnetic fields. Their magnetic moments are predicted
to be strongly influenced by electronic contributions. In magnetic
fields, chiral phonons preferably absorb polarized light of a given
handedness, resulting in magnetic circular dichroism. Chiral phonons
carry a finite magnetic moment that results in the phonon Zeeman effect,
which has been observed in the narrow-gap semiconductor PbTe, the
nonaxial CeF3 , and the Dirac semimetal
Cd3As2 , with the phonon magnetic moment values
ranging from hundredths to several Bohr magnetons. Large values of
phonon magnetic moment are predicted to be obtained when electronic
contributions are considered including a mechanism where the circular
motion of a chiral phonon induces an electronic orbital response that
contributes to the phonon magnetic moment. The chiral phonon could
induce inertial effects on the electrons, leading to an effective
spin-chiral phonon coupling. Electronic contributions to the phonon
magnetic moment open the possibility for the interplay of chiral phonons
and electronic topology. However, no experimental evidence has been
reported until now. The researchers here studied the connections between
the magnetic properties of large magnetic moment chiral phonons and a
material's underlying topology of the electronic band structure to
demonstrate that the magnetic moment of phonons is significantly
enhanced in topological materials. In a previous study, the group
applied a magnetic field to the semiconductor PbTe. When they did so,
the phonons stopped vibrating in a linear fashion and became chiral,
moving in a circular motion. After noting that chiral phonons' magnetic
moment was quite small in PbTe they tried to increase it by changing the
material's electronic band structure so they tested a crystalline
topological insulator. They added enough Sn to get band inversion,
creating topologically protected surface states. They measured that the
chiral phonons' magnetic moment was two orders of magnitude larger in
the topological material than in the material without such electronic
topology. The researchers studied one Pb1−xSnxTe
sample in the trivial phase (x = 0.24) and two samples in the
topological phase (x = 0.42 and 0.56). For each sample, they observed
two anharmonicity-split transverse optical (TO) phonon modes and
characterized their magnetic properties at temperatures low enough to
place the samples in their ferroelectric phases. Both the trivial and
topological samples exhibited chiral phonons, as a consequence of being
in a ferroelectric phase with broken inversion symmetry. However, across
the topological transition, the phonons switched chirality, and the
phonon magnetic moment increased by two orders of magnitude. The
ferroelectric transition might influence the increase effect. They
supplemented their experimental observations with a theoretical model
for the phonon magnetic moment arising from the electronic orbital
response, which captures the phonon chirality switching across the
topological transition. They studied two TO phonon modes across the
trivial to TCI transition. They observed the occurrence of a
ferroelectric phase in all the samples at a composition-dependent
critical temperature. In that phase and under intense magnetic fields,
the phonon modes exhibited circular polarization with opposite
handedness. While the sample in the topologically trivial phase (x =
0.24) showed magnetic properties in agreement with a previous report for
PbTe, the films in the TCI phase displayed unexpected results. A high
degree of magnetic circular dichroism could be reached at low magnetic
fields. The obtained effective g-factors for both phonons increased by
two orders of magnitude and changed sign for the lowest-energy phonon
mode across the topological transition, thus acquiring opposite signs
between the modes. The diamagnetic shift showed an increase of one order
of magnitude. The results here demonstrate that the magnetic properties
of phonons are largely enhanced in topological materials.
For more information:
Science Advances, December 15 (23); Phys.org, December 16 (2023).
WEEK OF DECEMBER 18, 2023 [No. 1554]
Strain-tunable, superconducting spin valve demonstrated:
a group lead by researchers at the University of
Washington in Seattle, WA has applied stress as a control switch between
a field tunable superconducting state and a robust non-field tunable
state, to demonstrate a strain- tunable, superconducting spin valve (a
heterostructure with ferromagnetic layers surrounding a superconducting
layer) with infinite magnetoresistance. An applied magnetic field
switches the sandwiching ferromagnetic layers between parallel and
antiparallel alignment, which strongly tunes the magnetic pairbreaking
effect and effectively turns the superconductivity on and off. This
enables the ultimate switchability of magneto-transport, between a
resistive and zero-resistance state, thus achieving infinite
magnetoresistance. The researchers combine tunable uniaxial stress and
applied magnetic field on the ferromagnetic superconductor
Eu(Fe0.88Co0.12)2As2 to
shift the field-induced zero-resistance temperature between 4 K and 10
K. They use XRD and spectroscopy measurements under stress and field to
reveal that strain tuning of the nematic order and field tuning of the
ferromagnetism act as independent control parameters of the
superconductivity. Combining comprehensive measurements with DFT
calculations, they propose that field-induced superconductivity arises
from the uniquely dominant effect of the Eu dipolar field when the
exchange field splitting is nearly zero. The researchers present
field-induced superconductivity between a range of temperatures by
combining XRD, XR circular dichroism, and transport measurements to show
how strain and magnetic field facilitated independent tuning knobs. The
high tunability of the system resulted in the simultaneous co-existence
of superconducting, nematic, and ferromagnetic phases. They expect even
higher field-induced superconducting temperatures in materials
engineered with a perfect balance between higher temperature
superconductivity and ferromagnetism. They combined tunable uniaxial
stress and applied a magnetic field on the ferromagnetic superconductor
to shift the field-induced zero resistance temperature. Using XRD, and
spectroscopy measurements under stress, the researchers proposed the
origin of field-induced superconductivity to result from dipolar fold.
They showed field-induced superconductivity in 12% co-doped
superconducting materials with varying temperature, with applied
uniaxial stress. The value is the highest reported temperature of
magnetic field-induced superconductivity in any material. The doped
materials existed as a naturally grown thin-film superconducting spin
valve architecture, with alternating ferromagnetic and superconducting
layers. They combined the synchrotron XR methods with transport
measurements to show strain tuning capacity and field tuning properties
to exist as features of independent superconductivity. They combined
strain tunability with high temperature and low switching fields. They
performed DFT calculations to highlight ferromagnetic and
antiferromagnetic exchange interactions. During these experiments, the
researchers grew single crystals with 12% co-doped materials in Sn flux
and noted how the non-stoichiometric growth composition yielded samples
with increased superconducting transition temperatures. They selected
samples from different growth batches and prepared them identically to
better compare field and strain tuning of resistivity. Then, they cooled
the samples through the superconducting and ferromagnetic temperatures,
respectively. After conducting these measurements, they mounted the
sample to a uniaxial stress device to measure the resistivity and strain
range. When they applied the field at fixed temperature stress, they
constructed a superconductivity strain field-tunable phase diagram. The
researchers noted the accessibility of the field-induced
superconductivity in a temperature window under zero strain. As the
temperature decreased, the increasing magnetic moment led to
ferromagnetism having a larger influence on superconductivity. To
identify the independence of strain and magnetic field to tune
superconductivity and resolve the mechanism of field-induced
superconductivity, they conducted transport measurements under applied
strain, concurrent with XRD or XR magnetic circular dichroism at the
Advanced photon source in ANL. XRD allowed them to study ferromagnetic
superconductors with element-specific magnetic information, under
fluorescence mode. They effectively strain-tuned the superconductivity
through its competition with strain-tunable nematicity and the
associated ferromagnetic order. They noted field-induced
superconductivity where a narrow strain range permitted field-induced
superconductivity. To investigate the origin of field-induced
superconductivity, the researchers performed simultaneous resistivity
and XR measurements to independently tune the parameters of
superconductivity. The researchers incorporated the antiferromagnetic
parent compound as a strong biquadratic interaction between the metallic
moments to manifest large magneto-structural coupling. They noted Zeeman
splitting induced by an external field to facilitate superconductivity
and the co-existence of superconductivity and ferromagnetism.
DFT)calculations show that the cancellation of Eu-Fe ferromagnetic and
antiferromagnetic exchange interactions results in a very weak exchange
field, which solves the question of how the superconducting order can
coexist with ferromagnetism. In the near absence of the exchange field,
the Eu dipole field has a dominant effect on the superconductivity. The
researchers introduce a new mechanism for field-induced
superconductivity, whereby an external field reorients this internal
dipole field from the direction of lower to higher upper critical field
Hc2, enabling the applied field to enhance T0. The
researchers present field-induced superconductivity between 4 and 10 K,
which is enabled with small fields (μ0H ≤ 0.1 to 0.3 T) and
tuned with accessible strain values (|ɛxx| < 0.2%).
Combined XRD, XMCD, and transport measurements show that strain and
magnetic field act as independent tuning knobs, with the former
affecting the nematic order and Fe antiferromagnetism and the latter
affecting the Eu ferromagnetism. These knobs tune the phase diagram
analogously to chemical doping, but without introducing additional
disorder. The high tunability of this system results from the close
competition between the simultaneously coexisting superconducting,
nematic, and ferromagnetic phases. They show how the external
field-tunable Eu dipole field has a dominant effect on superconductivity
when the Eu-Fe exchange splitting is sufficiently weak.
For more information:
Phys.org, December 12 (2023); Science Advances, November 24 (2023).
Electron transport measured in quantum Wigner solid:
researchers at Princeton University in Princeton, NJ have precisely
measured the electrical-transport properties of a highly ordered
crystalline state formed of electrons. When subjected to a strong
magnetic field, a 2D system of electrons can form a crystal structure
like a Wigner solid (WS) using an ultra-high-quality electron system. In
low-disorder, 2D electron systems (2DESs), the fractional quantum Hall
states at very small Landau level fillings (ν) terminate in a WS phase,
where electrons arrange themselves in a periodic array. The researchers
tailored the layer-by-layer growth of a semiconducting film such that
the film contained a precisely shaped version of a quantum well. This
structure was designed so that it hosted a 2DES with both a low density
and a high mobility (prerequisite for making a WS). Then they cooled
this electron system to near 0 K and then applied a high magnetic field
to it, to realize an ordered WS. Next, they applied an electric current
to the WS and measured how the current’s strength affected the crystal’s
electronic-transport properties, specifically, its differential
resistance and its electrical noise. They found that as the current’s
strength increased, the WS passed through three phases, each
identifiable by its differential-resistance and electrical-noise
characteristics. The crystal remained stationary and was an electrical
insulator in the first phase but could move freely and was an electrical
conductor in the other two phases. The WS is typically pinned by the
residual disorder sites and manifests an insulating behavior, with
nonlinear current-voltage (I−V) and noise characteristics. The
researchers here report measurements on an ultralow-disorder, dilute
2DES, confined to a GaAs quantum well. In the ν<1/5 range,
superimposed on a highly insulating longitudinal resistance, the 2DES
exhibits a developing fractional quantum Hall state at ν=1/7, attesting
to its exceptional high quality and dominance of electron-electron
interaction in the low filling regime. In the nearby insulating phases,
they observe nonlinear I−V and noise characteristics as a function of
increasing current, with current thresholds delineating three distinct
phases of the WS: a pinned phase (P1) with very small noise, a second
phase (P2) in which dV/dI fluctuates between positive and negative
values and is accompanied by very high noise, and a third phase (P3)
where dV/dI is nearly constant and small, and noise is about an order of
magnitude lower than in P2. In the depinned (P2 and P3) phases, the
noise spectrum also reveals well-defined peaks at frequencies that vary
linearly with the applied current, suggestive of washboard
frequencies.
For more information:
Physycs, December 5 (2023); Phys. Rev. Lett., December 5 (2023) page 236501.
WEEK OF DECEMBER 11, 2023 [No. 1553]
Molecules entangled:
researchers at Princeton University in Princeton, NJ and
independently at Harvard University in Cambridge, MA have produced
on-demand entanglement of molecules in a reconfigurable optical tweezers
array set up where cooling, controlling, and entangling is performed.
Ultracold polar molecules have long-lived molecular rotational states to
form robust qubits, and long-range dipolar interaction between molecules
to provide quantum entanglement. The researchers here demonstrate
dipolar spin-exchange interactions between CaF molecules trapped in an
optical tweezers array. They realized the spin- quantum XY model by
encoding an effective spin- system into the rotational states of the
molecules and used it to generate a Bell state through an iSWAP
operation. Conditioned on the verified existence of molecules in both
tweezers at the end of the measurement, they obtained a Bell state
fidelity of 0.89(6). They report using the dipolar interactions between
CaF molecules placed in optical tweezers for tailoring the quantum
states of individually addressable molecules to achieve quantum
entanglement on demand. They picked a polar molecular species that can
be cooled with lasers; and then, laser-cooled the molecules to ultracold
temperatures. Individual molecules were then picked up by optical
tweezers. By engineering the positions of the tweezers, they were able
to create large arrays of single molecules and individually position
them into any desired 1D configuration. Next, they encoded a qubit into
a non-rotating and rotating state of the molecule. They showed that this
molecular qubit remained coherent. They generated well-controlled and
coherent qubits out of individually controlled molecules. By using a
series of microwave pulses, they made individual molecules coherently
interact with one another. By allowing the interaction to proceed for a
precise amount of time, they implemented a two-qubit gate that entangled
two molecules.
For more information:
Science, December 7 (2023) pages 1143, 1138 and 1118; Phys.org, December 7
(2023.
Single-molecule ESR-AFM:
researchers at the University of Regensburg in Regensburg have
performed single-molecule electron spin resonance by means of atomic
force microscopy. Although ESR of single spins with nanoscale resolution
has been demonstrated, the understanding of decoherence in complex
solid-state quantum systems requires controlling the environment down to
atomic scales. The recent implementation of ESR in STM has been followed
by the demonstration of coherent oscillations and access to nuclear
spins with atomic resolution. However, the current-based sensing
inherent to this method limits coherence times. The researchers here
demonstrate pump–probe ESR-AFM detection of electron spin transitions
between non-equilibrium triplet states of individual pentacene
molecules. Spectra of these transitions exhibit < 1 neV spectral
resolution, allowing local discrimination of molecules that only differ
in their isotopic configuration. The electron spins can be coherently
manipulated over 10's µs. This single-molecule ESR-AFM can be combined
with atomic manipulation and characterization and allow research in the
atomistic origins of decoherence in atomically well-defined quantum
elements. ESR is detected directly with the AFM's tip, such that the
signal comes from one individual molecule only. This way, they can
characterize single molecules and determine the atoms composing the
imaged molecule. The researchers discriminate molecules that do not
differ in the type of atoms that they are composed of, but only in the
composition of the atoms' nuclei. They could operate the quantum state
of the spin in a single molecule many times before the state decohered.
To drive and probe ESR transitions, they first brought a closed-shell
pentacene molecule to the excited triplet state T1 by driving
two tunneling events with pump pulses, first extracting an electron from
the highest occupied molecular orbital (HOMO) and then injecting an
electron into the lowest unoccupied molecular orbital (LUMO). The two
unpaired electrons in the HOMO and LUMO form the triplet. The subsequent
decay from T1 into the singlet ground state S0 can
be measured by transferring the remaining population in T1
after a controlled dwell time tD to the cationic charge
state, whereas pentacene in S0 remains neutral. The two
charge states, cationic and neutral, can then be discriminated in the
AFM signal owing to their different electrostatic force acting on the
tip during a probe period, allowing the population decay of state
T1 to be measured as a function of tD . The
cationic state is only used to create the triplet state and facilitate
the readout, whereas the ESR spectroscopy and spin manipulation occurs
in the neutral triplet state.
For more information:
Nature, December 6 (2023) page 64; Physworld, December 6 (2023).
WEEK OF DECEMBER 4, 2023 [No. 1552]
Nuclear charge radius of 26mAl measured:
an international group of researchers have used the ISOLDE experiment
at CERN in Geneva to measure the nuclear charge radius (Rc)
of 26mAl by collinear laser spectroscopy thus probing the
assumption that there are only three families of quarks and the effects
of the weak force on quarks. The result turns out, to be much larger
than assumed in the calculation of the probability of the down quark
transforming into the up quark. The measured isotope shift to
27Al in the 3s23p2 P0
3/2 → 3s24s2 S1/2 atomic
transition enabled the experimental determination of Rc in
26mAl, giving Rc = 3.130(15)  fm. This differs by
4.5σ from the extrapolated value used to calculate the isospin-symmetry
breaking corrections in the superallowed β decay of 26mAl.
Its corrected Ft value, important for the estimation of Vud
in the Cabibbo-Kobayashi-Maskawa (CKM) matrix, is thus shifted by 1σ to
3071.4(1.0)Â s. The up quark, the lightest and most experimentally
accessible quark, provides the most stringent test of quark mixing
matrix (CKM-matrix) unitarity. Of the three matrix elements involving
this quark, the largest and most precisely known is the matrix element
of the transformation of an up quark into a down quark, Vud.
However, in the unitarity test, the square of that matrix element is
needed, and the uncertainty in Vud remains the main
contribution to the final uncertainty in the sum. Vud cannot
be determined directly but must be extracted from measurements of β
decay rates once those have been corrected for nuclear and atomic
factors, such as spin and nuclear charge distribution. Of the more than
3000 radioactive nuclei observed in the laboratory, there are a few in
which β decay is simpler than in the others and in which those
corrections are minimal. The long-lived excited state 26mAl
is one of these and it has one of the most precisely measured β decay
rates that constrains Vud. The researchers here explored how
the Rc of 26mAl directly affects the determination
of Vud and thus the testing of CKM-matrix unitarity. The
probability sum involving the up quark presently differs from unity.
This sum includes the respective probabilities of the down quark, the
strange quark and the bottom quark transforming into the up quark. The
first of these probabilities manifests itself in the β decay of an
atomic nucleus, in which a neutron changes into a proton or vice versa.
However, due to the complex structure of the atomic nuclei that undergo
β decays, an exact determination of this probability is generally not
feasible. The researchers here turned to a subset of β decays that are
less sensitive to the effects of nuclear structure to determine the
probability. Among the several quantities that are needed to
characterize β decays is the Rc of the decaying nucleus.
Although the Rc of the ground-state 26Al has
already been reported, that of the isomer had been more elusive, and the
value used to evaluate Vud has been extrapolated. The
challenge came from the half-life (6.35 s for the isomer vs. 717,000 y
for the ground state) and from the low production of the isomer. The
researchers here studied 26mAl using two different
experiments: COLLAPS at the radioactive-ion-beam facility ISOLDE at CERN
and IGISOL CLS at the Accelerator Laboratory of the University of
Jyväskylä, Finland. These facilities use different nuclear reactions to
generate and extract 26Al and 26mAl, resulting in
different ratios for the production yields of the two nuclear states. To
distinguish these states, the group used the difference in half-life at
COLLAPS and multiple atomic transitions in Al at IGISOL CLS. The
resulting Rc, a weighted average of the ISOLDE and IGISOL
datasets, is larger, and the probability sum involving the up quark is
closer to unity. Altogether, the two campaigns allowed them to extract a
value for 26mAl as Rc = 3.130 ± 0.015 fm (the
previously reported value was 3.040 ± 0.020 fm). In regard to the
CKM-matrix unitarity, they found a shift closer to unitarity for the top
row of the CKM matrix: from 0.99848 ± 0.00070 to 0.99856 ± 0.00070.
For more information:
hysics, November 27 (2023); Phys.org, November 28 (2023); Phys. Rev. Lett.,
November 27 (2023) page 222502.
Nanoparticle-assisted wakefield accelerator operated at 10 GeV/10cm:
an international group lead by researchers at the University of Texas
at Austin in Austin, TX has built a particle accelerator system with
system length < 20 m that produces a 10 GeV electron beam within a 10
cm gas cell. The researchers find that the nanoparticle-assisted laser
wakefield accelerator (LWFA) can generate 340 pC, 10 ± 1.86 GeV electron
bunches with 3.4 GeV rms convolved energy spread and 0.9 mrad rms
divergence. It can also produce bunches with lower energies in the 4–6
GeV range. From the 26 recorded electron spectra under various
experimental conditions, one electron spectrum showed an electron bunch
with 0.34 nC of charge and a centroid energy of 10 ± 1.86 GeV, while
another electron spectrum showed electron bunches with a tail extending
beyond 10 GeV. In system the main laser is the Texas Petawatt Laser (150
fs pulses) with an auxiliary laser used to strike a metal plate inside
the gas cell to inject a stream of metal nanoparticles to release
electrons and thus, boost the energy delivered to the gas electrons from
the plasma waves. They get more electrons into the wave when and where
the researchers want them to be, rather than statistically distributed
over the whole interaction. The group aims to use next a table-top Pw
laser in development. Due to the nonlinearity of the LWFA process, the
injection position and the number of electrons injected into the
wakefield depend very strongly on the laser and gas conditions before
the interaction. Small variations in the laser and gas conditions can
lead to shot-to-shot fluctuations of the accelerated electron beam
properties. Various schemes have been developed to address and control
the stability of LWFAs, including ionization injection, which increases
the charge; fast down-ramp injection, which reduces the energy spread
and controls the electron energy; and colliding laser beams, which
control the electron beam energy. As the injection process seems to be
the largest source of beam fluctuations, the idea here was to inject
electrons into the nonlinear plasma wave (NPW) using nanoparticles. The
use of nanowires and nanoparticles has been shown theoretically and
experimentally to trigger the injection of electrons into the NPW and
increase the charge density, thus providing a possible method for
controlling the parameters of the accelerated electron beam. The
nanoparticles here are generated inside a gas cell through laser
ablation of a metal surface and are assumed to be mixed uniformly with
the He gas fed into the gas cell. The researchers cannot control when
the injection happens due to the random distribution of nanoparticles in
the experiment. They do not have a satisfactory model or experimental
explanation for the generation of the resulting high electron energies.
The researchers plan to focus on characterizing the system in terms of
the output electron parameters by using better statistics and by probing
the wakefields using few-cycle lasers and electron beams. Due to the
high number of shots required for this experiment (statistics and
probing), access to a high repetition rate Pw-class laser (0.01–1 Hz)
will be required.
For more information:
Phys.org, November 28 (2023); Matt. Rad. Extremes, November 15 (2023).
WEEK OF NOVEMBER 27, 2023 [No. 1551]
Electron current with liquid-like flow detected:
researchers at Rice University in Houston, TX and in TU Wien in
Vienna have performed measurements of quantum charge fluctuations
showing shot noise suppression, thus providing evidence that electricity
seems to flow through strange metals in a liquid-like form that cannot
be readily explained in terms of quantized packets of charge
(quasiparticles). They measured shot noise to probe the granularity of
the current-carrying excitations in nanowires of the heavy fermion
strange metal YbRh2Si2. When compared with
conventional metals, shot noise in these nanowires is strongly
suppressed. This suppression cannot be attributed to either
electron-phonon or electron-electron interactions in a Fermi liquid,
which suggests that the current is not carried by well-defined
quasiparticles in the strange-metal regime probed here. The
quasiparticle description of electron transport is expected to break
down in strange metals, leading to a reduction in shot noise. The
researchers tested this prediction by measuring shot noise in
YbRh2Si2 nanowires. In these samples, shot noise
was reduced compared with the values measured in a comparable Au
nanowire and in consistency with the theoretical expectations for a
system of quasiparticles. The sample material used here contains a high
degree of quantum correlation that produces linear temperature-dependent
resistivity behavior. If cooled below a critical temperature, the
material instantly transitions from non-magnetic to magnetic. At
temperatures slightly above the critical threshold, is a heavy-fermion
metal, with charge-carrying quasiparticles that are hundreds of times
more massive than bare electrons. Some prior theoretical studies have
suggested that strange metal charge carriers might not be
quasiparticles. Shot noise experiments allowed researchers here, to
gather direct empirical evidence to test the idea. In the theory of
quantum criticality they used by, the electrons are pushed to the verge
of localization, and the quasiparticles are lost everywhere on the Fermi
surface. The larger question is whether similar behavior might arise in
any or all of the dozens of other compounds that exhibit strange metal
behavior. This strange metallicity shows up in many different physical
systems despite the fact that the microscopic, underlying physics is
very different.
For more information:
Science, November 23 (2023) page 907; Phys.org, November 23 (2023).
Nuclear ground state with molecular-like structure detected:
an international group lead by researchers at Hong Kong University in
Hong Kong has determined the 10Be ground-state molecular
structure using the reaction 10Be(p,pα)6He triple
differential reaction cross-section measurements. The cluster structure
of the neutron-rich isotope 10Be has been probed via the
(p,pα) reaction at 150  MeV/nucleon in inverse kinematics and in
quasi-free conditions. The populated states of 6He residues
were investigated through missing mass spectroscopy. The triple
differential cross section for the ground-state transition was extracted
for quasi-free angle pairs (θp,θα) and compared to distorted-wave
impulse approximation reaction calculations performed in a microscopic
framework using successively the Tohsaki-Horiuchi-Schuck-Röpke product
wave function and the wave function deduced from antisymmetrized
molecular dynamics calculations. The agreement between calculated and
measured cross sections in both shape and magnitude validates the
molecular structure description of the 10Be ground-state,
configured as an α−α core with two valence neutrons occupying π-type
molecular orbitals. Ground-state cluster structures were considered in
1938 for the analysis of binding energies in α-conjugate nuclei,
suggesting the possible existence of α-molecule-like cluster structures
in the ground states of some nuclei (8Be, 12C, and
16O). The use of the classical shell model's single-particle
description put that aside. Using the inverse kinematics knockout
reaction, researchers here have validated the presence of a
molecular-type structure in the ground state of 10Be. The
experiment was conducted at the Radioactive Isotope Beam Factory (RIBF)
at the RIKEN Nishina Center in Japan. In the experiment, a 0.5c
secondary beam of 10Be, bombarded a 2-mm-thick solid H
target. The α-clusters bound within 10Be nuclei were knocked
out by p's (with almost no momentum transferred to the residual nucleus)
thus preserving information about the cluster structure in the ground
state of Be-10. Detecting all three products (the p, the α, and the
remaining 6He fragment) of this interaction allowed the
researchers to directly measure the locations of the α's in the original
nucleus and compare the results with calculations. They showed that the
ground state of 10Be is analogous to a diatomic molecule,
with two α particles acting like atoms and two neutrons orbiting like a
pair of electrons forming a covalent bond. The experimental results
demonstrated agreement between the experimental cross-sections of
knockout reactions and theoretical predictions. This verification
supports the long-standing hypothesis regarding the molecular-state
structure of 10Be's ground state, suggesting the formation of
an α–α dumbbell-shaped core with two valence neutrons rotating
perpendicular to the core axis.
For more information:
Physics, November 21 (2023); Phys.org, November 28 (2023); Phys. Rev. Lett.,
November 21 (2023) page 212501.
WEEK OF NOVEMBER 20, 2023 [No. 1550]
Neutron skin in 208Pb nucleus measured using ultrarelativistic
nuclear collisions:
an international group has used the LHC at CERN in Geneva to
determine the thickness of the neutron skin in 208Pb nuclei.
They determined the neutron skin from measurements of particle
distributions and their collective flow in 208Pb
+208Pb collisions at ultrarelativistic energies, which were
mediated by interactions of gluons, and thus, are sensitive to the
overall size of the colliding 208Pb ions. By means of global
analysis tools within the hydrodynamic model of heavy-ion collisions,
they inferred a neutron skin Δrnp = 0.217 ± 0.058  fm, consistent with
nuclear theory predictions, and competitive in accuracy with a recent
determination using parity-violating asymmetries in polarized electron
scattering. When 208Pb nuclei are collided, the neutron
distribution affects the shape of the quark–gluon plasma matter
produced, leaving measurable imprints in the distributions of detected
particles. Using data from heavy-ion runs of the LHC the researchers
determined the neutron skin thickness to be 0.217 ± 0.058 fm based on
strong force experiments. The Lead Radius Experiment (PREX)
collaboration at the Jefferson Lab in Newport News, VA calculated it at
0.283 ± 0.071 fm in 2021 based on electroweak force experiments. In
total, the researchers here used 670 data points taken from Runs 1 and 2
of the LHC, mostly from the ALICE experiment, with some points coming
from the ATLAS and CMS experiments.
For more information:
Phys.org, November 15 (2023); Phys. Rev. Lett., November 15 (2023) page 202302.
Near QSL-state observation in 2D triangular antiferromagnet:
a group lead by researchers at ORNL in Oak Ridge, TN has provided
evidence of quantum spin liquid (QSL) behavior in KYbSe2, a
delafossite material with layered triangular lattice. Phil Anderson
hypothesized (1973) that a QSL state existed on some triangular lattices
where pairs of quantum entangled particles fluctuated among multiple
combinations. The researchers here observed multiple features of a QSL
including quantum entanglement, quasiparticles and the right balance of
exchange interactions, which control how a spin influences its
neighbors. By examining the spin dynamics with the Cold Neutron Chopper
Spectrometer at the SNS and comparing the results to theoretical models,
they found evidence that the material was close to the quantum critical
point for the QSL state. They analyzed its single-ion magnetic state
with SNS's Wide-Angular-Range Chopper Spectrometer. The data from the
neutron scattering experiments showed strong correlations between
KYbSe2 and the simulated spectrum of a QSL state. Although
KYbSe2 is not a true QSL, the fact that ~ 85% of the
magnetism fluctuates at low temperature means that it has the potential
to become one. The researchers anticipate that slight alternations to
its structure or exposure to external pressure should get 100%
fluctuation. The researchers here demonstrate that a spin-half
delafossite material (KYbSe2), shows close proximity to the
triangular-lattice Heisenberg QSL. Using neutron scattering, they
identify a diffuse continuum with a sharp lower bound within the
measured spectra. Applying entanglement witnesses to the data indicates
multipartite entanglement spread between its neighbors, and an analysis
of its magnetic-exchange couplings reveals close proximity to the
theoretical QSL phase. The key features of the data are reproduced by
Schwinger boson theory and tensor network calculations with a
substantial next-nearest-neighbor coupling. The strength of the
dynamical structure factor at the Brillouin-zone K point shows a scaling
collapse down to 0.3 K, indicating the existence of a second-order
quantum phase transition. Comparing this with previous theoretical work
suggests that the proximate phase at a larger next-nearest-neighbor
coupling is a gapped spin liquid.
For more information:
Phys.org, November 16 (2023); Nat. Phys., November 6 (2023) page 74.
WEEK OF NOVEMBER 13, 2023 [No. 1549]
Top quark - photon observed in pp collision:
the ATLAS Collaboration have used the LHC in CERN at Geneva to observe
the coincident production of a single top quark and a photon (tqγ event)
from a pp collision, directly probing the electroweak coupling of the
top quark. The analysis uses 139 fb-1 of 13Â TeV pp collision
data collected with the ATLAS detector at the LHC. Requiring a photon
with transverse momentum larger than 20Â GeV and within the detector
acceptance, the cross section is measured to be 688±23(stat) +
75−71(syst) fb (the SM prediction is 515+36−42 fb at next-to-leading
order in QCD). On a Feynman diagram, the initial state consists of a
gluon and quark constituents of the colliding protons. The gluon splits
into a bottom–antibottom quark pair. Then the b quark interacts with the
initial quark via the weak force, mediated by the W boson, to produce a
single top quark and a different quark. The top quark radiates a photon
before decaying into a b quark and a W boson, which subsequently decays
into a charged lepton (electron or muon) and its partner neutrino.
Previously, a single top quark had only been seen in association with a
W or a Z boson with the photon absent despite its higher probability of
occurring. The production processes behind single-top-quark events are
highly sensitive to top-quark electroweak interactions. But they are
much harder to observe than production processes that create particle
pairs (ex: a top–antitop quark pair). That difficulty comes because
single-top-quark events are rarer than pair ones and the presence of
similar looking top–antitop quark-pair events can swamp single-top-quark
signals. The tqγ measurement achieved here targeted the leptonic decay
of the W boson after a tqγ event. After its production, the top quark
immediately decays into a W boson and a bottom quark. The W boson then
decays into either a pair of quarks or a charged lepton (electron or
muon) and a neutrino. Identifying and measuring the properties of
charged leptons is easier than doing the same for the quarks, which join
in pairs to create hadrons, so lepton channels are favored for measuring
top-quark processes. For its observation, the researchers analyzed only
the subset of events that contained all the final-state particles
expected to be produced in tqγ: a photon, a b jet (a cluster of
energetic hadrons deemed likely to have originated from a b quark), a
lepton, and some missing energy, the hallmark of a neutrino (which does
not leave a trace in the detectors). They selected out these events
using machine-learning algorithms trained on simulated data. The
machine-learning algorithm outputs were also used to extract the
experimental tqγ rate, along with its uncertainty. The researchers
determined that the tqγ observation had a significance of 9.8 σ. The
quoted uncertainty in the measured rate is ~ 10% (with the statistical
confidence of the tension between the data and the model being 2 σ) A
previous analysis made by CERN’s CMS Collaboration concurred on the tqγ
rate being higher than SM predicted.
For more information:
November 7 (2023); Phys. Rev. Lett., October 30 (2023) page 181901.
Controllable splitting of a Cooper pair demonstrated:
an international group of researchers at TU Delft in Delft has
demonstrated the controllable splitting of a Copper pair into its two
constituent electrons within a hybrid quantum dot system, including
holding onto them after the split. The researchers can controllably
split and retain single Cooper pairs in a multi-quantum-dot device
isolated from lead reservoirs, and detect the electrons emerging from a
split pair. They identify a coherent Cooper pair splitting charge
transition using dispersive gate sensing at GHz. They utilize a double
quantum dot as an electron parity sensor to detect parity changes
resulting from electrons emerging from a superconducting island.
Conventional devices to split electrons bound in Cooper pairs consist of
a superconductor-based electrical contact and two ordinary metallic
contacts, separated by quantum dots. Quantum dots typically only receive
one electron at a time, while electrical current flowing through
superconductors is carried by electron Cooper pairs. If one forces a
current between the superconductor and the metal contacts, Cooper pairs
have no choice but to split in order to make it through the quantum dots
towards the other metal terminals of the circuit. The researchers here
replaced the superconducting lead with an isolated chunk of
superconductor and got rid of the electrical contacts entirely. By
applying electric fields to the quantum dots and superconductor, they
were able to push a single Cooper pair out of the superconductor,
forcing it to split onto the two quantum dots. The hybrid quantum dot
system here has no electrical current flowing through it. When the
researchers pushed a single Cooper pair out of the superconductor, the
electrons became isolated onto the quantum dots. Through this process,
the researchers were able to hold on to split electrons that were
previously part of a single Cooper pair. The researchers also
demonstrated a method for detecting single electrons jumping on to a
quantum dot without external charge sensors. They measured the shift in
the resonant frequency of a microwave resonator coupled to the
superconductor in their device as a function of the voltages applied to
the surrounding quantum dots. The frequency shifts when electrons move
back and forth between the dots and the superconductor.
For more information:
Phys.org, November 6 (2023); Phys. Rev. Lett., October 12 (2023) page 157001.
WEEK OF NOVEMBER 6, 2023 [No. 1548]
Ground-state nuclear decay by pentaproton emission detected:
a group lead by researchers at Washington University in St. Louis, MO
and Michigan State University in East Lansing, MI has observed a highly
unstable nucleus (9N, for which more than
half of nucleons are unbound) that decays by emitting five protons in a
three-step process. The 9N nucleus is
composed of a small He-like core surrounded by five untethered protons
that quickly escape after the nucleus’s formation. Previous experiments
have seen at most four unbound protons in a nucleus. The researchers had
to sift through a large volume of nuclear-collision data to identify the
9N decays. The observation of this barely
bound nucleus beyond the drip line challenges theories of nuclear
structure. While analyzing nuclear-collision data, they stumbled on a
signal that suggested a five-proton decay of
9N, a nucleus that is so far beyond the drip
line that no one thought it would be detectable. To confirm this
detection, the group modeled the 9N
structure. The core of the nucleus consists of two protons and two
neutrons. The five remaining protons are not bound to this core, but
they still must tunnel through an energy barrier to escape, The
researchers modeled this tunneling behavior using a theory in which the
nucleus emits protons in a way that resembles an atom emitting photons.
Their calculations revealed a three-step process in the
9N decay. First, one proton is emitted,
leaving a 8C nucleus behind. Then, two
protons exit, producing 6Be. Finally,
another two protons escape, so that only the
4He core remains. This five-proton emission
occurs in ~1 zs. The experiments were performed in 2016 as part of a
study of several proton-rich nuclei at the NSCL (presently FRIB). The
researchers started with a beam of 16O
nuclei and smashed it into a Be target. Out of the debris they isolated
13O nuclei, which they smashed into another
Be target. Detectors recorded the outgoing particles from this second
collision. For the 9N study, the group
selected events in which the collision products included one
4He nucleus and five protons. After sifting
out background events from other nuclear decays, the researchers found
that these six-particle events produced a range of total energies, with
the peak in the energy spectrum matching the predicted
9N mass. There is some ambiguity about the
detection because the peak in the energy spectrum may actually result
from two unresolved peaks, possibly representing the ground state and an
excited state of the nucleus. The invariant-mass spectrum of its decay
products can be fit with two peaks whose energies are consistent with
the theoretical predictions of an open-quantum-system approach; however,
the researchers cannot rule out the possibility that only a single
resonance-like peak is present in the spectrum. The researchers are
considering follow-up experiments that would discriminate between one-
and two-peak scenarios.
For more information: Physics, October 27 (2023); Phys.org, October 30
(2023); Science News, October 27 (2023) ; Phys. Rev. Lett., October 27 (2023)
page 172501.
Organic polariton condensate controlled at RT:
an international group lead by researchers at the Skolkovo Institute
of Science and Technology in Moscow has demonstrated active spatial
profile, density, and energy reversible control of a RT polariton
condensate without relying on the gain-inducing excitation polariton
profiles. This is realized by introducing an extra intracavity copolymer
semiconductor layer, nonresonant to the cavity mode. The researchers use
a double-dye organic microcavity (a weakly coupled absorber that remains
nonresonant to the cavity mode) with two-color excitation profiles
creating a polariton condensate in the center of a ring shape. By
partially saturating the optical absorption in the uncoupled microcavity
layer using the two-color beam excitation, the researchers achieved the
ultrafast modulation of the effective refractive index simultaneously
with the formation of a polariton condensate and, through excited-state
absorption, of the polariton dissipation as well.
For more information: Phys.org, October 31 (2023); Phys. Rev. Lett., October 30
(2023) page 186902
WEEK OF OCTOBER 30, 2023 [No. 1547]
Simultaneous scattering of two photons with a single two-level atom observed:
researchers at Humboldt Universität zu Berlin in Berlin have
revisited the scattering of light by a single fluorescent atom to
observe an effect predicted four decades ago. The quantum-mechanical
description of this interaction shows that the scattered light exhibits
photon antibunching; that is, it never contains two photons at the same
time and place. This property can be seen as a consequence of the photon
emission being associated with quantum jumps from the emitter’s excited
state to its ground state. It thus seems natural to state that a single
two-level atom will never scatter two photons simultaneously. However, a
more in-depth inspection of the optical Bloch equations reveals the
existence of two distinct components of the scattered field, referred to
as coherently and incoherently scattered light, reflecting their
respective ability and inability to interfere with the driving field.
Taken together, these components form the well-known Mollow structure in
the fluorescence spectrum. Interestingly, when considered individually,
each of these components also contains higher photon-number components,
that is, two or more photons at the same time and place. In view of this
fact, it has been argued that the origin of antibunching in resonance
fluorescence stems from the destructive interference between the higher
photon-number components of the coherently and incoherently scattered
light. Cross-correlation measurements between the sidebands of the
Mollow structure have revealed a time ordering between the detected
photons, whereby the photon originating from the sideband furthest from
the emitter’s resonance typically arrives first. This is commonly
interpreted as a cascaded two-photon emission from the dressed states of
the emitter. Recently, motivated by theoretical works, it has been
experimentally shown that by spectrally rejecting the incoherently
scattered component of the fluorescence light of a single quantum dot,
one can modify the photon statistics of the scattered field in such a
way that all correlations are lost, and the remaining light again
recovers the spectral and temporal characteristics of the classical
driving field. The researchers here show that when performing the
opposite, that is, rejecting the coherent component via spectral
filtering, the remaining incoherent component of the fluorescence light
of a single two-level atom consists of pairs of photons that appear to
have been simultaneously scattered by the atom. The results here
validate the picture that antibunching in resonance fluorescence arises
from fully destructive interference between the two-photon components of
the coherently and incoherently scattered light. As indicated before,
the interaction of light with a single two-level atom emitter, two
photons are never detected simultaneously in the light scattered by the
emitter. This is normally interpreted by saying that a single two-level
quantum emitter can only absorb and emit single photons. However, it has
been theoretically proposed that the photon anticorrelations can be
thought of as arising from quantum interference between two possible
two-photon scattering amplitudes, which are referred to as coherent and
incoherent. This is in stark contrast to the usual interpretation in
that it assumes that the atom has two different mechanisms at its
disposal to scatter two photons at the same time. The researchers here
experimentally validate this interference picture by showing that, when
spectrally rejecting only the coherent component of the fluorescence
light of a single two-level atom, the remaining light consists of photon
pairs that have been simultaneously scattered by the atom. When the
researchers removed a certain color component from the light with the
help of a filter, the single photon stream transformed into pairs of
photons that were detected simultaneously. Thus, by removing the correct
ones from a stream of single photons, the remaining photons suddenly
appear as pairs. The photon pairs generated are quantum mechanically
entangled. Thus, a single atom can be a source for entangled photon
pairs. The previous certainty that a single atom can only scatter one
photon at a time also seems to have been disproved here: When viewed
through the right color filter, the atom is able to scatter two photons
at the same time. In the experimental set up, a single atom is excited
by laser light and scatters one photon after another. An optical filter
removes certain color components from this stream of single photons.
This causes the remaining photons to become pairs that leave the filter
simultaneously. If the light of a single atom, excited to fluoresce by a
laser beam, hits a highly sensitive photodiode, two photons will never
be detected simultaneously. In this respect, the fluorescent light from
a single atom differs from the laser light with which it is excited, as
photons do occur simultaneously in laser light. But if two laser photons
impinge on a single atom at the same time, the atom will absorb only one
photon and allow the second to pass. Subsequently, the atom will radiate
the absorbed laser photon in a random direction, and only then will it
be ready to absorb another laser photon. A single atom can scatter only
one photon at a time, and the photons in the fluorescent light of a
single atom strike the detector as if lined up. The researchers here use
a weak and detuned light field to excite the atom such that the coherent
and incoherent components are spectrally separated while also ensuring
that the latter consists purely of photon pairs. To collect only the
incoherently scattered light, they make use of an adjustable, narrowband
optical notch filter to continuously reduce the amplitude of the
coherently scattered light. Subsequently, they measure the second-order
correlation function, g(2)(Ï„), of the residually transmitted
light. By tuning the relative magnitude of the two scattered components,
they observe an evolution from photon antibunching of
g(2)(0) = 0.43 ± 0.05 without filtering to a strong photon
bunching of up to g(2)(0) = 7.65 ± 1.21 when maximally
rejecting the coherently scattered light. The latter indicates that,
counter intuitively, the incoherently scattered light only consists of
pairs of simultaneously scattered photons. In their experiment they
prepare a single 85Rb atom in an optical dipole trap that is
loaded from a magneto optical trap (MOT). The MOT lasers, with frequency
ωL, are red-detuned with respect to the Stark-shifted atomic
resonance by Δ/2π = −57.9 ± 3.7 MHz. In this setting, the atom scatters
photons from the MOT laser beams into free space. The quantum state of
this fluorescence light can be separated into a coherent state, |α〉,
and an incoherently scattered component, |ϕ〉, whereby the latter
originates from the saturable nature of the emitter. They weakly drive
the atom with a saturation parameter S = 0.025 ± 0.004. In this
low-saturation regime, the incoherently scattered part consists solely
of photon pairs.
For more information:
Phys.org, October 26 (2023); Nat. Phot., July 27 (2023) page 1579.
Superconducting control of spin waves on a chip demonstrated:
researchers at Delft University of Technology in Delft have shown how
to control and manipulate spin waves on a chip using a superconducting
electrode. They report the observation and control of hybrid
spin-wave–Meissner-current transport modes. The researchers here show
that the diamagnetism of a superconducting gate can be used to shape the
magnetic environment that governs the transport of spin-waves in a
magnetic thin film. Using diamond-based magnetic imaging, they observed
hybridized spin-wave–Meissner-current transport modes with strongly
altered, temperature-tunable wavelengths and then demonstrated local
control of spin-wave refraction using a focused laser. A spin wave
generates a magnetic field that in turn generates a supercurrent in the
superconductor. That supercurrent acts as a mirror for the spin wave.
The superconducting electrode reflects the magnetic field back to the
spin wave. The superconducting mirror causes spin waves to move up and
down more slowly, and that makes the waves easily controllable. When
spin waves pass under the superconducting electrode, it turns out that
their wavelength changes completely. And by varying the temperature of
the electrode slightly, the researchers can tune the magnitude of the
change very accurately. They started with a thin magnetic layer of YIG.
On top of that they laid a superconducting electrode and another
electrode to induce the spin waves. By cooling to 5 K, they got the
electrode into a superconducting state. The spin waves got slower and
slower as it got colder. The set up had two Au electrodes on top of a
thin magnetic layer with the superconducting electrode in the middle.
With the left Au electrode, the researchers generate spin waves in the
magnetic material, which travel to the right. On top of the electrodes
is a square diamond membrane, which allows the researchers to see right
through the superconducting electrode. That gives them a unique handle
to manipulate the spin waves including that they can deflect them,
reflect them, make them resonate and others. The researchers imaged the
spin waves by measuring their magnetic field with a special diamond
sensor essential to the experiment.
For more information:
Science, October 26; Physworld, October 26 (2023).
WEEK OF OCTOBER 23, 2023 [No. 1546]
Ï„-lepton anomalous magnetic moment measured:
the ATLAS Collaboration in CERN in Geneva has observed Ï„-lepton-pair
production in the γγ→ττ process using ultraperipheral Pb+Pb→Pb(γγ→ττ)Pb
collisions of Pb to obtain constraints on the Ï„-lepton anomalous
magnetic moment aτ. Surprisingly, this approach enables more accurate
measurements of aτ than previous techniques. To date, the best
measurement of aτ with two decimal places was made in 2004 using an
electron collider at CERN. From 2015 to 2018, there was an experiment at
CERN that was designed primarily to allow study of hot matter created in
Pb head-on collisions. In 2019 the researchers here realized the data
from the same Pb experiments could be used to measure the Ï„ 's magnetic
moment by looking at passing-by collisions producing Ï„ 's. The dataset
corresponds to an integrated luminosity of 1.44  nb-1 of LHC
Pb+Pb collisions at √sNN = 5.02  TeV recorded by the ATLAS
experiment in 2018. Selected events contain one µ from a τ-lepton decay,
an electron or charged-particle track(s) from the other Ï„-lepton decay,
little additional central-detector activity, and no forward neutrons.
The γγ→ττ process is observed in Pb+Pb collisions with a significance
exceeding 5 standard deviations and a signal strength of
μττ=1.03+0.06−0.05 assuming the SM value for aτ. To measure aτ, a
template fit to the µ transverse-momentum distribution from τ-lepton
candidates is performed, using a dimuon (γγ→μμ) control sample to
constrain systematic uncertainties. The observed 95% confidence-level
interval for aτ is −0.057 < aτ < 0.024. This method tied the
previous best measurement using just one month of data recorded in 2018.
This result opens a path toward the tenfold improvement in precision
needed to test SM predictions. The LHC restarted Pb data collection on
Sept. 28, 2023, after routine maintenance and upgrades. The group here
plans to quadruple the sample size of Pb near-miss data by 2025. This
increase in data will double the accuracy of the measurement of the aτ,
and more with improvements to analysis methods.
For more information:
Phys.org, October 18 (2023); Phys. Rev. Lett., October 12 (2023) page 151802.
Nanophotonic electron accelerator built:
researchers at the Friedrich-Alexander-Universität Erlangen-Nürnberg
in Erlangen have built a coherent electron accelerator (dielectric laser
acceleration) on a chip. The demonstration has been simultaneous with a
demonstration by researchers at Stanford University. The researchers
here demonstrate a scalable nanophotonic electron accelerator that
coherently combines particle acceleration and transverse beam
confinement, and accelerates and guides electrons over a distance of
500 μm in a just 225-nm-wide channel. They observe a maximum coherent
energy gain of 12.3 keV, a 43% energy increase of the initial 28.4 keV
to 40.7 keV. Two years ago the researchers succeeded in using the
alternating phase focusing (APF) method to control the flow of electrons
in a vacuum channel over long distances. Using this technique, they have
now succeeded in guiding electrons and in accelerating them in the
nano-fabricated structures. Particles are accelerated by ultrashort
laser pulses illuminating the nano-structures. They combined the APF
method with specially developed pillar-shaped geometrical structures. In
the dual pillar accelerator structure, the incident laser light along
the viewing direction generates an optical mode inside of the structure
comoving with the electrons. In the acceleration process, one can
consider the synchronous Lorentz force components Fz and
Fx acting on a design electron, that is, an electron
synchronous with the propagating nearfield mode and initially positioned
at a phase of φs = 60°. Before the phase jump, the electron experiences
an acceleration force (Fz positive). At the same time, the
transverse forces act in a transversally defocusing way on the electrons
(Fx negative for electrons at negative x coordinates, for
example). After an abrupt phase jump of Δφ = 120°, the electron enters
the same nanophotonic mode in the next macrocell, but is now
phase-shifted to φs = −60°. Also here the electron experiences an
acceleration force (positive Fz), but now the transverse
forces act in a focusing manner. This repeats with every period of the
laser field, that is, every 6.45 fs, as the electron propagates through
the structure. simultaneously arising longitudinal bunching and
de-bunching. In order to achieve higher electron currents at higher
energies at the output of the structure, the researchers will have to
expand the structures or place several channels next to each other.
For more information:
Nature, October 18 (2023) page 476; Physorg, October 18 (2023).
WEEK OF OCTOBER 16, 2023 [No. 1545]
Orbital Hall effect observed:
researchers at Ohio State University in Columbus, OH and ETH Zurich
in Zurich have independently performed the magneto-optical detection of
the orbital Hall effect (OHE) in Cr and the orbital Hanle
magnetoresistance in a 3d transition metal. The two different
experiments on two different transition metals reveal that a current of
electron orbital angular momentum (OAM) flows in response to an electric
field. In the spin Hall effect, an applied electric field drives a
magnetic current of electron spin in a direction transverse to the
field. In a transition metal, theory predicts that an OAM current can
also flow.. Electrons are also capable of generating electricity
through: OAM (OHE). Orbital currents are mixed with spin currents in
typical heavy metals and it is difficult to tell them apart.
Theoretically by using light transition metals (weak spin Hall currents)
magnetic currents generated by the OHE would be easier to spot flowing
alongside them. For their demonstration, the ETH group used Hanle
magnetoresistance. In a conductor, when a magnetic field is applied
parallel to the direction of electron OAM, orbital moments should
accumulate at the edges of the sample because of the OHE. If instead,
the field is applied perpendicular to electron OAM, the orbital moments
should precess. The orbital moments should then fall out of phase with
each other, which boosts the material’s magnetoresistance. The ETH group
observed these effects in thin films of Mn. The Hanle magnetoresistance
is a telltale signature of spin precession in nonmagnetic conductors, in
which strong spin-orbit coupling generates edge spin accumulation via
the spin Hall effect. They report the existence of a large Hanle
magnetoresistance in single layers of Mn with weak spin-orbit coupling,
which they attribute to the OHE. The simultaneous observation of a
sizable Hanle magnetoresistance and vanishing small spin Hall
magnetoresistance in BiYIG/Mn bilayers corroborates the orbital origin
of both effects. They estimate an orbital Hall angle of 0.016, an
orbital relaxation time of 2Â ps and diffusion length ~ 2Â nm in
disordered Mn. Their findings indicate that current-induced orbital
moments are responsible for magnetoresistance effects comparable to or
even larger than those determined by spin moments. The OSU group used
the magneto-optical Kerr effect (MOKE). When linearly polarized light is
reflected off a magnetic surface, its plane of polarization is rotated
depending on the direction of the local magnetization. A strong OHE
should cause electrons of opposite OAM to accumulate at either edge of
the sample, which is what the group observed in thin films of Cr. They
demonstrated the OHE by reflecting laser polarized light onto various
thin films of the light metal Cr to probe the metal's atoms for a
potential build-up of OAM. They detected a magneto-optical signal
showing that electrons gathered at one end of the film exhibited strong
OHE characteristics. In the OHE, the charge current generates a
transverse orbital current, leading to orbital accumulation on the
sample's surfaces. The measurement setup utilized the longitudinal MOKE
to detect the in-plane orbital accumulation. The OSU researchers report
the magneto-optical detection of current-induced orbital accumulation at
the surface of a light 3d transition metal, Cr. The orbital polarization
is in-plane, transverse to the current direction, and scales linearly
with current density, consistent with the OHE. Comparing the
thickness-dependent magneto-optical measurements with ab initio
calculations, they estimate an orbital diffusion length in Cr ~
6.6±0.6  nm.
For more information:
Phys.org, October 13 (2023); Physics, October 11 (2023); Phys. Rev. Lett.,
October 11 (2023) pages 156702 and 156703.
Cross section of the CNO's cycle onset nuclear reaction revisited:
an international group lead by researchers at INFN in Padova has
obtained an improved S-factor of the 12C(p,γ) 13N
reaction at E=320–620 keV and the 422 keV resonance. With the high
precision evaluation of reaction cross section at the Dresden
Felsenkeller accelerator they concluded that H burning in stars proceeds
at a slower pace than thought until now. The previously accepted cross
section value has to be corrected down by ~ 25%. Thus, the burn-in phase
of the CNO cycle takes longer, and the emission of 13N
neutrinos occurs closer to the center of the sun than previously
assumed. The data, obtained at the Felsenkeller shallow-underground
laboratory in Dresden, encompass the 320–620 keV center of mass energy
range to include the wide and poorly constrained E=422 keV resonance
that dominates the rate at high temperatures. This work's S-factor
results are included in a comprehensive R-matrix fit, and the energy of
the 12+ first excited state of 13N is found to be 2369.6(4)
keV with a radiative and proton width of 0.49(3) eV and 34.9(2) keV,
respectively. A reaction rate, based on the present R-matrix fit and
extrapolation, is provided. The new data allows more precise theoretical
predictions for the ratio 12C/13C in stars, which
in turn help to better benchmark models for processes in the interior of
stars. The researchers use Ta disks with C evaporated on the surface as
targets. The disks were hit by p from the 5 MV Pelletron accelerator
(which can cover a large energy range). The γ created in the reaction
were detected with 20 high-purity Ge detectors with 45 m of rock
protecting the detectors from cosmic radiation.
For more information:
Phys.org, October 12 (2023); Phys. Rev. C, June 14 (2023) page L062801.
WEEK OF OCTOBER 9, 2023 [No. 1544]
Magnetic moment measured for a nucleus bound electron:
researchers at the Max-Planck-Institut für Kernphysik in Heidelberg
have precisely measured the g factor of electrons in
118Sn49+ ions. The bound-electron g factor was
measured with a precision of 0.5 ppb. With this measurement, they
challenge the best QED tests by means of the Lamb shift and, with
anticipated advances in the g-factor theory, will surpass them by more
than an order of magnitude. The researchers report on high-precision
g-factor measurement in the electron hydrogen-like
118Sn49+, reaching directly into the
medium-to-high-Z range. They produce the hydrogen-like ions
externally in the Heidelberg EBIT, which can reach higher charge states
than the ion sources that were previously available for this type of
measurement. From there, the ions are transported into the ALPHATRAP
apparatus, in which they are captured to perform high-precision
spectroscopy of the bound-electron g factor. The researchers
compare the measured value with its theory prediction, which tests
bound-state QED in a mean electric field of
1.6 × 1015 V cm−1 (x 60 stronger compared with the
28Si13+ measurement case which has so far the
strongest field for a precise g-factor measurement). The
researchers have succeeded in producing 118Sn49+
ions and storing them for months in the ALPHATRAP ion trap. Due to the
long storage time, they were able to measure the electron's magnetic
moment with unprecedented precision. To achieve the required high-degree
of ionization, they used an electron beam ion trap (the Heidelberg-EBIT)
with the experimental setup for production, trapping and detection of
118Sn49+ nuclei produced in the trap. The ions
were transported into the ALPHATRAP magnet, using a RT beamline. The
ions were captured in the capture section by pulsing the applied voltage
at the moment the ions were in the trap, a seven-electrode trap in which
the frequency ratio Γ0 = νL/νc is
measured. An image-current detector is used to detect the particle
motion in the trap. The voltage applied to the center electrode is ~ −59
V. In this experimental setup, a cloud of about 105 ions is bombarded
with high-energy electrons. In the process, the ions successively lose
their bound electrons. Afterwards the ions that have only one electron
left in their shells are filtered and fed into the particle trap of the
ALPHATRAP experiment (based on a Penning trap). There, the magnetic
properties of the electrons are measured. The system has a cryogenic UHV
system. This is necessary as the highly charged
118Sn49+ ions would immediately acquire electrons
from any surrounding atoms which would make longer-term measurements
impossible. The preparation of 118Sn49+ ions is
very time-consuming. An enriched sample of 118Sn was heated
in an oven source for injection into the Heidelberg EBIT. In the EBIT, a
200-mA electron beam focused by a 7-T magnetic field to a waist of
10-µm's crosses the atomic beam in the center electrode. With kinetic
energy ≈ 45 keV (> binding energy of the K-shell ≈ 35 keV), electrons
striking the 118Sn atoms sequentially generate higher charge
states until the charge-state distribution reaches a steady state. For
the production and extraction of hydrogen-like
118Sn49+, a charge-breeding time of 60 s was used.
After this, a fast pulse on the central electrode ejects the trapped
highly charged ions. The ion bunch, with kinetic energy ~
7 keV × Nq (Nq is the charge
state), is transported through a RT beamline, in which the required
charge state is separated with a dipole magnet. Various ion-optical
elements guide the ion cloud into the experimental setup. Before
entering the ALPHATRAP magnet, the ion bunch passes a pulsed drift tube,
in which the kinetic energy is reduced to 100's
eV × Nq, which is necessary to capture the ions in
the trap. The cryogenic valve is opened briefly for the ion injection.
This way, the inflow of gas from the RT beamline is blocked, achieving
UHV for long ion storage. For this measurement campaign, four
hydrogen-like 118Sn49+ ions were loaded once. One
of these was stored for three months, which allowed to precisely measure
the magnetic moment of the bound electron. The particles are trapped in
a Penning-trap setup, which consists of a superconducting magnet with
B ~ 4 T for radial confinement. This is overlapped with an
electrostatic field, which confines the ions in the axial direction.
Once trapped, they are cooled by means of image currents to 5.4(3) K. In
the magnetic field, the Zeeman effect splits the energy levels of the
electron spin. The energy difference is h
νL with
νL = (geB)/(4πme) ≈ 107.6 GHz
the Larmor frequency. The free-space cyclotron frequency
νc =
(qionB)/(2πmion) ≈ 25.7 MHz
governs the motion of the stored ion. Because both result from the
magnetic field B, their relation allows access to the
g factor of the bound electron. The researchers measured the g
factor of the electron on the captured 118Sn49+
ions using irradiated microwaves. At the matching frequency, the
electrons in the applied magnetic field in the trap make spin flips.
This effect allows highly accurate measurements of the g factor.
Electric fields of ~ 1015 V/cm are present on the position of
the electron around the nucleus. The calculations of the g factor at
strong fields provide a less precise predictions than for the free
electron due to the interaction with the nucleus. According to theory,
the g factor should be gtheo = 1.910 561 821 (299). The experimental
value measured in the ALPHATRAP apparatus has much higher accuracy and
is gexp = 1.910 562 058 962 (73)stat (42)sys (910)ext.
For more information:
Nature, October 4 (2023) page 53; Phys.org, October 4 (2023).
Orbital magnetic properties in moiré quantum matter analyzed:
researchers at NIST in Gaithersburg, MD have developed a quantum ruler to
measure and analyze the properties of moiré quantum matter. They used STM
spectroscopy to measure the Landau levels in twisted double-bilayer graphene
with an intermediate twist angle of 1.74° between the two bilayers. By comparing
their results against the theoretical expectations, the researchers were able to
measure deviations from the standard form of the Onsager relation stemming from
the orbital magnetism in the system. They used Landau-level spectroscopy to
study the energy-resolved valley-contrasting orbital magnetism and large orbital
magnetic susceptibility that contribute to the energies of Landau levels of
twisted double-bilayer graphene. These orbital magnetism effects led to
substantial deviations from the standard Onsager relation, which manifested as a
breakdown in scaling of Landau-level orbits. These substantial magnetic
responses emerged from the nontrivial quantum geometry of the electronic
structure and the large length scale of the moiré lattice potential. The
researchers took two layers of graphene of ~ 20 µm across and twisted them and
cooled to ~0.01 K. Then, they used a home-made STM to examine how the energy
levels of electrons in the layers of graphene changed when they varied the
strength of a strong external magnetic field. When the researchers applied a
voltage to the graphene bilayers in the magnetic field, the STM recorded the
current from the electrons that tunneled out from the material to the microscope
probe tip. Ordinarily, the magnetic flux enclosed by the circular orbits of the
electrons in solid materials subject to a magnetic field take on a set of fixed,
discrete values. The difference in energy between successive energy levels that
follow this pattern can be used like tick marks on a ruler to measure the
material's electronic and magnetic properties. Any subtle deviation from this
pattern would represent a new quantum ruler that can reflect the orbital
magnetic properties of the analyzed quantum moiré material. When the researchers
varied the magnetic field applied to the moiré graphene bilayers, they found
evidence of a new quantum ruler at play. The area enclosed by the circular orbit
of electrons multiplied by the applied magnetic field no longer equaled a fixed
value. Instead, the product of those two numbers had shifted by an amount
dependent on the magnetization of the bilayers. This deviation translated into a
set of different tick marks for the energy levels of the electrons. The findings
here help understand how electrons confined to twisted sheets of graphene give
rise to new magnetic properties. In moiré quantum materials, electrons have a
range of possible energies that are determined by the electric field of the
materials. with the electrons concentrated in the valleys. The large spacing
between the valleys in the bilayers, bigger than the atomic spacing in any
single layer of graphene or multiple untwisted layers, accounts for some of the
unusual magnetic properties the researchers found. Landau-level STM spectroscopy
offers a complete quantum ruler that resolves the full energy dependence of
orbital magnetic properties in moiré quantum matter.
For more information:
Science, October 5 (2023) page 81; Phys.org, October 5 (2023).
Zero-field composite Fermi fluid in twisted bilayer predicted:
researchers at MIT in Cambridge, MA have computational evidence of a composite
Fermi fluid in a semiconductor bilayer system with no magnetic field applied to
it. Recent experiments have produced evidence for fractional quantum anomalous
Hall (FQAH) states at zero magnetic field in the semiconductor moiré
superlattice system tMoTe2. The researchers here suggest that a
composite fermion description, already a unifying framework for the
phenomenology of 2D electron gases at high magnetic fields, provides a similarly
powerful perspective in this new context. They present exact diagonalization
evidence for composite Fermi liquid states at zero magnetic field in
tMoTe2 at fillings n=12 and n=34. They call these non-Fermi liquid
metals anomalous composite Fermi liquids (ACFLs), and propose that they play a
central organizing role in the FQAH phase diagram. They proceed to develop a
long wavelength theory for this ACFL state that offers experimental predictions
upon doping the composite Fermi sea. Theoretically, certain twisted
semiconductor bilayers should host a Fermi liquid of composite fermions without
an applied magnetic field. When the layers of a MoTe2 semiconductor
bilayer are twisted with respect to each other, the system’s electronic band
structure creates a pseudomagnetic field. Electrons confined to the bilayer
capture an even number of field quanta and become composite fermions. When
electrons are confined to 2D, cooled to ~ 0 K, and exposed to a strong magnetic
field, they capture part of this field and turn into weakly interacting
composite fermions (CFs) that display fractional quantum Hall effect (FQHE).
Recently, experiments showed that a twisted MoTe2 semiconductor
bilayer exhibits CFs and FQHE without an applied magnetic field. Now, based on a
theoretical analysis of these experiments, researchers predict that this twisted
bilayer should also realize a CF Fermi liquid. The FQHE refers to the existence
of a plateau at RH = h/(fe2), where f is a fraction. Such
a plateau occurs when electrons fill approximately this fraction of a Landau
level. The number of observed fractions has swelled from one (f = 1/3) in 1982
to ~100 today. The FQHE is caused by strong electron correlations resulting from
the repulsive Coulomb interaction. To avoid one another as efficiently as
possible, electrons capture an even number of magnetic-field quanta and
transform into CFs. The CFs see only the uncaptured field, and their integer
IQHE manifests as the FQHE of electrons at sequences of odd-denominator filling
fractions. The strategy for producing CFs and the FQHE at zero magnetic field
is, then, to identify a material that has an electronic band mimicking a Landau
level. This band must be topological (have a nonzero Chern number), be of
roughly constant kinetic energy, and generate a pseudomagnetic field that is as
uniform as possible. Researchers at MIT and Harvard University predicted that
these desired properties would be exhibited by certain bands in a
MoTe2 semiconductor bilayer whose layers are slightly twisted
relative to each other. Recent optical and transport experiments showed that
this system displays the FQHE at the odd-denominator filling fractions of f =
2/3 and f = 3/5 without a magnetic field. The researchers here present numerical
and analytical studies of the interacting electrons that partially populate one
of the topological bands in twisted bilayer MoTe2. The results
strongly suggest that CFs also form at the even-denominator filling fractions of
f = 1/2 and f = 3/4 without any external magnetic field. At these fractions, CFs
would see no pseudomagnetic field and are predicted to condense into a CF Fermi
liquid (a strongly correlated metallic state distinct from the ordinary, weakly
correlated, electron Fermi liquid found in metals). Recent transport experiments
showed a gapless state at f = 1/2. That observation is consistent with the
behavior of a CF Fermi liquid but does not prove its existence. The numerical
studies suggest several ways to experimentally distinguish a CF Fermi liquid
from an ordinary electron Fermi liquid. These methods include observing
oscillations in the system’s electronic properties as one varies either the
concentration of charge carriers or the strength of an applied magnetic field.
For more information:
Physics, October 27 (2023); Phys. Rev. Lett., September 27 (2023) page 136501.
WEEK OF OCTOBER 2, 2023 [No. 1543]
Resonant excitation of 45mSc nucleus by XR-FEL observed:
an international group lead by researchers at Texas A&M University in College
Station, TX and ANL in Lemont, IL has used the XR-FEL in Schenefeld, Germany to
induce resonant excitation of the 45mSc nucleus and determine its
transition energy. Thus, they have created a much more precise pulse generator
based on Sc, which enables an accuracy of 1 s / 300 By (x 103 more precise than
the current standard Cs atomic clock). They report on resonant XR excitation of
the long-lived ultra-narrow 12.4-keV 45Sc nuclear isomeric state by
irradiation of Sc-metal foil with 12.4-keV photon pulses from a XR FEL and
subsequent detection of nuclear decay products. The transition energy was
determined with uncertainty 10-2 than the ±50 eV todate. They obtained
estimates for the resonance cross-section and for the coefficients of internal
conversion of this transition. The high average spectral flux of the incident XR
and the very low background noise in the detection of the nuclear decay products
were crucial for the success of the experiment. The scientific potential of the
45Sc resonance (ultra-narrow nuclear resonance transition in
45Sc between the ground state and the 12.4-keV isomeric state with a
long lifetime of 0.47 s), together with the possibility of its resonant
excitation by photons from accelerator-based sources of hard XR (no radioactive
parent isotope is available for 45Sc), was identified more than 30 y
ago. The required accelerator-driven, high-brightness XR sources have become
available only recently. Earlier attempts to induce resonant excitation at
third-generation synchrotron radiation sources were not successful, mainly
because of the lack of sufficient spectral flux. This flux constraint was
overcome only recently with the advent of narrow-band XR-FELs working at a high
repetition rate. 45Sc has a transition energy of E0 = 12.4
keV, an isomer lifetime of τ0 = 0.47 s, and a natural linewidth of
Γ0 = ħ/τ0 = 1.4 feV, resulting in an extremely high
quality factor of Q = E0/Γ0 ≃ 1019. This
quality factor is x 106 the 14.4-keV resonance of 57Fe
(the workhorse of Mössbauer spectroscopy) and surpasses other measurable
Mössbauer resonances by orders of magnitude. 45Sc is a stable isotope
with 100% natural abundance and is readily available either as ultra-pure Sc
metal or as Sc2O3, in which its 12.4-keV transition has a
high Lamb–Mössbauer factor fLM ≃ 0.75 at RT. All of these facts make
45Sc superior to any other candidate for a Mössbauer nuclear clock.
The researchers excited the selected transition in the nucleus of Sc where the
nonradiative internal conversion decay channel dominates, resulting in the
emission of Sc characteristic XR fluorescent photons and Auger electrons.
Kα (4.09 keV) and Kβ (4.46 keV) fluorescent photons were
detected in this study. In the experiment, the team irradiated a 25-µm-thick Sc
foil with XR laser light and was able to detect a characteristic afterglow
emitted by the excited atomic nuclei (evidence of Sc's extremely narrow
resonance line). Extreme noise suppression and high-resolution crystal optics
allowed the value of the Sc resonance energy in the experiments to be determined
at 12.38959 keV (> x 250 previous accuracy). The experiment involved firing
XR pulses at the Sc foil target. After a pulse stroked the target, the target
was quickly removed from the beamline to a nearby region where the photon
detectors were located. This isolation from the beamline allowed the researchers
to measure the small signal produced by the decay of the resonant excitation.
This process was repeated as frequency of the incident light pulses was scanned
in order to find the exact frequency at which the resonance occurs. 93 nuclear
decay events were detected in response to 1020 near-resonant photons
directed at the target, Because of the extremely low detector noise, this number
was enough to detect the resonance and allow the energy of the transition to be
measured with uncertainty much smaller than the previous best value. Upstream
and downstream XR counters detected time-delayed nuclear decay products (both
inelastic Kα,β fluorescence and elastically and coherently scattered
delayed 12.4-keV photons in the forward direction). To minimize the detection
background, the decay product detectors were offset from the beam path and the
sample was moved to the detectors after irradiation with each pulse train. The
resonance energy was measured with XR single-shot and Bond spectrometers. A next
step is the time-resolved observation of XR coherently scattered off the nuclei,
which would reveal the actual spectral width of the resonance.
For more information:
Nature, September 27 (2023) page 471; Physicsworld, October 19 (2023); Phys.org,
September 27 (2023).
Metastable negative pressure liquid state analyzed:
an international group of researchers at the Friedrich-Alexander Universität
Erlangen-Nürnberg, in Erlangen has used liquid-filled optical fibers and sound
waves to develop a method to measure negative pressure. They demonstrate the
Brillouin–Mandelstam measurements of nl volumes of liquids in extreme
thermodynamic regimes. This is enabled by a fully sealed liquid-core optical
fiber containing CS2. Within this waveguide, which exhibits tight optoacoustic
confinement and a high Brillouin gain, they were able to conduct spatially
resolved measurements of the local Brillouin response, giving access to a
resolved image of the temperature and pressure values along the liquid channel.
They measured the material properties of the liquid core at very large positive
pressures (above 1,000 bar) and substantial negative pressures (below –300 bar),
as well as explore the isobaric and isochoric regimes. The extensive
thermodynamic control allows the tunability of the Brillouin frequency shift of
more than 40% using minute volumes of liquid. Thus, non-contact measurements
that encapsulate the material of interest from the environment can be performed
by measuring optical or acoustic parameters of the material. They show that by
combining optics and acoustics via stimulated Brillouin–Mandelstam scattering
(SBS, a nonlinear interaction of light and sound waves) changes in temperature,
pressure and density can be probed with high precision. SBS has been
demonstrated before in liquids, gases and optical fibers. Access to extreme
thermodynamic ranges of liquids, such as very high or even negative pressures,
has so far been beyond reach with fiber optics. The full description of such
dynamics requires a method sensitive to thermodynamic parameters like the
refractive index and speed of sound, spatially resolved along the fiber axis.
SBS provides both by exploiting the interaction of optical and acoustic waves.
The researchers have analyzed extreme thermodynamics in nl volumes through SBS.
Liquids can exist in a specific metastable state corresponding to a negative
pressure value. In this metastable state, even a tiny external influence can
cause the system to collapse into one state or another. In the experiments
here, nl of a liquid were encapsulated in a fully closed optical fiber, allowing
both highly positive and negative pressures. Subsequently, the specific
interaction of optical and acoustic waves in the liquid enabled the sensitive
measurement of the influence of pressure and temperature in different states of
the liquid. The influence of negative pressure on a liquid should decrease its
volume, but the liquid is retained in the glass fiber capillary by adhesive
forces resulting in a stretching of the liquid. The sound waves used here can
detect temperature, pressure, and strain changes very sensitively along the
optical fiber and can provide an image of the situation inside the optical fiber
at cm-scale resolution along its length. The observation of the negative
pressure regime becomes clear when looking at the frequency of the sound waves.
The researchers here have developed a tiny, simple setup in which they can make
very precise pressure measurements using light and sound waves with no hazard
even with the toxic liquid CS2 utilized in the experiments here.
For more information:
Phys.org, September 23 (2023); Nat. Phys. September 25 (2023) page 1805.
WEEK OF SEPTEMBER 25, 2023 [No. 1542]
SARPES analysis of a topological superconducting interface:
researchers at ORNL in Oak Ridge, TN have used molecular beam epitaxy (MBE) to
build an interface of a topological insulator and a superconductor, tailored the
electronic structure at the interface, and probed the interface with spin- and
angle-resolved photoemission spectroscopy (SARPES). The researchers report
superconductivity in monolayer
FeTe1–ySey (Fe(Te,Se)) grown on
Bi2Te3 by MBE. SARPES was used to directly resolve the
interfacial spin and electronic structure of
Fe(Te,Se)/Bi2Te3 heterostructures. For y = 0.25, the
Fe(Te,Se) electronic structure is found to overlap with the
Bi2Te3 topological interface states (TIS) and the desired
spin-momentum locking is not observed. In contrast, for y = 0.1, reduced
inhomogeneity measured by STM and a smaller Fe(Te,Se) Fermi surface with clear
spin-momentum locking in the topological states are found. The interface between
2D topological Dirac states and an s-wave superconductor is expected to support
Majorana-bound states (MBS) that can be used for quantum computing applications.
Realizing these states of matter and their applications requires control over
superconductivity and spin-orbit coupling to achieve spin-momentum-locked TIS
which are simultaneously superconducting. While signatures of MBS have been
observed in the magnetic vortex cores of
bulkFeTe0.55Se0.45, inhomogeneity and disorder from doping
make these signatures unclear and inconsistent between vortices. The researchers
here analyzed monolayer superconductivity and tunable topological electronic
structure at the Fe(Te,Se)/Bi2Te3 interface. They
co-deposited Fe, Se and Te to make a superconducting monolayer. A key to getting
their results was understanding how to combine Bi2Te3 with
Fe(Te,Se) at an atomic interface to gain the desired electronic behavior. The
superconductor's lattice of Fe, Se and Te comprises ordered square cells,
whereas the topological insulator is a network of adjoining triangles. To
understand the topological properties of the synthesized interface, they used
SARPES, to probe quantum spin-dependent electronic structure at the interface of
the topological insulator and the superconductor. To confirm its superconducting
behavior, they performed measurements of electrical resistance. The researchers
were able to see how the different electronic structures were interacting at the
interface, and to control those interactions to ensure all the ingredients for
topological superconductivity existed. They found that the desired topological
properties only exist for specific Se doping ranges. They used STM to
characterize disorder in the materials. The researchers plan to explore for
possible Majorana particles using an ultralow-temperature STM.
For more information:
Phys.org, September 20 (2023); Adv. Mat., March 15 (2023).
Mott insulator excitations observed at RT:
an international group lead by researchers at Yousei University in Seoul have
uncovered RT unconventional charge carriers in a triangular-lattice single
crystal TbInO3 Mott insulator specimen studied with terahertz
time-domain spectroscopy (THz-TDS). The power-law behavior σ1(ω)
∝ ω2 (where ω is the angular frequency) in the THz region was
obtained over the entire temperature range of 1.5–300 K, with the power index
equal to 2.0009 ±â€‰0.0057. A Fano interference pattern was observed in an
IR-active phonon mode, indicating a continuous spectrum of emergent excitations
strongly interacting with the phonon mode. They found that the excitation
continuum is insensitive to the formation of crystal-field transitions below 150
K and robust against external magnetic fields up to 7 T. Their THz spectroscopic
analysis establishes TbInO3 as a Mott insulator whose exotic emergent
carriers are coherent even at RT. TbInO3 crystallizes into a
hexagonal structure, consisting of alternating layers of TbO6 and
InO5 polyhedra along the c axis. Tb+ ions form a
quasi-two-dimensional triangular lattice with two different sites: the Tb2 sites
form a honeycomb structure, and the Tb1 sites are at the center of the hexagons.
An imbalance in the populations and opposite shifts along the c axis of the two
Tb sites lead to ferroelectric polarization and topological vortex domains. The
magnetic susceptibility shows strong in-plane anisotropy without any magnetic
ordering down to 0.46 K. Studies of muon spin rotation and specific heat in
TbInO3 did not find any sign of magnetic order or spin freezing down
to ~0.1 K, suggesting that TbInO3 is a quantum spin liquid (QSL)
candidate. Since Tb+ is a non-Kramers ion, it may have a non-magnetic
ground state. Analysis of the low-energy spectra from inelastic neutron
scattering from powder samples suggests that the ground states of the Tb1 and
Tb2 ions are singlets and doublets, respectively, thus realizing a honeycomb
spin structure. However, inelastic neutron scattering and Raman spectroscopy for
single crystals suggest that both Tb sites are equivalent, thus maintaining the
triangular spin structure. Mott insulators contain strongly correlated
electrons, which can generate highly entangled many-body states marked by
unconventional excitations. A QSL describes a liquid-like Mott insulator without
magnetic orderings. As they were categorized as Mott insulators, many
researchers assumed that QSLs do not interact with external electromagnetic
fields and thus optical measurements at RT would be unable to detect their
unconventional excitations. The dynamics of exotic emergent excitations in Mott
insulators is qualitatively different from that of electrons in band insulators.
Whereas electrons couple to an external electromagnetic field via the minimal
coupling with the electromagnetic potentials, exotic emergent excitations are
charge-neutral, which prohibits the minimal coupling. In this context, THz
optical conductivity σ(ω) has been measured for a number of Mott insulators
during searches for relevant excitations in the real part of the optical
conductivity σ1(ω). A decade ago, researchers at MIT proposed that a
class of QSLs may see external electromagnetic fields indirectly, further
predicting that the optical conductivity would be proportional to the square of
the frequency at low frequencies. Despite numerous efforts over the last 10
years, QSL candidate materials exhibiting this signature characteristic were not
found. The researchers here set out to analyze the long-standing belief that
exotic excitations only exist in Mott insulators at low temperatures. They grew
high-quality single-crystals of the Mott insulator TbInO3, using
laser floating-zone growth. They selected TbInO3 because their
previous studies had captured unique signatures in this material using neutron
scattering techniques, which indicated that it exhibited QSL behavior. The
samples were analyzed, using THz-TDS. The researchers observed that the a.c.THz
conductivity in the material is precisely proportional to the square of the
frequency of light, even at RT. The researchers devised theoretical
interpretations that could explain these surprising experimental observations.
They have discovered unconventional charge carriers, made of a macroscopic
number of quantum spins. In contrast to the understanding that charge carriers
do not exist at low energy in an insulator, they prove the existence of charge
carriers by measuring the optical conductivity proportional to the square of the
frequency of light with the charge carriers surviving and being coherent up to
RT. They have demonstrated that unconventional charge carriers could also be
found in Mott insulators at RT. In the temperature range 1.5–300 K, they observe
a quadratic frequency dependence in the real part of the in-plane optical
conductivity as well as Fano asymmetry of an optical phonon mode strongly
interacting with the excitation continuum. These features are robust even under
external magnetic fields of up to 7 T. Their data confirm the presence of
emergent charge carriers within the Mott charge gap of TbInO3,
suggesting that it is possible to probe and manipulate highly entangled quantum
many-body states at RT.
For more information:
Phys.org, September 18 (2023); Nat. Phys., August 17 (2023) page 1611.
WEEK OF SEPTEMBER 18, 2023 [No. 1541]
Microwave shielded bosonic dipolar molecular gas cooled to 36 nK:
researchers at Columbia University in New York, NY have used microwaves to
create shields around dipolar molecules in a molecular gas. The shielding
stabilized the molecules and helped the researchers to cool them down to the
coldest temperatures yet close to the temperature needed to create a molecular
BEC. Stable ultracold ensembles of dipolar molecules had not been produced yet
due high inelastic loss rates. Recently, gases of fermionic molecules in their
ground state have been effectively stabilized by applying external fields. It
has been unknown whether a electromagnetic field suppression of losses can be
achieved in a bosonic gas. This is due to the high inelastic loss rates for
bosonic molecules, which are intrinsically one to two orders of magnitude larger
than those for their fermionic counterparts. The researchers here stabilized a
bosonic gas of strongly dipolar NaCs molecules via microwave shielding,
decreasing losses by more than a factor of 200 and reaching lifetimes ~1s. In
addition, they measured the high elastic scattering rates and characterized
their anisotropy, which arises from strong dipolar interactions. They also
demonstrated evaporative cooling of a bosonic molecular gas. They increased the
phase-space density by a factor of 20, reached a temperature of 36(5) nK and
brought the system to the brink of quantum degeneracy. They used microwaves
emitted from a custom-built antenna to extend the lifespan of a bosonic gas of
large dipole moment ground-state NaCs molecules from a few ms to > 1 s. The
microwaves created a shield that prevents the molecules from sticking to each
other and getting lost from the sample. Once held in place, the molecules could
be subjected to evaporative cooling. With their long-lasting
collisionally-stable sample, the researchers dropped the temperature to 36 nK.
The group had previously developed techniques for assembling ultracold NaCs
molecules from ultracold gases of Na and Cs atoms.
For more information:
Phys.org, September 13 (2023); Nat. Phys., September 4 (2023) page 1579.
Broadband coherent wave control through photonic collisions at time interfaces:
researchers at CUNY, NY have found that photons can collide as if they were
massive objects at time interfaces (metamaterials that undergo abrupt large
changes in their optical properties). Coherent wave control exploits the
interference among multiple waves impinging on a system to suppress or enhance
outgoing signals based on their relative phase and amplitude. This process
inherently requires the combination of energy exchanges among the waves and
spatial interfaces to tailor their scattering. The researchers here explore the
temporal analogue of this phenomenon, based on time interfaces that support
instantaneous non-conservative scattering events for photons. Based on this
mechanism, they demonstrate ultra-broadband temporal coherent wave control and
the photonic analogue of mechanical collisions with phase-tunable elastic
features. They apply them to erase, enhance and reshape arbitrary pulses by
suitably tailoring the amplitude and phase of counter propagating signals. The
researchers have shown they can create a new form of control over energy
exchanges between photons. In the process, they realized the photonic analogue
of a mechanical collision for electromagnetic waves using metamaterials that can
undergo abrupt and large changes in their electromagnetic properties. These
variations allowed them to create a time interface. When two waves propagating
in opposite directions experience such an interface while they are overlapping,
they experience extremely fast energy exchanges, as if they were colliding
objects. The relative phase of the two waves can control the nature of this
collision, which can either conserve energy, dissipate it or amplify it. In this
form of temporal coherent wave control, the waves reflected off the time
interface destructively interfere with refracted waves. Under suitable
conditions, this allows one or even both waves to be cancelled out. The
researchers got the idea for this work when they wondered whether it might be
possible to erase an unwanted mechanical wave by another similar wave against
it to counteract it. While such an outcome is impossible in conventional wave
physics, they thought it was possible, in principle, with a temporal
metamaterial. This photonic analogue of mechanical collisions could also be used
to shape electromagnetic pulses by colliding them against each other. The
researchers have demonstrated such sculpting for electromagnetic waves in the
microwave regime and are now aiming to achieve this at higher frequencies by
using devices such as high-speed graphene transistors instead of time
interfaces. Their findings provide a pathway to effectively sculpt broadband
light with light without requiring spatial boundaries, within an ultrafast and
low-energy platform.
For more information:
Physworld, September 14 (2023); Nat. Phys., August 14 (2023) page 1703.
WEEK OF SEPTEMBER 11, 2023 [No. 1540]
Pines' demon quasiparticle detected using EELS:
an international group lead by researchers at University of Illinois, at
Urbana-Champaign, IL has spectroscopically detected in
Sr2RuO4 a collection of electrons in a metal that behaves
like a massless wave, neutral and unable to interact with light, as predicted in
1956. They observed the Pine's demon as a 3D acoustic plasmon and presented
evidence using momentum-resolved electron energy-loss spectroscopy (EELS). The
demon, formed of electrons in the β and γ bands, is gapless with critical
momentum qc= 0.08 reciprocal lattice units and RT velocity v = (1.065
±â€‰0.12) × 105 m s-1 that undergoes a 31%
renormalization upon cooling to 30 K because of coupling to the particle–hole
continuum. The characteristic excitation of a metal is its plasmon (a quantized
collective oscillation of its electron density). David Pines predicted that a
distinct type of plasmon (demon), could exist in 3DÂ metals containing more than
one species of charge carrier. Consisting of out-of-phase movement of electrons
in different bands, demons are acoustic, electrically neutral and do not couple
to light, and had never been detected in an equilibrium, 3D metal. What makes
demons difficult to detect is their inherent charge neutrality. The out-of-phase
currents of the two electron fluids exactly cancel as, extinguishing the
long-ranged part of the Coulomb interaction. For this reason, a demon has no
signature in the dielectric function of a metal, in the limit of small q, and
does not couple to light. The most promising way to detect a demon is to measure
the excitations of a multiband metal at non-zero q, where a demon modulates the
density and may be experimentally observable using EELS techniques that observe
plasmons originally. Sr2RuO4, has three nested bands, α, β
and γ, crossing the Fermi energy. At a temperature T ≲ 40 K,
Sr2RuO4 is a good Fermi liquid showing resistivity,
well-defined quantum oscillations and the expected scattering rate in optics. At
higher temperatures, T ≳600 K, Sr2RuO4 crosses over
into a strongly interacting strange metal phase in which the quasiparticles are
highly damped, the resistivity and its value exceeds the Mott–Ioffe–Regel limit
at high temperature. The strong interactions arise from Hund’s coupling and are
described well by dynamical mean field theory. The momentum dependence of the
intensity of the demon confirms its neutral character. Pines hypothesized the
existence of a plasmon which would require no initial burst of energy, forming
when electrons in different bands of a metal move out of phase with one another
such that they keep the overall charge static. This demon quasiparticle is the
collective motion of neutral quasiparticles whose charge is screened by
electrons from another band. Pines predicted that it was possible to create a
plasmon excitation with no Coulomb energy cost. The collective mode arises when
electrons in different bands move out of phase, thereby resulting in no net
transfer of charge but a modulation in the band occupancy. With the two electron
currents out of phase with one another, they cancel out and eliminate long-range
Coulomb interactions. That precludes any signature from the demon in the metal’s
dielectric properties, meaning that the quasiparticle does not interact with
light. The researchers were using EELS to explore the electronic properties of
Sr2RuO4 to use the material as a kind of surrogate for
HTS. EELS involves firing a beam of electrons with a narrow range of energies
and recording how much energy is lost at which momenta after the electrons pass
through a target material. The technique is well-suited to studying plasmons
because electrons are very sensitive to fluctuations in charge density. Using
mm-sized single crystals of Sr2RuO4 grown at Kyoto
University, the researchers recorded quite different spectra using low-energy
and high-energy electrons. In the latter case they found energy loss peaking at
around 1.2Â eV, which they identify as an interaction with a typical (charged)
plasmon. On the other hand, at lower energies they observed an oscillation with
a tiny energy gap < 8Â meV at zero momentum. This second feature is an
acoustic mode with a velocity about 100 times that of sound, which is far too
high to be associated with phonons. At the same time, the velocity of the mode
is about three orders of magnitude lower than that of a surface plasmon. The
figure is ~ 10% of that calculated for a quasiparticle made up of two electron
bands in Sr2RuO4 oscillating out of phase with one
another. To make sure they had found the demon, the researchers checked its
neutrality by examining how its intensity varied with momentum as seen as
variations in the electron scattering angle. They worked out that the intensity
of a conventional plasmon should vary inversely with momentum raised to the
power 5. The intensity of neutral plasmon should also vary inversely with
momentum, but with a smaller power. They established that the new quasiparticle
was characterized by an inverse power of 1.83. The demon is thought to be
present in other metals with sufficiently different electron bands.
For more information:
Physworld, September 4 (2023); Nature, August 9 (2023) page 66.
Neutrino mass limit set by cyclotron radiation emission spectroscopy:
the Project 8 Collaboration has obtained the first ν -mass constraint derived
from a frequency-based technique and thus, shown that they can use their
proposed technique to reliably track and record a T β decay
(3T1 → 3He2 + +
e− + νa). Thus, their distinctive β spectrum
measurement strategy to determine the ν mass becomes a realistic contender to be
the first to measure the ν mass. With only a cm3-scale physical detection
volume, a limit of mβ < 155  eV/c2 (152  eV/c2) is
extracted from the background-free measurement of the continuous β spectrum in a
Bayesian (frequentist) analysis. Using 83mKr calibration data, a
resolution of 1.66±0.19  eV (FWHM) is measured, the detector response
model is validated, and the efficiency is characterized over the multi-keV T
analysis window. These measurements establish the potential of CRES for a
high-sensitivity next-generation direct ν mass experiment featuring low
background and high resolution. By precisely determining the maximum total
energy of the e−, they can infer the minimum total energy of the ν
and, in turn, set an upper bound on its mass. Currently, the strongest such
bound is 0.8 eV, which was reported by the Karlsruhe Tritium Neutrino (KATRIN)
Collaboration last year. For that experiment, the KATRIN Collaboration measured
the e− energy by directly detecting the particle. The Project 8
Collaboration instead used an indirect method. The researchers applied a uniform
magnetic field to T as it decayed, causing the generated e− to move
in a spiral path and emit radiation. Then. they determined the e−
energy by measuring the frequency of this radiation. Using a small-sized
(cm-scale) version of the apparatus, the collaboration set an upper limit on the
ν mass of ~ 150 eV. With a full-sized (m-scale) version, the team expects to
measure masses as low as 0.04 eV thanks to the precise electron-energy
measurements that should be possible with their indirect method. When they
measure a free e− generated by β decay, and they know the total mass,
the missing energy is the ν mass and motion. Since the ν is >
0.6×106 times lighter than the e−, when ν and
e− are created at the same time, the ν mass has only a tiny effect on
the e− motion. The technique uses a cryogenic CRES (Cyclotron
Radiation Emission Spectroscopy) cell, where e− are produced in
radioactive decay and magnetically trapped. While this strategy has been used
previously, the CRES detector here measures that crucial e− energy
with the potential to scale up beyond any existing technology. The cell
waveguide has a cold interior diameter of 10.03 mm and length of 132 mm
(distance between RF windows). Cyclotron radiation travels axially up the
waveguide, toward the amplifiers and readout electronics. It captures the
microwave radiation emitted from emerging e− as they spiral around in
a magnetic field. These e− carry away most of the energy released
during a β decay event. In their most recent experiment, built at the University
of Washington in Seattle, the group tracked 3,770 T β decay events over an
82-day trial window in a sample cell the size of a single pea. The sample cell
is cryogenically cooled and placed in a magnetic field that traps the emerging
e− long enough for the system's recording antennas to register a
microwave signal. The researchers registered zero false signals (background
events) that could be confused for the sought trackings. They have developed a
suite of specialized software to take the raw data and convert them to signals
that can be analyzed. Now that the team has shown their design and experimental
system works using T molecules, they will set a system to produce, cool and trap
individual T atoms. Tritium prefers to form molecules that would make the
ultimate goals of the research unachievable. The researchers are working on
testing designs for scaling up the experiment from the pea-size sample chamber
to one 103 times larger to capture many more β decay events.
For more information:
Phys.org, September 6 (2023); Physics, September 6 (2022); Phys. Rev. Lett.,
September 6 (2023) page 102501.
WEEK OF SEPTEMBER 4, 2023 [No. 1539]
28O nucleus observed unbound:
an international group lead by researchers at Tokyo Institute of Technology has
determined that the 28O nucleus exists in an unbound state that
disintegrates rapidly even if expected to be doubly magic stable, with strongly
bound neutrons and protons. Light isotopes with neutron-to-proton ratio
significantly different from that of stable nuclei, exist as very short-lived
resonances, decaying through spontaneous neutron emission. They report the
observation of two such isotopes , 28O and 27O with 8
protons, through their decay into 24O with four and three neutrons,
respectively. It is shown that the cross-section for the production of
28O from a 29F beam is consistent with it not exhibiting a
closed N = 20 shell structure. The experiments used multi-neutron-decay
spectroscopy in the RIKEN RI Beam Factory in Wako, which uses a superconducting
ring cyclotron to produce intense neutron-drip-line beams of unstable nuclei
coupled to an active target of thick LH2 and multi-neutron detection arrays. The
scientists fired a beam of 48Ca isotopes at a Be target, which
created a 29F isotope. The nucleus of this isotope has one more
proton than does 28O but the same number of neutrons. The researchers
next smashed 29F into a thick barrier of LH2, knocking a proton out
of the nucleus and generating 28O. Proton-induced nucleon knockout
reactions from a high-energy 29F beam generated the neutron-unbound
isotopes 27O and 28O. This rare forms of O were too
short-lived to be observed directly. Instead, the researchers detected their
decay products: 24O plus neutrons To observe several individual
neutrons, they used a detector built for that purpose, on loan from the GSI
Helmholtz Centre for Heavy Ion Research in Darmstadt. In this specialized
detector, incoming neutrons are revealed when they knock protons around.
Although the researchers could not get an exact measurement of the lifetime of
28O, the isotope did not behave as if it were doubly magic and it
fell apart almost as soon as it came into existence. They found that both
27O and 28O exist as narrow low-lying resonances and
compared their decay energies to the results of theoretical models based on
effective field theories of QCD. Most theoretical approaches predicted higher
energies for both isotopes. Comparison of the measured energies of
27O and 28O with respect to 24O with a broad
range of theoretical predictions showed that in almost all cases theory
underbinds both systems. The statistical coupled-cluster calculations indicated
that the energies of 27O and 28O can provide valuable
constraints of such ab initio approaches and, in particular, the interactions
used. Although 28O is expected in the standard shell-model picture to
be a doubly magic nucleus (Z = 8 and N = 20), the single-proton removal
cross-section measured here, when compared with theory, was found to be
consistent with it not having a closed neutron shell character. The researchers
investigated the cross-section for the production of 28O from the
29F beam, finding it to be consistent with 28O not
exhibiting a closed N = 20 shell structure. This result suggests that the island
of inversion, whereby the energy gap between neutron orbitals weakens or
vanishes, extends beyond the isotopes 28F and 29F into the
O isotopes.
For more information:
Nature, August 30 (2023) page 965; Physicsworld, September 12 (2023); Phys.org,
August 30 (2023).
Quantum-entanglement-enhanced sensing on optical transitions observed:
researchers at Universität Innsbruck in Innsbruck and JILA in the University of
Colorado Boulder in Boulder, CO have identified a way of creating quantum
entanglement through finite-range interactions that can improve the accuracy of
measurements integral to an optical atomic clock's function. They illustrate a
pathway for harnessing large-scale entanglement in an optical transition using
1D chains of up to 51 ions with interactions that decay as a power-law function
of the ion separation. They show that their sensor can emulate many features of
the one-axis-twisting (OAT) model, a fully connected model known to generate
scalable squeezing and Greenberger–Horne–Zeilinger-like states. The collective
nature of the state manifests itself in the preservation of the total transverse
magnetization, the reduced growth of the structure factor (spin-wave
excitations) at finite momenta, the generation of spin squeezing comparable with
OAT (a Wineland parameter of −3.9 ±â€‰0.3 dB for only N = 12 ions), and the
development of non-Gaussian states in the form of multi-headed cat states in the
Q-distribution. They demonstrate the metrological utility of the states in a
Ramsey-type interferometer, in which they reduce the measurement uncertainty by
−3.2 ±â€‰0.5 dB below the standard quantum limit for N = 51 ions.
Observations of quantum systems are subject to statistical uncertainty. The
control over quantum states in atomic systems has led to the most precise
optical atomic clocks so far. Their sensitivity is bounded at present by the
standard quantum limit for uncorrelated particles, which can be overcome when
operated with entangled particles. The researchers here used lasers to tune the
interaction of ions lined up in a vacuum chamber. They entangled the ensemble in
a chain with each other, produced a squeezed quantum state and tested the
measurement accuracy on the entangled ensemble demonstrating that measurement
error is halved in their experiment. The interaction between neighboring
particles decreases with the distance between the particles. Therefore, they
used spin-exchange interactions to allow the system to behave more collectively.
Thus, all particles in the chain were entangled with each other and produced a
squeezed quantum state. Using this, they showed that measurement errors can be
roughly halved by entangling 51 ions in relation to individual particles.
Previously, entanglement-enhanced sensing mainly relied on infinite
interactions, limiting its applicability to only certain quantum platforms. They
used an optical transition that is employed in atomic clocks, in their
experiments. They obtained very similar results using neutral atoms
For more information:
Nature, August 30 2023) page 740; Phys.org, July 20 (2023).
WEEK OF AUGUST 28, 2023 [No. 1538]
3D structure of excited nucleon resonances studied:
the CLAS Collaboration at the Jefferson Lab in Newport News, VA have used the
Continuous Electron Beam Accelerator Facility (CEBAF) in Experimental Hall B in
2018-2019 and the CLAS12 detector to measure hard exclusive
π−Δ++ electroproduction beam-spin asymmetries off the
proton. A high-energy electron beam was sent into a chamber of cooled
H2 gas. The electrons impacted the target's protons to excite the
quarks within and produce nucleon resonance in combination with a
quark-antiquark state (meson). The researchers plan experiments using different
targets and polarizations. By scattering electrons from polarized protons, they
can access different characteristics of the scattering process. The polarized
cross-section ratio σLT/σ0 from hard exclusive
π−Δ++ electroproduction off an unpolarized H target has
been extracted based on beam-spin asymmetry measurements using a 10.2  GeV/10.6
GeV incident electron beam and the CLAS12 spectrometer. The study provides an
observation of this channel in the deep-inelastic regime, and focuses on very
forward-pion kinematics in the valence regime and photon virtualities ranging
from 1.5  GeV2 up to 7  GeV2. The reaction provides an
access to the d -quark content of the nucleon and to p →Δ++
transition generalized parton distributions. They provide a comparison to
existing results for hard exclusive π+n and π0p
electroproduction showing the impact of the excitation mechanism, encoded in
transition generalized parton distributions, on the asymmetry.
For more information:
Phys.org, August 21 (2023); Phys. Rev. Lett., July 11 (2023) page 021901.
Circularly polarized photon emitter built:
a group of international researchers at LANL in Los Alamos, NM has produced a
quantum light emitter (QE) that generates a stream of single circularly
polarized photons. They stacked two different atomically thin materials to
realize the chiral quantum light source. The researchers showed that a monolayer
semiconductor can emit circularly polarized light without an external magnetic
field. This effect had been achieved before by using high magnetic fields, or by
coupling quantum emitters to nanoscale photonics structures, or by injecting
spin-polarized carriers into quantum emitters. The researchers here stacked a
single-molecule-thick layer of WSe2 semiconductor onto a thicker
layer of NiPS3 magnetic semiconductor. They used AFM to create a
series of indentations (~ 400 nm in diameter ) on the thin stack of materials.
The indentations created were useful for two effects when a laser was focused on
the stack of materials. The indentation formed a well in the potential energy
landscape. Electrons of the WSe2 monolayer got into the well. That
stimulated the emission of a stream of single photons from the well. The
nanoindentation also disrupted the magnetic properties of the underlying
NiPS3 crystal, creating a local magnetic moment pointing up out of
the materials. That magnetic moment circularly polarized the photons being
emitted. The researchers performed high magnetic field optical spectroscopy
experiments and measured the minute magnetic field of the local magnetic
moments. The experiments proved the way to control the polarization state of a
single photon stream. The proximity-effect approach has low-cost fabrication and
reliability. The strain-engineered WSe2/NiPS3
heterostructures hosting QE displayed sharp, localized photoluminescence (PL)
peaks with a strong degree of spontaneous circular polarization. The portion of
the WSe2 monolayer that did not overlap with NiPS3 emitted
bright PL, whereas PL was quenched when the WSe2 was coupled to the
NiPS3. Strong PL was restored by indentations. The group is now
exploring ways to modulate the degree of circular polarization of the single
photons with the application of electrical or microwave stimuli. They are also
looking into coupling of the photon stream into waveguides.
For more information:
Phys.org, August 24 (2023); Nat. Mat., August 17 (2023) page 1311.
WEEK OF AUGUST 21, 2023 [No. 1537]
Electron pairing in quantum corral observed:
researchers at the Universität Hamburg have observed electron quantum pairing
state on a quantum corral as predicted in the early 70's. They locked the
electrons into Ag quantum dots (QD). By coupling the locked electrons to an
elemental superconductor, the electrons inherited the tendency towards pairing
from the superconductor. The researchers related the experimental signature, a
spectroscopic peak at very low energy, to the quantum state already predicted
theoretically. They investigated artificial QDs defined by a cage of Ag atoms on
thin Ag(111) islands grown on superconducting Nb(110) using STM / STS. They
analyzed the minimum example of the proximity effect (gapless materials in
electronic contact with superconductors acquire proximity-induced
superconductivity in a region near the interface) on only a single
spin-degenerate quantum level of a surface state confined in a quantum corral on
a superconducting substrate, built atom by atom by using a STM. Whenever an
eigenmode of the corral is pitched close to the Fermi energy by adjusting the
size of the corral, a pair of particle–hole symmetric states enters the gap of
the superconductor. They identified these as spin-degenerate Andreev bound
states theoretically predicted 50 years ago by Machida and Shibata, which had
eluded detection by STM so far although were recently shown to be relevant for
transmon qubit devices. They found that the observed anticrossings of the in-gap
states are a measure of proximity-induced pairing in the eigenmodes of the
quantum corral. States of matter are formed when superconductivity is induced
into intrinsically non-superconducting materials by the proximity effect based
on Andreev reflection processes at the interface. If the transparency of the
interface between a normal metal in the clean limit and the superconductor is
high, superconductivity is induced over > 10 nms. However, for many
heterostructures, superconductivity has to be induced through interface states
or into surface states. These are typically well decoupled from the bulk bands
and, thus, it is unclear a priori whether they acquire sufficient pairing if
their distance to the superconductor is > nms. To study this effect in
detail, the researchers downscaled the problem as much as possible by
investigating only a single resonance mode of a surface state. This is achieved
by laterally confining the surface state in a quantum corral, forming a
particular QD. This can naturally occur in nanoscopic islands or, in a more
tunable platform, in artificially designed adsorbate arrays , in which the QD
walls are built atom by atom using the tip of a STM as a tool. Although the
surface states are typically well decoupled from metallic bulk states in the
direction perpendicular to the surface plane, scattering at step edges or at the
adsorbates introduces a measurable coupling to the bulk electronic states,
leading to a lifetime broadening of the QD’s eigenmodes (meVs). In contrast to
the usual cases of typical semiconductor or molecular QDs, the electron density
screening in the metallic QDs investigated here is larger by orders of
magnitude, which leads to largely suppressed electron–electron interactions,
that is, the QD charging energy U is negligible and, thereby, the QD can be
described by spin-degenerate single-particle eigenmodes. The researchers believe
that the concept of impurity-supported proximity-induced Cooper pairing could be
helpful in general to induce superconductivity into arbitrary surface states,
potentially also combined with non-trivial topology. Although electron–electron
interactions inside the noble-metal QDs studied here are typically screened well
by the charge carriers in the system’s bulk, the researchers might try to extend
this platform towards reduced screening, potentially enabling atomic-scale
studies of the crossover from spin-degenerate to spinful QDs coupled to
superconductors.
For more information:
Nature, August 16 (2023) page 60; Phys.org, August 17 (2023).
Shape inversion observed in µs excited state nucleus:
an international group of researchers at the ORNL in Oak Ridge, TN has studied
nuclear shapes far from stability. and observed a µs isomer at the N=20 island
of nuclear shape inversion. Researchers may have found an unstable
32Na nucleus that has an excited state with a spherical wave
function. They report excited-state spectroscopy from the first experiment at
the Facility for Rare Isotope Beams (FRIB). A 24(2)μs isomer was observed with
the FRIB Decay Station initiator (FDSi) through a cascade of 224- and 401-keV γ
in coincidence with 32Na nuclei. This is the only known µs isomer (1
μs ≤ T1/2 <1  ms) in the region. This nucleus is at the heart
of the N=20 island of shape inversion and is at the crossroads of the spherical
shell-model, deformed shell-model, and ab initio theories. It can be represented
as the coupling of a proton hole and neutron particle to 32Mg,
32Mg + π-1 + ν+1. This odd-odd coupling and
isomer formation provides a sensitive measure of the underlying shape degrees of
freedom of 32Mg, where the onset of spherical-to-deformed shape
inversion begins with a low-lying deformed 2+ state at 885 keV and a
low-lying shape-coexisting 0+2 state at 1058 keV. The
researchers suggest two possible explanations for the 625-keV isomer in
32Na: a 6− spherical shape isomer that decays by E2 or a
0+ deformed spin isomer that decays by M2. The present results and
calculations are most consistent with the latter, indicating that the low-lying
states are dominated by deformation. The wave functions of a nucleus are either
spherical or deformed. Typically, for a nucleus with a long-lived (more than 5
ns) isomer excited state, either the ground state’s wave function is spherical
and the isomer’s wave function is deformed, or the ground state’s and the
isomer’s wave functions are both deformed. A nucleus with a deformed
ground-state wave function and a spherical isomer wave function has yet to be
definitively observed. Numerical calculations performed by the group offer two
possible explanations for the wave-function shape of such a nucleus: it is
either spherical with six units of angular momentum or deformed with none.
Measurements of the spatial angular correlations between the two emitted γ
could help determine the correct explanation. Such measurements may be possible
after upcoming upgrades to the FRIB facility. The FRIB experiment involved
aiming a beam of excited 32Na nuclei at a detector. The detector
stopped the nuclei, causing them to decay to their ground states via the
emission of two γ. By measuring the time delay between the arrival of the nuclei
at the detector and the emission of the γ, the group deduced that the
32Na isomer has a long lifetime of 24 µs (ps nuclear excited state
lifetime being typical). This lifetime is the longest for an isomer with ~ 20 to
28 neutrons that decays by emitting γ. In data collected during the experiment,
the researchers found the previously unseen isomer of 32Na that has a
wave function that calculations indicate could be spherical or deformed. The
finding leaves open the possibility that they have observed the elusive excited
spherical system. The energy of an excited deformed state can drop below that of
a spherical ground state, making the spherical shape the high-energy one.
Unexpectedly, this role reversal appears to be happening for some exotic nuclei
when the natural ratio of neutrons to protons becomes unbalanced. Yet, the
post-reversal excited spherical states have never been found. It is as though
once the ground state becomes deformed, all the excited states do, too. Many
examples exist of nuclei with spherical ground states and deformed excited
states. Similarly, plenty of nuclei have deformed ground states and subsequent
excited states that are also deformed, sometimes with different amounts or kinds
of deformation. However, nuclei with both deformed ground states and spherical
excited states are much more elusive. Using data collected in 2022 from the
first experiment at FRIB, the researchers discovered a long-lived isomer excited
state of radioactive Na32. The long lifetime indicates something
unanticipated like difficulty in returning to a deformed ground state when the
excited state is spherical. The experiment employed the FDSi, a modular
multidetector system that is extremely sensitive to rare isotope decay
signatures. It shows that the long-lived excited state of 32Na is
delivered within the FRIB beam and that it then decays internally by emitting γ
to the ground state of the same nucleus. To stop FRIB's highly energetic
radioactive beam (speed ~ 0.5c), an implantation detector was positioned at
FDSi's center. North of the beam line was a γ detector array (DEGAi), comprising
11 Ge clover-style detectors and 15 fast-timing LaBr detectors. South of the
beam line were 88 modules of the NEXTi detector to measure time of flight of
neutrons emitted in radioactive decay. A beam of excited 32Na nuclei
stopped in the detector and decayed to the deformed ground state by emitting γ.
Analysis of γ spectra to discern the time difference between beam implantation
and γ emission revealed how long the excited state existed. The new isomer's
24-µs existence was the longest lifetime seen among isomers with 20 to 28
neutrons that decay by γ emission. Approximately 1.8% of the 32Na
nuclei were observed to be the new isomer. The researchers came up with two
different models that fit equally well the energies and lifetime observed in the
experiment. An experiment with higher beam power is needed to determine whether
the excited state in 32Na is spherical. If it is, then the state
would have six quantized units of angular momentum, which is a quality of a
nucleus related to its whole-body rotation or the orbital motion of its
individual protons and/or neutrons about the center of mass. However, if the
excited state in 32Na is deformed, then the state would have zero
quantized units of angular momentum. The two possibilities have very different
angular correlations between the two γ that are emitted in a cascade.
For more information:
Phys.org, August 16 (2023); Physics, June 13 (2023); Phys. Rev. Lett., June 13
(2023) page 242501.
WEEK OF AUGUST 14, 2023 [No. 1536]
Collective accelerated reactions in quantum state observed:
researchers at the University of Chicago in Chicago, IL have reported evidence
for quantum-enhanced chemical reactions with particles in the same quantum state
undergoing collective accelerated reactions. They report the observation of
coherent and collective reactive coupling between Bose-condensed atoms and
molecules near a Feshbach resonance. Starting from an atomic condensate, the
reaction begins with the rapid formation of molecules, followed by oscillations
of their populations during the equilibration process. The researchers observe
faster oscillations in samples with higher densities, indicating bosonic
enhancement. The researchers cooled down Cs atoms and placed them into the same
quantum state. Then, they observed as the atoms reacted to form molecules. The
reaction happens faster than it would under ordinary conditions. The more atoms
in the system, the faster the reaction happens. The final molecules share the
same molecular state. They observed many-body chemical reactions in the quantum
degenerate gas with reactions taking place as a three-body interaction more
often than as a two-body interaction. That is, three atoms would collide; two
would form a molecule, and the third remained single. But the third played some
role in the reaction. The researchers use a dynamics quantum field model to
identify three-body recombination as the dominant reaction process.
For more information:
Phys.org, August 7 (2023); Nat. Phys., July 24 (2023) page 1466.
2D sub-wavelength photonic waveguide built:
researchers at the University of Chicago in Chicago, IL have made a sub-nm thick
sheet of van der Waals glass crystal that can efficiently trap and carry light.
They built wafer-scale δ waveguides for integrated 2D photonics and showed that
they could confine and guide light in the ultrathin 2D material. They
demonstrated the efficient, large-scale generation and control of photonic modes
guided by van der Waals materials . Using MoS2 monolayers, they
trapped a laser beam and generated a 2D photonic wave propagating along the film
over mm distances. By integrating microfabricated thin-film optical components,
they demonstrated various optical functionalities. The researchers report
three-atom-thick waveguides (δ waveguides) based on wafer-scale MoS2
monolayers that can guide VIS and NIR light over mm-scale distances with low
loss and an efficient in-coupling. The extreme thinness provides a
light-trapping mechanism analogous to a δ-potential well in quantum mechanics
and enables the guided waves that are essentially a plane wave freely
propagating along the in-plane, but confined along the out-of-plane, direction
of the waveguide. They demonstrated key functionalities essential for 2D
photonics, including refraction, focusing, grating, interconnection, and
intensity modulation, by integrating thin-film optical components with δ
waveguides using microfabricated dielectric, metal, or patterned MoS2
For more information:
Science, August 10 (2023) page 648; Phys.org, August 11 (2023).
WEEK OF AUGUST 7, 2023 [No. 1535]
Spin density tuned:
researchers at MIT in Cambridge, MA have found a way to tune the spin density in
diamond a nitrogen vacancy (NV) center by applying an external laser or
microwave beam. They basically use a NV defect to sense and control the
surrounding electronic and nuclear spins. This quantum sensor reveals the nearby
spin environment and how that is affected dynamically by the charge flow, which
in this case is pumped up by the laser. By exploiting the cycling process of
ionization and recombination of NV centers in diamond, they pump electrons from
the valence band to the conduction band. These charges are then transported to
modulate the spin concentration by changing the charge state of material
defects. By developing a wide-field imaging setup integrated with a fast single
photon detector array, they achieve an efficient characterization of the charge
redistribution process by measuring the complete spectrum of the spin bath with
µm-scale spatial resolution. They demonstrate a two-fold concentration increase
of the dominant spin defects while keeping the T2 of the NV center
relatively unchanged, which also provides a potential experimental demonstration
of the suppression of spin flip-flops via hyperfine interactions. A major spin
defect in the environment (P1 center), can usually be 10 to 100 times more
populous than the NV center, and thus, can have stronger interactions, making
them good for studying many-body physics. The researchers exploit NV center
photoionization and charge transport to tune the density of the P1 spins in
diamonds. Using the NVs as quantum sensors of their spin and charge environment,
they demonstrate a factor of two increase in the P1 concentration, while
preserving the quantum sensor coherence. The charge redistribution is imaged by
a wide-field imager by measuring the spin bath spectrum. The group developed a
wide-field imaging setup that allows measurement of many different spatial
locations within the crystalline material simultaneously, using a fast
single-photon detector array in a microscope. They can spatially image the
density distribution over different spin species and the charge transport
dynamics. The researchers effectively have used charge ionization dynamics to
continuously tune the local spin defect density. The work was done using
lab-grown diamond, but the principles could be applied to other crystalline
solid-state defects.
For more information:
Phys.org, August 2 (2023); PNAS, August 1 (2023) page 251901.
Fermionic critical slow down in a phase transition directly observed:
a group lead by researchers of ETH Zurich in Zurich has proved that the concept
of electrons as carriers of quantized electric charge does mot apply near
quantum phase transitions. They have directly observed critical slowing down
(CSD) near a magnetic quantum phase transition with fermionic breakdown. Valence
- conduction electron boson quasiparticles can be destroyed during a phase
transition so a continuous phase transition on them can be observed and with it,
critical slowing down (with continuous transitions, the two phases get
energetically closer and closer together) can be observed too. So far, this
effect could be observed only indirectly in experiments. The researchers have
developed a method, which allows direct identification of the collapse of
quasiparticles at a phase transition, including the associated CSD. This has
enabled them to show directly that such a slow down can also occur in fermions.
When a system close to a continuous phase transition is subjected to
perturbations, it takes an exceptionally long time to return to equilibrium.
This CSD is observed universally in the dynamics of bosonic excitations, such as
order-parameter collective modes, but it is not generally expected to occur for
fermionic excitations. The researchers here use time-resolved THz (TRTHz)
spectroscopy to find evidence for fermionic CSD in
YbRh2Si2 close to a quantum phase transition between an
antiferromagnetic phase and a heavy Fermi liquid. In the latter phase, the
relevant quasiparticles are a quantum superposition of itinerant and localized
electronic states with a strongly enhanced effective mass. As the temperature is
lowered on the heavy-Fermi-liquid side of the transition, the heavy-fermion
spectral weight builds up until the Kondo temperature TK ≈ 25 K, then
decays towards the quantum phase transition and is, thereafter, followed by a
logarithmic rise of the quasiparticle excitation rate below 10 K. A two-band
heavy-Fermi-liquid theory shows that this is indicative of the fermionic CSD
associated with heavy-fermion breakdown near the quantum phase transition. At a
continuous phase transition, the ordered and the disordered phases have the same
energy. As a consequence, the fluctuations between these two states become
infinitely slow. This CSD is universally observed in the dynamics of classical
fields that are bosonic in nature but vanishes at the phase transition, like the
magnetization associated with bosonic magnons, in the case of ferromagnetic
order. In contrast, the CSD of fermionic excitations or quasiparticles is
generally not expected to occur since fermions, as elementary particles, are
thought to be indestructible. However, heavy-fermion (HF) compounds host
composite fermionic quasiparticles. These are quantum superpositions of
itinerant and localized (heavy) electron states generated by the Kondo effect
and have low binding energy parameterized by the Kondo energy scale, the lattice
Kondo temperature TK. At a quantum phase transition (QPT) in such
materials, these brittle, heavy quasiparticles are assumed to disintegrate
despite their fermionic nature. Defining their spectral weight, that is, the
probability of their existence, as the order parameter of such a fermionic QPT,
one may expect the CSD of the fermionic quasiparticle oscillations, a unique
signature of critical HF quasiparticle destruction, as opposed to bosonic
order-parameter fluctuations. Using TRTHz spectroscopy, the researchers directly
observe such fermionic CSD as a suppression of the heavy-particle hybridization
gap and a flattening of the associated band. This softening expands the region
in momentum space where resonant THz absorption is allowed. They observe this as
an increase instead of a Kondo-weight-loss-generated decrease of the
Kondo-related THz signal towards the quantum-critical point, before the heavy
quasiparticle band vanishes altogether below a breakdown temperature. They
identify a critical exponent in this behavior, which may, thus, lead to the
classification of fermionic quantum criticality in analogy to the criticality of
thermodynamic phase transitions. The researchers observed a logarithmic
low-temperature increase in the resonant quasiparticle excitation probability
near a magnetic QPT in HF materials. They identified this logarithmic increase
as a unique signature of fermionic quasiparticle CSD, that is, a vanishing
quasiparticle frequency near a QPT with fermionic breakdown. Since, in contrast
to the thermodynamic and transport properties, TRTHz spectroscopy is exclusively
sensitive to the HF quasiparticle dynamics as opposed to thermal fluctuations,
they could further extract the fermionic critical exponent α of the vanishing
quasiparticle weight. The critical behavior of α suggests that they can define
the heavy quasiparticle weight as an order parameter for QPTs with fermionic
breakdown. This may lead to the classification of fermionic QPTs in terms of
their critical exponent, analogous to thermodynamic phase transitions.
For more information:
Phys.org, July 31 (2023); Nat. Phys., July 31 (2023).
WEEK OF JULY 31, 2023 [No. 1534]
Nuclei vibration measured with record precision:
researchers at Heinrich-Heine-Universität Düsseldorf in Düsseldorf have tested
the charged baryon interaction with high-resolution vibrational spectroscopy of
molecular H ions. They have used ultra-high-precision laser spectroscopy on the
molecular H ion HD+ to measure the vibration of atomic nuclei with an
unprecedented level of precision. The long-range baryon–baryon interaction and
the degrees of freedom of baryon rotational and vibrational motion are key
features of molecular H ions. The researchers present the measurement of a
one-photon transition of HD+, a fourth overtone vibrational
transition. Through a comparison of experimental and ab initio frequencies of
this and previously measured transitions, they implement a test of the
low-energy quantum physics of baryon interaction and motion. The results may
also be interpreted as a test of Weinberg’s quantum mechanics extension.
Furthermore, they compare the value of the fundamental constant combination
μ/me = mpmd/(mp +
md)me determined from their measurement with the value
obtained from mass spectrometry experiments. This may also be regarded as a test
of the quantum behavior of baryons, revealing a moderate tension of 1.7 times
the combined uncertainty. Combining their measurement result with some previous
ones on HD+, they obtain a least-squares-adjusted value for
μ/me as well as a bound for the force between p and D in a scenario
that is beyond the SM. No evidence of a deviation from conventional quantum
physics is found. They have found no evidence of any deviation from the
established force between atomic nuclei. Over the years, the researchers have
refined the laser spectroscopy of the HD+, developing techniques that
have improved the experimental resolution of the spectra by orders of magnitude.
To achieve this, they confine some 100 HD+ in an ion trap in an
ultra-high vacuum container, using laser cooling techniques to cool the ions
down to 1 mK. This enables extremely precise measurement of the molecular
spectra of rotational and vibrational transitions. Following earlier
investigations of spectral lines with wavelengths of 230 μm and 5.1 μm, the
researchers now present measurements for the 1.1 μm spectral line. The
experimentally determined transition frequency and the theoretical prediction
agree. In combination with previous results, they have established the most
precise test of the quantum motion of charged baryons. Any deviation from the
established quantum laws must be < 10 ppt.
For more information:
Phys.org, July 28 (2023); Nat. Phys., June 22 (2023) page 1263.
Enhanced photodetection by photon trapping metasurface:
a group lead by researchers at UC in Davis, CA has built ultrafast thin Si-based
photodetectors with integrated photon-trapping micro- and nano-surface surface
structures with higher photoabsorption than GaAs and other group III-V
semiconductors. They have devised a way to boost the light absorption of thin Si
films by using photon-trapping micro- and nano-sized holes in Si that make
normally incident light bend by near 90°, making it propagate laterally along
the plane and increasing light absorption in the NIR band. Owing to its weak NIR
absorption, Si-based photonic devices need a thick substrate layer (for 95%
photon absorption of 850 nm wavelength, a 50μm thick Si layer is required). Even
with a thick Si absorption layer, a long transit time of the generated
electron–hole pair and its inherent low carrier mobilities result in low
photodetection. Commonly used III-V compounds like GaAs, InP, have 15× and
40× higher absorption at 850 nm, respectively. GaAs shows 90% absorption
of 850 nm incident wavelength in a 2.5μm thick layer. However, CMOS
incompatibility in the fabrication process, requires a costly hybrid integration
with CMOS electronics. The proposed photodetectors consist of a 1-µm-thick
cylindrical Si slab placed over an insulating substrate, with metallic
extensions from the contact metals atop the slab. The bulk Si is filled with
circular micro-holes arranged in a periodic pattern that act as photon-trapping
sites. The overall structure of the device causes normally incident light to
bend by almost 90° upon hitting the surface, making it travel laterally along
the Si plane. These laterally propagating modes increase the propagation length
of light and effectively slow it down, leading to more light–matter interaction
and an increase in absorption. This high-absorption phenomenon is explained by
finite-difference time-domain analysis, that shows an enhanced photon density of
states while substantially reducing the optical group velocity of light compared
to Si without photon-trapping structures, leading to enhanced light–matter
interaction. The researchers conducted optical simulations and theoretical
analyses to better understand the effects of the photon-trapping structures, and
performed several experiments comparing photodetectors with and without them.
They found that photon trapping led to an increase in the absorption efficiency
over a wide band in the NIR spectrum, staying above 68% and peaking at 86%. The
observed absorption coefficient of the photon-trapping photodetector was several
times higher than that of plain Si and exceeded that of GaAs in the NIR band.
Simulations of 30-nm and 100-nm Si films compatible with CMOS electronics showed
a similarly enhanced performance. By achieving high absorption even in ultrathin
Si layers, the parasitic capacitance of the circuit can be kept low for
high-speed systems. They have fabricated metal–semiconductor–metal
photodetectors on a 1μm-thin Si layer and integrated periodic photon-trapping
hole arrays. They have utilized CMOS-compatible processes to fabricate the
photodetectors and have fabricated two sets of devices with and without a
photon-trapping hole array. They demonstrate high enhancement of 80%, 85%, and
65% in the absorption efficiency in the photon-trapping-equipped photodetectors
for the NIR wavelength spectrum at 800, 850, and 905 nm, respectively. They show
20% reduction in the device capacitance due to a reduced effective device
volume of photon-trapping-equipped Si photodetectors that can result in
ultrafast performance due to the reduction in the resistance and capacitance
time constant. The researchers have reported an experimental demonstration of
photoabsorption enhancement > 20× in Si that effectively exceeds the
intrinsic absorption limit of GaAs for a broad wavelength spectrum between 800
and 905 nm.
For more information:
Phys.org, July 26 (2023); Physics, July 18 (2022); Ad. Phot. Nexus, July 24
(2023) page 056001; Phys. Rev. Lett., July 18 (2023) page 033601.
WEEK OF JULY 24, 2023 [No. 1533]
3D crystals built with 2D materials:
an international group lead by researchers at the University of Washington in
Seattle, WA has studied mixed-dimensional moire systems of twisted graphitic
thin films and observed that, by stacking a sheet of graphene onto bulk graphite
at a small twist angle, properties present at the graphene-graphite interface
can be transferred into the graphite itself. Thus, it is possible to imbue
graphite with physical properties similar to graphene. The researchers performed
transport measurements of dual-gated devices constructed by slightly rotating a
monolayer graphene sheet atop a thin bulk graphite crystal. They found that the
moire potential transforms the electronic properties of the entire bulk
graphitic thin film. In zero and small magnetic fields, transport is mediated by
a combination of gate-tuneable moire and graphite surface states, as well as
coexisting semi-metallic bulk states that do not respond to gating. At high
field, the moire potential hybridizes with the graphitic bulk states due to the
unique properties of the two lowest Landau bands of graphite. These Landau bands
facilitate the formation of a single quasi-2D hybrid structure in which the
moire and bulk graphite states are inextricably mixed. The researchers believe
that their approach could be used to test whether similar types of bulk
materials can also take on 2D-like properties. That is, it might be possible to
mix 2D properties into 3D materials. The researchers adapted an approach
commonly used to probe and manipulate the properties of 2D materials: stacking
2D sheets together at a small twist angle. They placed a single layer of
graphene on top of a thin, bulk graphite crystal, and then introduced a twist
angle of ~ 1° between graphite and graphene. They detected novel and unexpected
electrical properties not just at the twisted interface, but deep in the bulk
graphite as well. The twist angle induced a moire pattern and is critical to
generating these properties. Even though only a single sheet of graphene atop
the bulk crystal was twisted, researchers found that the electrical properties
of the whole material differed from typical graphite. And when they turned on a
magnetic field, electrons deep in the graphite crystal adopted unusual
properties similar to those of electrons at the twisted interface. The single
twisted graphene-graphite interface became inextricably mixed with the rest of
the bulk graphite. The entire crystal took on this 2D state. This is a way to
affect electron behavior in a bulk material.
For more information:
Nature, July 19 (2023) page 750; Phys.org, July 19 (2023).
Double magic nucleus 100Sn studied:
an international group at CERN in Geneva has used the ISOLDE facility and the
upgraded ISOLTRAP experiment to determine the energy necessary to bring the
nucleus of 99In from its ground state to a long-lived excited isomer
state 99Inm. The excitation energy of the 1/2− isomer in
99In at N=50 is measured to be 671(37) keV and the mass uncertainty
of the 9/2+ ground state is significantly reduced. The results reveal constancy
of the 1/2− isomer excitation energies in neutron-deficient 99In that
persists down to the N=50 shell closure, even when all neutrons are removed from
the valence shell. That is, the isomeric excitation energy for
99Inm obtained from mass spectrometry reveals constant
trend next to doubly magic 100Sn. The result follows an earlier
ISOLTRAP measurement of 99In in the ground state, offering an even
closer look at the doubly-magic nucleus of 100Sn, that it is also
the heaviest such nucleus comprising protons and neutrons in equal number so it
is particularly difficult to produce in the lab and is relatively short-lived.
The researchers looked into the 99Inm nuclear isomer,
which has a slightly different orbital occupation, and hence higher energy, than
the ground state and results in a slightly larger nuclear mass. Using the
upgraded version of the ISOLTRAP experiment's multireflection time-of-flight
mass spectrometer, the researchers were able to measure the difference in the
time-of-flight of confined 99Inm nuclei in their ground
and isomeric states. This small difference, which is caused by the different
mass of the nucleus in these two states, made it possible to determine the
energy necessary to excite the isomer. The researchers compared their result
with measurements of isomer excitation energies for other nuclei close to In
(including a recent measurement of 101Inm in ISOLTRAP).
This comparison showed that the excitation energies are essentially the same
down to the magic neutron number 50. The result contrasts with recent results on
the magnetic moments of In nuclei from the ISOLDE's CRIS experiment, which saw
their constant value undergoing an abrupt change at magic neutron number 82.
They compared the results with large-scale shell model, ab initio, and density
functional theory calculations. They found that the calculations do not predict
the isomer excitation energies and the ground-state electromagnetic magnetic
moments simultaneously.
For more information:
Phys.org, July 20 (2023); Phys. Rev. Lett., July 14 (2023) page 022502.
WEEK OF JULY 17, 2023 [No. 1532]
Quantum fluctuations controlled:
researchers at MIT in Cambridge, MA have demonstrated control over quantum
randomness by biasing the quantum vacuum to control macroscopic probability
distributions. The researchers here focused their work on vacuum fluctuations.
The researchers have shown that injecting a weak laser bias into an optical
parametric oscillator (OPO), an optical system that naturally generates random
numbers, can serve as a controllable source of biased quantum randomness. Using
the two-state phase output from an OPO as a bit, they have shown that the bit
probabilities change in response to a small bias field injected into the system.
By changing the attenuation level of the bias field, they traverse the
continuous space between perfectly random and deterministic state selection.
They show that vacuum-level bias fields injected into multistable optical
systems enable a controllable source of quantum randomness. By injecting bias
pulses with less than one photon on average, they controlled the probabilities
of the two possible OPO output states. The potential of this method for sensing
sub–photon-level fields was demonstrated by reconstructing the temporal shape of
fields below the single-photon level. The approach provides the prospect of
using vacuum fluctuations as a source of controllable randomness for photonic
probabilistic computing. The random quantum fluctuations of optical systems can
be used for the generation of random bit strings. Their use in probabilistic
computing, requires the probability distributions to be controllable. Practical
implementation of probabilistic computing and weak field sensing has been
hampered by lack of control over the probability distributions associated with
quantum randomness. The work here has shed light on a possible solution. The
researchers have shown the ability to manipulate the probabilities associated
with the output states of an OPO, creating a controllable photonic probabilistic
bit (p-bit). Their system has shown sensitivity to the temporal oscillations of
bias field pulses, far below the single photon level. Their p-bit generation
system currently allows for the production of 104 p-bits/s, each of
which can follow an arbitrary binomial distribution.
For more information:
Science, July 13 (2023) page 205; Phys.org, July 13 (2023).
Enhanced spin-exchange carrier multiplication in doped quantum dots:
researchers at LANL in Los Alamos, NM have produced spin-exchange carrier
multiplication (CM) in Mn-doped colloidal quantum dots (QD). In CM, the kinetic
energy of a carrier relaxes via generation of additional electron-hole pairs
(excitons). CM is driven by carrier–carrier interactions that lead to excitation
of a valence-band electron to the conduction band. Normally, the rate of
phonon-assisted relaxation exceeds that of Coulombic collisions, which limits
the CM yield. The researchers here show that this limitation can be overcome by
exploiting not direct but spin-exchange Coulomb interactions in Mn-doped
core/shell PbSe/CdSe QDs. In these structures, CM occurs via two spin-exchange
steps. First, an exciton generated in the CdSe shell is rapidly transferred to a
Mn dopant. Then, the excited Mn ion, Mn*, undergoes spin-flip relaxation via a
spin-conserving pathway, which creates two excitons in the PbSe core. The
researchers here incorporated magnetic dopants into specially engineered
colloidal QDs to achieve enhanced spin-exchange CM. Specifically, Mn-doped QDs
with a PbSe core and CdSe shell convert a single photon into two excitons in
spin-exchange CM. The Mn ions' magnetic spins strongly interact with both the
core and the shell of the QD. In the course of these interactions, energy can be
transferred to and from the Mn ion by flipping its spin by spin exchange. In
spin-exchange CM, a single absorbed photon generates two excitons, which occur
as a result of spin-flip relaxation of an Mn*. Due to the extremely fast rate of
spin-exchange interactions (subps timescales), the Mn-doped QDs show a
three-fold enhancement in the CM multiexciton yield compared to similarly
structured undoped QDs (the enhancement is especially large in the range of
photon energies within the solar spectrum). Mn dopants help tackle the problems
of fast phonon emission. Building off previous research that demonstrated the
sub-ps timescales (faster than phonon emission) of spin-exchange interactions,
the researchers realized that using these interactions would boost the CM
efficiency. To enact spin-exchange CM, properly engineered QDs are required. The
bandgap of these dots must be less than half of the energy of the Mn spin-flip
transition and the spin structure of the QDs should match that of the Mn*. The
energy conditions can be satisfied with Mn-doped QDs containing a PbSe core and
CdSe shell. In these structures, The researchers use the Mn-doped core/shell
PbSe/CdSe QDs to demonstrate that spin-exchange interactions open a highly
efficient CM pathway. In this pathway CM occurs via two fast spin-exchange
steps. First, the energy of the exciton, generated by an absorbed photon in the
CdSe shell, is transferred to the Mn ion (a very fast spin-exchange excitation
transfer from the light-harvesting CdSe shell to a Mn dopant). Followed by
spin-flip relaxation of the Mn* back to the unexcited state by creating two
excitons in the PbSe core. They also detect weak signatures of radiative decay
of Mn* leading to the formation of a PbSe-core exciton and a NIR photon whose
energy is defined by the difference between the energy of the Mn spin-flip
transition and the PbSe-core bandgap. The magnetic ions act as highly effective
mediators of spin-exchange interactions between the CdSe and PbSe QD components
with the Mn* undergoing spin-flip relaxation accompanied by the generation of
two core-based excitons. They detect weak signatures of radiative decay of Mn*
leading to the formation of a PbSe-core exciton and emission of a NIR photon
whose energy is defined by the difference between the energy of the Mn spin-flip
transition and the PbSe-core bandgap. Due to the spin-exchange contribution to
the CM process, Mn-doped QDs exhibit a considerable reduction in the exciton
creation energy compared to the undoped structures, which translates into an
up-to-threefold enhancement of the CM yield at near-CM-threshold photon
energies.
For more information:
Phys.org, July 14 (2023); Nat. Mat., July 13 (2023) page 1013.
WEEK OF JULY 10, 2023 [No. 1531]
Zero electron electric dipole moment confirmed with record precision:
researchers at the University of Colorado Boulder (JILA) in Boulder, CO have set
and performed a high-precision HfF+ molecular spectroscopy table-top
experiment to measure the electron electric dipole moment (eEDM). They found no
evidence that the electron has an electric dipole moment. The measurement
lowered the previously obtained, with a different technique limit on the
moment’s strength by a factor of 2.4. The researchers present the symmetry
breaking eEDM measurement, using electrons confined inside molecular ions,
subjected to a high intramolecular electric field, and evolving coherently for
up to 3s . They found that any imbalance in eEDM must be < 4.1 ×
10-30 e cm, with an uncertainty of 2.1×10-30 e cm
(with electron charge asymmetry bounded to < 10-17). An EDM in an
electric field causes an energy shift when its orientation is flipped with
respect to the field. A non-zero eEDM would imply time-reversal symmetry
violation (TSV): If time were reversed, its magnetic moment would flip and the
EDM would not, looking fundamentally different from before time-reversal. All
eEDM measurements so far are consistent with zero, indicating that no particle
has been found to induce enough TSV to generate an eEDM that is within the
current experimental sensitivities. However, tighter experimental bounds on the
eEDM probes increasingly higher particle masses (analogous to how higher-energy
particle colliders can search for more-massive particles). The researchers here
looked for symmetry-breaking particles indirectly in a tabletop experiment, by
looking for signs of new particles in the charge distribution of the electron
(rather than by direct high energy collisions in colliders). In the experiment
the scientists consider placing an electron with spin pointing horizontally in a
vertical magnetic field. The field would make the electron spin perpendicular to
the field. If they applied a strong electric field to the molecules, non-round
electrons would want to align with the field, shifting around inside the
molecule. If they were round, then the electrons would not budge. If the
electron also has an electric dipole moment, then applying an electric field
pointing the same way as the magnetic field would make the electron spin faster.
Flipping the electric field the opposite way would make the electron spin
slightly slower. In their experiment they apply such electric fields and look
for the small shift in the frequency of the electron’s spinning. They found a
sufficiently strong electric field in the extreme conditions inside
HfF+ molecular ions confined within an ion trap. Within the bond
between the Hf and F atoms, an electron experiences an electric field ~ 20
GeV/cm. They applied an external electric field to flip the molecular ion and
its powerful internal field one way or the other relative to the applied
magnetic field, while watching for any changes in the frequency at which the
electron within spinned. Thus, they determined the eEDM by measuring the
frequency difference of the electron precession inside the molecular ions. The
high internal electric field of these ions makes the frequency difference much
larger, and by confining the ions in a trap, they were able to measure the
precession of the electron for up to 3 s. They had good control over the
molecules so as to measure the precession frequency to 10's µHz precision. The
experiment required layers of technology to trap the ions, set the electrons
within them spinning, and, after a variable delay, determine which way the
electrons were pointing by blowing the molecules up with a laser. Critical was
the ability to hold the ions for up to 3 s in a one-of-a-kind trap, enabling the
electrons within the molecules to spin hundreds of times. Using a UV laser, they
stripped electrons off molecules, making and trapping a set of positively
charged ions. Once they produced and trapped the HfF+ molecules in
their ion trap, they polarized them with a rotating electric field (forcing them
to either align or not align with the field). The polarization controls the
orientation of the large internal molecular field that the molecule’s valence
electron experiences. With an additional applied magnetic field, lasers, MW, and
RF fields, they measured the energy of HfF+ molecules in the two
configurations (when the electron’s magnetic moment was aligned and when it was
anti-aligned to the molecule’s massive electric field). If the levels were
different between them, that would indicate that the electrons were
asymmetrical. Their experiment allowed them to have longer measurement times
than in past eEDM measurements, which gave them greater sensitivity. However,
the measurements showed that the electrons did not move energy levels,
indicating that electrons are round within the precision measured.
The researchers meticulously studied their experimental apparatus and
measurement technique to understand systematic uncertainties in minute detail to
ensure that no spurious signals were mistakenly introduced. After 620 h of data
collection, during which they changed multiple experimental parameters to
investigate and reduce systematic errors, they reduced the upper limit on the
eEDM strength to 4.1×10-30 e cm (37 times smaller than their
own previous measurement and 2.4 times smaller than the previous best limit; the
SM, unable to address the UBA outstanding problem, predicts eEDM <
10-38 e cm). That bound implies a higher lower bound on the mass of
any not-yet-observed particles that would induce TSV. The new limit imposed here
contradicts predictions for the eEDM made by some extensions to the SM (ex:
split SUSY and spin-10 GUT). The predicted eEDM strength scales inversely with
the energy scale of the proposed new particles and in that sense, more precise
measurements of the eEDM will probe higher and higher energy scales. That energy
scale is bound here at 40 TeV, with the implication that any undiscovered vacuum
particles might be too massive for production in the foreseable colliders. The
next generation of the eEDM experiment here will use ThF+ allowing to
measure electron precession for 10-20s.
For more information:
Science, July 6 (2023) pages 46 and 28; Phys.org, July 6 (2023); Physicsworld,
August 22 (2023).
Pair density wave observed at zero field:
an international group lead by researchers at BNL in Upton, NY has performed a
direct visualization of a pair density wave (PDW) with no magnetic field
present, and thus, confirmed smectic PDW order in
EuRbFe4As4 in the Fe-based superconductor
EuRbFe4As4 (Eu-1144), a material with layered crystalline
structure that features co-existing superconductivity (Tc
≈ 37 K) and ferromagnetism (Tm ≈ 15 K). Using
spectroscopic imaging scanning tunneling microscopy (SI-STM) measurements, they
show that the superconducting gap at low temperature has long-range,
unidirectional spatial modulations with an incommensurate period of about eight
unit cells. Upon increasing the temperature above Tm, the
modulated superconductor disappears, but a uniform superconducting gap survives
to Tc. When an external magnetic field is applied, gap
modulations disappear inside the vortex halo. The SI-STM and bulk measurements
show the absence of other density-wave orders, indicating that the PDW state is
a primary, zero-field superconducting state in this compound. Both four-fold
rotational symmetry and translation symmetry are recovered above
Tm, indicating that the PDW is a smectic order. A PDW (coupled
pairs of electrons constantly in motion) has been thought to only arise when a
superconductor is placed within a large magnetic field. Evidence for a
zero-field PDW state that exists independent of other spatially ordered states
had so far been elusive. It had been theorized before that a PDW could exist on
its own, but the evidence had been ambiguous until now. Since superconductors
are generally destabilized by magnetic order, when both superconductivity and
magnetism exist together in a single compound, it is of interest to see how the
two of them coexist. It's conceivable that the two phenomena exist in different
parts of the compound and have nothing to do with each other. But, instead, the
researchers here found that there is a connection between the two. The spatially
modulated superconductivity was detected upon appearance of the magnetism. The
research took place in an ultra-low vibration laboratory using a SI-STM. to
measure how many electrons at a specific location in the material tunnel back
and forth between the sample's surface and the tip of the SI-STM as the voltage
between the tip and the surface is varied. They have obtained a wave pattern
above the crystal lattice representing how the energy level of the electron
pairs spatially modulates as these electrons move through the crystal. These
measurements allow to create a map of both the sample's crystal lattice and the
number of electrons at different energies at each atomic location. They
performed measurements on their sample as its temperature was increased, passing
through Tm and Tc. Below the sample's
Tc, the measurements revealed a gap in the spectrum of
electron energies. This gap' size is equivalent to the energy it takes to break
apart the electron pairs that carry the superconducting current. Modulations in
the gap reveal variations in the electrons' binding energies, which oscillate
between a minimum and maximum. These energy gap modulations are a direct
signature of a PDW.
For more information:
Nature, June 28 (2023) page 940; Phys.org, July 3 (2023).
WEEK OF JULY 3, 2023 [No. 1530]
Spatially modulating superconducting state discovered:
an international group lead by researchers at Cornell University in Ithaca, NY
has discovered that UTe2 is a special type of superconductor,
possibly topological, where some of the electron pairs form an electron
pair-density wave state (a crystal structure embedded in the quantum background
fluid) with the pairs of electrons having intrinsic angular momentum. Bulk
Cooper-pair condensates are definitely topological when their superconductive or
superfluid order parameters exhibit odd parity Δ(k) = −Δ(−k) with spin-triplet
pairing. This situation is epitomized by liquid 3He, the only known
bulk topological Cooper-pair condensate. Although no bulk superconductor
exhibits an unambiguously topological Δ(k), UTe2 (Tc= 1.65
K ) might. Its extremely high critical magnetic field and the minimal
suppression of the Knight shift on entering the superconductive state imply
spin-triplet superconductivity. Temperature, magnetic field, and angular
dependence of the superconductive quasiparticle thermal conductivity are all
indicative of a superconducting energy gap with point nodes. In the
superconductive phase, evidence for time-reversal symmetry breaking is provided
by polar Kerr rotation measurements but is absent in muon-spin-rotation studies.
The superconductive electronic structure when visualized at opposite mesa edges
at the UTe2 (0–11) surface breaks chiral symmetry. Dynamically,
UTe2 seems to contain both strong ferromagnetic and antiferromagnetic
spin fluctuations relevant to superconductivity. Together, these results are
consistent with a spin-triplet and, thus, odd-parity, nodal, time-reversal
symmetry breaking, chiral superconductor. Although UTe2 may embody
bulk topological superconductivity , its superconductive order parameter Δ(k)
remains unknown. Many diverse forms for Δ(k) are physically possible in a heavy
fermion materials Intertwined density waves of spin (SDW), charge (CDW) and
pair (PDW) may interpose, with the latter exhibiting spatially modulating
superconductive order parameter Δ(r), electron-pair density and pairing energy
gap. The recently discovered CDW state in UTe2 implies that a PDW
state might exist on it. The researchers here visualize the pairing energy gap
with μeV-scale energy resolution using superconductive STM tips. The researchers
here have demonstrated that PDWs occur at three incommensurate wavevectors
Pi=1,2,3 on the (0–11) surface of UTe2. These wavevectors
are indistinguishable from the wavevectors Qi=1,2,3 of the prevenient
normal-state CDW at the equivalent surface. All three PDWs exhibit peak-to-peak
gap energy modulations in the range ≅ 10 μeV. When the Pi=1,2,3
PDW states are visualized at 280 mK in the identical FOV as the
Qi=1,2,3 CDWs visualized > Tc, every Qi:Pi pair is
spatially registered to each other, but with a relative phase shift of |δϕi|
≅Ï€ throughout. Given the premise that UTe2 is a spin-triplet
superconductor, the PDW phenomenology detected and described here sets it as
spin-triplet PDW superconductor. From these observations, and given
UTe2 as a spin-triplet superconductor, this PDW state should be a
spin-triplet PDW. Although such states do exist in superfluid 3He,
for superconductors, they are unprecedented.
For more information:
Nature, June 28 (2023) page 921; Phys.org, June 28 (2023).
Superradiant state observed in α decay of mirror nuclei:
an international group lead by researchers at Texas A&M University in College
Station, TX has found evidence of the superradiance effect in the differences
between the α decaying states in 18O and 18Ne. They
studied α clustering in 18Ne and compared it with what is known about
clustering in the mirror nucleus 18O. The excitation function for
14O+α resonant elastic scattering was measured in inverse kinematics
using the Texas active target detector system. The data cover the
excitation-energy range from 8 to 17 MeV. The analysis was performed using a
multichannel R-matrix approach. The detailed spectroscopic information was
obtained from the R-matrix analysis: excitation energy of the states, spin, and
parity as well as partial α and total widths. This information was compared with
theoretical models and previous data. Correspondence between the levels in
18O and 18Ne was established. The researchers carried out
an extensive shell-model analysis of the 18O and 18Ne
mirror pair. There was good agreement between theory and experiment, useful to
understand the clustering strength distribution. The comparison of the
experimental data with theory showed that certain states especially at high
excitation energies are significantly more clustered than predicted. This
indicates that the structure of these states is collective and is aligned
towards the corresponding α reaction channel. Superradiance can take place when
the excitation energy and the number of excited states so large that the nuclear
excited states are so close to each other that neighboring excited states
overlap with each other. If that happens, instead of observing many states, one
sees only one superradiant state. The wave functions for the analogous states in
mirror nuclei are expected to be nearly identical in bound state approximation
due to proton-neutron symmetry. On the other hand, coupling to the continuum may
be very different as a result of different electric charges and energy above the
respective decay threshold. To find evidence of superradiance in nuclei,
researchers look for two systems that have the same internal structure but
different decay channels. Mirror nuclei have the same total number of protons
and neutrons, but the number of protons in one equals the number of neutrons in
the other. The internal structure of mirror nuclei is the same since the nuclear
force is charge independent. However, the decay channels are different due to
the different electric charge repulsion in the two systems. The researchers here
studied the structure of 18Ne by scattering a radioactively unstable
14O beam on a thick 4He gas target. Using a gas target
allowed them to measure the tracks of the incoming and outcoming particles and
produce a complete reconstruction of the nuclear events. The structure of
18O had been previously studied elsewhere by scattering
14C on a 4He target using a particle accelerator so data
on 14C + α and 14O + α allows to compare α resonances in
the 18O and 18Ne mirror nuclei. The α particle decay
thresholds are 6.23 MeV in 18O and 5.11 MeV in 18Ne; this
and the difference in the Coulomb barrier results in the difference in
penetrability factors. Levels with substantial reduced α width and thus well
coupled to α channels were observed in both measurements. These studies allowed
the researchers here to use the information about the 18O excited
states to find the initial parameters for the analysis of the 18Ne
data.. As expected from the charge independence of the nuclear force, the
researchers found a correspondence between mirror states in the two nuclei,
although some differences emerged when comparing the strength of mirror states.
If the internal structure of the nuclei is the same, one would expect mirror
levels to have the same strength, but in these cases alignment with slightly
different decay channels produces observed differences. The researchers
interpreted these differences as evidence of the superradiance effect.
For more information:
Phys.org, June 28 (2023); Phys. Rev. C, November 8 (2022) page 054310; Comm.
Phys., December 19 (2022).
WEEK OF JUNE 26, 2023 [No. 1529]
Positive gluon polarization determined:
researchers at the PHENIX Collaboration in RHIC at BNL in Upton, NY have
obtained definitive evidence that gluon spins are aligned in the same direction
of their proton.. The researchers present measurements of direct-photon cross
section and double-helicity asymmetry at 510  GeV in longitudinally polarized pp
collisions. The measurements have been performed at midrapidity (|η|<0.25)
with the PHENIX detector at RHIC. At relativistic energies, direct photons are
dominantly produced from the initial quark-gluon hard scattering and do not
interact via the strong force at leading order. Therefore, at 510  GeV, where
leading-order-effects dominate, these measurements provide clean and direct
access to the gluon helicity in the polarized proton in the
gluon-momentum-fraction range 0.02<x<0.08, with direct sensitivity to the
sign of the gluon contribution. The research here is a comparison of the number
of direct photons emitted when RHIC collides protons with their spins pointing
in opposite directions (either at or away from each other) with the number of
direct photons produced when the protons in the two beams are pointing in the
same direction (colliding head to tail). Given the way quarks and gluons can
interact to emit photons (and knowing that net quark spins are positively
aligned with proton spin), seeing a difference would indicate that gluon spins
are also polarized and in which direction. The collision condition producing
more direct photons would tell them whether the gluon spin alignment was
positive (pointing in the same direction as the proton spin and contributing to
that value) or negative (pointing in the opposite direction and counteracting
the quarks' contributions to spin). To investigate gluons' contribution to spin,
the researchers compared the number of direct photons emitted from two different
types of longitudinally polarized pp collisions: Ones where the proton spins
were pointing in opposite directions (in this case, at each other) and ones
where the two proton spins were pointing in the same direction, colliding head
to tail. The direct photon production at RHIC is dominated by quark-gluon
interactions (they come directly from the interaction of a quark in one proton
beam with a gluon in the other rather than from the decay of some other particle
produced in the collision). Because photons do not carry any color charge they
do not interact with the surrounding nuclear matter. One reason for the
difficulty of the experiment is that many other photons stream out of RHIC
collisions. Finding those that come directly from quark-gluon interactions
requires a systematic process of elimination. The PHENIX detector has the most
finely grained electromagnetic calorimeter for a collider experiment, so it
provides the capabilities needed to separate out direct photons from those
coming from other sources. If a photon picked up in the detector is surrounded
by other particles, it likely came from radiative processes that happened after
the collision so those photons are not direct. Likewise if the energy and angles
of a pair of photons can be reconstructed to have originated from the decay of a
parent particle. After all the eliminations, the photons with no other obvious
source are assumed to have originated from a quark-gluon scattering event. A
very high number of collisions is needed to get enough direct photons for a
proper statistical analysis after filtering out other photon-producing
processes. When RHIC first started colliding polarized protons in the early
2000s, it did not provide the required number of collisions. Meanwhile the
RHIC's two large detectors, PHENIX and STAR, made other measurements related to
gluon spin. STAR's analysis of particle jets and PHENIX's analysis of
Ï€0 mesons showed that gluons were polarized. But those measurements
could not reliably reveal the direction of the gluons' spin alignment, plus, the
particles tracked in those measurements do interact via the exchange of color
charges, so their signals were not as clean as the direct photon measurements.
By 2013, RHIC's luminosity for both polarized proton and heavy ion collisions
were increased and the degree of polarization in the proton beams was ~ 60%.
Where reconstructing photons resulting from the decay of other particles was not
feasible, the researchers instead relied on dedicated simulations to determine
the frequency of these occurrences and remove them from the data. That allowed
them to zero in on a high enough number of direct photons to perform a
statistically valid analysis. The results here are based on data collected in
2013, the last year high-energy longitudinally polarized protons were run while
RHIC's PHENIX detector was still collecting data. (PHENIX shut down in 2016 to
make way for sPHENIX). The data analyzed corresponds to about 5.4 x
1012 p-p collisions. The PHENIX results show a clear difference in
direct photon yields, with higher yields coming from the collisions where the
proton spins are pointing in opposite directions (at or away from one another).
This is evidence that the gluon spins are aligned in the same direction as the
proton spin and make a significant contribution to the proton's overall spin
value. The final contribution to proton spin is likely the orbital motion of
quarks and gluons. Those details are inaccessible in collisions of
longitudinally polarized protons. Collisions at RHIC where protons are collided
with their spins pointing up (transversely), have shown hints of the gluons'
orbital motion. Precision measurements of orbital motion will be made at the
Electron-Ion Collider in BNL. The EIC will use polarized e-p collisions to
measure the spin contribution of gluons. While RHIC has given access to the
highest momentum fraction gluons (carrying a large fraction of the overall
proton momentum), the EIC will give access to low momentum fraction gluons where
each carry a small fraction of the overall proton momentum, but they may make an
outsized contribution to proton spin due to their relative abundance.
For more information:
Phys.org, June 21 (2023); Phys. Rev. Lett., June 21 (2023) page 251901.
Mechanism for nematic transition in superconductors observed:
researchers at MIT in Cambridge, MA and ANL in Lemont, IL have observed
spontaneous orbital polarization in the nematic phase of FeSe. The researchers
use XR linear dichroism at the Fe K pre-edge to measure the anisotropy of the
3d orbital occupation as a function of in situ applied stress and temperature
across the nematic transition. Using XR diffraction to precisely quantify the
strain state, the researchers observed a lattice-independent, spontaneously
ordered, orbital polarization within the nematic phase, as well as an orbital
polarizability that diverges as the transition is approached from above. They
studied the 2D FeSe material (the highest-temperature Fe-based superconductor
with Tc ~ 70 K). In other Fe-based superconducting materials,
researchers have observed that this switch occurs when individual atoms
spontaneously shift their magnetic spin towards a coordinated, preferred
magnetic direction. Strong interactions between electrons cause the material as
a whole to stretch infinitesimally in one particular direction that allows
electrons to flow freely in that direction. The researchers here discovered that
FeSe rather than undergoing a coordinated shift in atom spins, undergoes a
collective shift in their orbital energy, that is, FeSe has no spin-driven
transition and no coordinated magnetic behavior. Understanding the origin of
nematicity here requires studying how the electrons arrange themselves around
the Fe atoms, and what happens as those atoms stretch apart. Using ultrabright
XR, the researchers tracked how the atoms in each sample were moving, as well as
how each atom's electrons were behaving. After a certain point, they observed a
definite, coordinated shift in the atoms' orbitals (electronic energy levels).
In FeSe, electrons can occupy one of two orbital states around an Fe atom.
Normally, the choice of which state to occupy is random. The researchers found
here that as they stretched the FeSe, its electrons began to overwhelmingly
prefer one orbital state over the other. This signaled a coordinated shift, and
a mechanism of nematicity and superconductivity. The researchers worked with
ultrathin, mm-long samples of FeSe, which they glued to a thin Ti strip. They
mimicked the structural stretching that occurs during a nematic transition by
physically stretching the Ti strip, which in turn stretched the FeSe samples. As
they stretched the samples by a fraction of µm at a time, they looked for any
properties that shifted in a coordinated fashion. To understand what drives the
nematicity, one needs to determine which electronic degree of freedom admits a
spontaneous order parameter independent from the structural distortion. The
results here provide strong evidence that spontaneous orbital polarization
serves as the primary order parameter of the nematic phase.
For more information:
Phys.org, June 22 (2023); Nat. Matt., June 22 (2023).
WEEK OF JUNE 19, 2023 [No. 1528]
Optical-lattice clock operation below quantum projection noise:
researchers at JILA in Boulder, CO have realized a direct comparison of
spin-squeezed optical lattice clocks at record precision level. The researchers
present an optical clock platform integrated with collective strong-coupling
cavity QED for quantum non-demolition (QND) measurement. Optimizing the
competition between spin measurement precision and loss of coherence, they
measure a Wineland parameter of -1.8(7) dB for 1.9x107 atoms, thus
verifying the presence of entanglement. A moving lattice allows the cavity to
individually address two independent sub-ensembles, allowing the researchers to
spin squeeze two clock ensembles successively and compare their performance.
This differential comparison between the two squeezed clocks directly verifies
enhanced clock stability of 2.0(3) dB below quantum projection noise (QPN), and
0.6(3) dB above the standard quantum limit (SQL), at the measurement precision
level of 10-17, without subtracting any technical noise
contributions. Optical atomic clocks are limited by the QPN from the spin
statistics of the many atoms they interrogate. By leveraging the quantum nature
of these systems, it is possible to entangle the atomic sample to circumvent
this QPN limit. These clocks all use large samples of non-interacting atoms;
when measuring the quantum state of these atoms to extract information about the
clock frequency, each atom is projected into a discrete state, contributing
quantum projection noise to frequency measurements. Optical atomic clocks are
limited by the QPN defined by many uncorrelated atoms. Work on producing spin
squeezed states of atoms has shown a path towards integrating entanglement into
the best performing clocks. To directly demonstrate advantage of quantum
entanglement in a working clock one must prevent backaction effects that degrade
quantum coherence and introduce uncontrolled perturbations, as well as minimize
the influence of technical noise arising from the interrogating clock laser. The
researchers present direct observations of an optical-lattice clock operating
below the classical QPN limit, averaging down to a measurement precision level
of 10-17. These clocks can be improved using entanglement (difficult
due to fragility of entanglement in increasingly larger sized quantum systems)
to reduce the QPN of the collective ensemble. They have demonstrated the best
stability to date of a spin-squeezed clock, thus getting one step closer to
making spin squeezing practical for optical lattice clocks. The researchers
prepared spin-squeezed atoms in an optical-lattice clock to achieve improved
precision. Although proof-of-principle experiments have generated squeezing on
optical clock transitions and inferred sub-QPN operation after subtracting
noise, an optical-lattice clock has not been directly observed to operate below
the QPN limit todate. The researchers here combined a vertical 1D Sr optical
lattice clock with an optical cavity that provides strong collective coupling.
This enabled atom squeezing through cavity-mediated quantum non-demolition
measurements. A conveyor-belt lattice allowed independent sub-ensembles to be
shuttled into the cavity to generate distinct spin-squeezed samples, while a
clock laser simultaneously interrogated the entire atom cloud. The researchers
then performed a self-synchronous clock comparison between two spin-squeezed
ensembles in the Sr optical lattice clock. The researcher plan to further
control atomic motion within their lattice to improve rotation fidelities and
collective coupling to the cavity, thus leading to better clock stability and
less decoherence of the atomic ensemble
For more information:
Phys.org, June 15 (2023); arXiv (2022). DOI: 10.48550/arxiv.2211.08621.
Measurement and tuning of moire potential:
an international group lead by researchers at MIT in Cambridge, MA has realized
pressure tuning of minibands in MoS2/WSe2 heterostructures
probing it by moire phonons. They applied extreme pressure to a moire system
while shining light through it, analyzing the effects with Raman spectroscopy.
They used high pressure to tune the minibands in a rotationally aligned
MoS2/WSe2 moire heterostructure, and showed that their
evolution can be probed via moire phonons. The latter are Raman-inactive phonons
from the individual layers that are activated by the moire potential. Moire
phonons manifest themselves as satellite Raman peaks arising exclusively from
the heterostructure region, increasing in intensity and frequency under applied
pressure. Theoretical analysis reveals that their scattering rate is directly
connected to the moire potential strength. By comparing the experimental and
calculated pressure-induced enhancement, they obtained numerical estimates for
the moire potential amplitude and its pressure dependence. The researchers
compressed (~5 GPa) the two sheets of a transition metal dichalcogenide between
two diamond tips in a chamber with size ~ 100 µm. The technique also allowed
the team to measure and tune the moire potential. Hydrostatic pressure allowed
to continuously and reversibly enhance the moire potential. By studying the
optical properties of a semiconducting moire bilayer under pressure, the team
probed and manipulated the effects of the moire superlattice. By measuring the
difference between the energies of photons coming in and out of the sample, they
probed the energy of vibrations created in the material indicating how strongly
the electrons in one sheet communicate with the electrons in the other. The
moire potential indicates the strength of that coupling between the 2D layers.
By comparing the experimental enhancement of the intensity of the out-going
light associated with these vibrations, versus the calculations of their
theoretical model, the researchers obtained the strength of the moire potential
and its evolution with pressure. The results here show how moire phonons are a
sensitive probe of the moire potential and the electronic structure of moire
systems.
For more information:
Phys.org, June 15 (2023); Nat. Nanot., June 15 (2023).
WEEK OF JUNE 12, 2023 [No. 1527]
Bosonic correlated insulator discovered:
an international group lead by researchers at UCSB in Santa Barbara, CA has made
a correlated insulator of excitons in a WSe2/WS2 moire
superlattice and, thus, they have discovered a bosonic correlated insulator, a
quantum electronic phase consisting of highly ordered crystal interlayer
excitons in a WSe2/WS2 moire bilayer. They develop a CW
pump probe spectroscopy method that they use to observe an exciton
incompressible state at exciton filling νex = 1 and charge
neutrality, indicating a correlated insulator of excitons. With varying charge
density, the bosonic correlated insulator continuously transitions into an
electron correlated insulator at charge filling ν = 1, suggesting a
mixed correlated insulating state between the two limits. The exciton and
electron density are independently controlled by pump light and electrostatic
gate with measurements performed at base temperature ~ 1.65 K. Moire multilayers
of graphene and transition metal dichalcogenides have been found to host a
number of exotic electronic phases. In addition to charge carriers (electrons
and holes) these 2D structures support their pairs (excitons), which are also
expected to form correlated phases. The researchers here observed a correlated
insulator of interlayer excitons in a bilayer consisting of WSe2,
which hosted holes, and WS2, which hosted electrons. They measured
the optical response of the material to excitation by light and found signatures
of an incompressible state of excitons. To create and identify excitons in their
system, the researchers layered the two WSe2/WS2 lattices
and shone strong light on them in doing pump-probe spectroscopy. The combination
of particles from each of the lattices and the light, created a favorable
environment for the formation of and interactions between the excitons while
allowing the researchers to probe these particles' behaviors. When these
excitons reached a certain density, they could not move anymore. Strong
interactions and the collective behavior of these particles forced them at a
certain density into a crystalline state, and their immobility created an
insulating effect. The researchers analyzed the correlation that drove the
bosons into a highly ordered state. Generally, a loose collection of bosons
under ultracold temperatures will form a condensate, but in this system, with
both light and increased density and interaction at relatively higher
temperatures, they organized themselves into a symmetric solid and
charge-neutral insulator.
For more information:
Phys.org, June 7 (2023); Science, May 11 (2023) page 860.
High-resolution bolometric β spectral measurements on 115In:
an international group lead by researchers at MIT in Cambridge, MA has measured
the β decay spectrum of 115In with high resolution using high-quality
low-background cryogenic bolometric LiInSe2 crystal detectors. The
researchers present measurements performed on a LiInSe2 bolometer in
a source=detector configuration to measure the spectral shape of the fourfold
forbidden β decay of 115In. The value of the axial vector coupling
constant (gA/gV) is determined by comparing the spectral shape of theoretical
predictions to the experimental β spectrum taking into account various simulated
background components as well as a variety of detector effects. They find
evidence of quenching of gA/gV at >5σ with a model-dependent quenching factor
of 0.655±0.002 as compared to the free-nucleon value for the interacting
shell model. They measured the 115In half-life to be
[5.18±0.06(stat)+0.005−0.015(sys)]×1014  yr within the
interacting shell model framework. Using background simulation/subtraction
techniques pioneered in other ton-scale nuclear decay experiments, researchers
have extracted the energy spectrum of outgoing electrons from decays of
115In that occurred inside a LiInSe2 crystal. The physics
processes that determine the decay rate of mid-sized nuclei are difficult to
probe due to the large number of intermediate nuclear energy states. The
researchers collected data at ~0 K to detect and record the very smallest spikes
in temperature due to particle interactions (such as those from 115In
β decays), via highly sensitive thermometers. The study rejected background
events such as external gamma rays using a combination of particle simulations
and examination of individual recorded decays. The result was a clean
115In decay spectrum of the emitted electrons. The researchers
compared this spectrum to a library of predicted spectra and found the predicted
spectrum that most closely matched the collected data here. This study obtained
the most precise measurement to date of the 115In decay rate and
showed the viability of obtaining clean electron energy spectra from long-lived
nuclei using low temperature crystal detectors.
For more information:
Phys.org, June 6 (2023); Phys. Rev. Lett., December 2 (2022) page 232502.
WEEK OF JUNE 5, 2023 [No. 1526]
Excitation and probing of nuclear states using lasers:
an international group lead by researchers at Princeton University in Princeton,
NJ and Johannes Gutenberg-Universität Mainz in Mainz has studied the excitation
and probing of low-energy nuclear states at high-energy storage rings . They
have explained how to excite the first nuclear transition in 229Th
using lasers in the VIS wavelength range, and how to produce 229mTh
isomers through resonant excitation and excitation via the second nuclear
excited state (this requires a large Lorentz factor of the ion bunch). The
researchers propose electric dipole transitions changing both the electronic and
nuclear states that are opened by the nuclear hyperfine mixing. They suggest
using them for efficient isomer excitation in Li-like 229Th ions, via
stimulated Raman adiabatic passage or single-laser excitation. They also propose
schemes for probing the isomers, utilizing nuclear radiative decay or laser
spectroscopy on electronic transitions, through which the isomeric-state energy
can be determined with an orders-of-magnitude higher precision than the current
value. With relativistic 229Th ions in storage rings, high-power
lasers with wavelengths in the VIS range or longer can be used to achieve high
excitation rates of 229Th isomers. This can be realized through
direct resonant excitation or excitation via an intermediate nuclear or
electronic state, facilitated by the tunability of both the laser-beam and
ion-bunch parameters. The researchers considered 229Th ions that have
3 electrons left in their shell (out of 90 in a neutral atom) circulating in a
relativistic storage ring. Highly charged 229Th ions have
nuclear-state mixing with the significantly reduced isomeric-state lifetime
corresponding to a much higher excitation rate by direct resonant excitation.
The metastable isomeric state 229mTh has by far the lowest excited
energy level of all the 3,800 known nuclei. The idea is to accelerate these ions
to ~ c in a particle accelerator ( so that due to relativistic effects, they
perceive a laser beam directed at them from the front as a beam with a much
shorter wavelength, with VIS laser light appearing like UV laser light), in
order to excite them as effectively as possible with a VIS laser and thus be
able to study them very precisely. Thus, multiple excited states can be
addressed and used to populate the isomeric state that is of interest. Most
previous studies of 229mTh have dealt with non-relativistic atoms or
ions in low charge states so that a DUV laser is needed. The researchers
describe the generation of an accelerated beam of highly charged
229Th ions and then they discuss in detail various scenarios for
obtaining the most complete possible excitation of 229Th nuclei, the
detection of the excited states produced , and the transferability to similar
systems. The researchers estimate that the energy of the isomeric state can be
measured with a precision 10-4 to 10-6.
For more information:
Phys.org, June 1 (2023); Phys. Rev. Res., May 30 (2023) page 023134.
SX-STM spectroscopy of a single atom:
researchers at ANL in Lemont, IL have shown how to use XR to characterize the
elemental and chemical state of one atom. Using a specialized tip as a detector,
they detect XR-excited currents generated from an Fe and a Tb atom coordinated
to organic ligands. The fingerprints of a single atom, the L2,3 and
M4,5 absorption edge signals for Fe and Tb, respectively, are
observed in the XR absorption spectra. The chemical states of these atoms are
characterized by means of near-edge XR absorption signals, in which XR-excited
resonance tunneling (X-ERT) is dominant for the Fe atom. The XR signal can be
sensed only when the tip is located directly above the atom in extreme
proximity, which confirms atomically localized detection in the tunneling
regime. Until now, the smallest sample size that could be XR analyzed was 1 ag
(~ 104 atoms); the XR signal produced by a single atom is extremely weak and
conventional detectors are not sensitive enough to detect it. The researchers
here added a sharp metallic tip to a conventional XR detector to detect
XR-excited electrons in samples containing Fe or Tb atoms. The tip is placed 1
nm above the sample to collect XR-excited electrons that are core-level
electrons (fingerprints unique to each element). The researchers used a
purpose-built synchrotron XR instrument at the XTIP beamline of the Advanced
Photon Source. SX-STM (synchrotron XR scanning tunneling microscopy) combines
the ultrahigh-spatial resolution of STM with the chemical sensitivity provided
by XR illumination. As the sharp tip is moved across the surface of a sample,
electrons tunnel through the space between the tip and the sample, creating a
current. The tip detects this current and the microscope transforms it into an
image that provides information on the atom under the tip. The elemental type,
chemical state and even magnetic signatures are encoded in the same signal. XR
spectroscopy in SX-STM is triggered by photoabsorption of core level electrons,
which constitutes elemental fingerprints and is effective in directly
identifying the elemental type of material. Although atoms can be routinely
imaged with SPM, but XR are needed to find what they are made of and their
chemical state. The researchers have developed XR excited resonance tunneling
(X-ERT) that allows to detect how orbitals of a single molecule orient on a
material surface using synchrotron XR. When XR illuminate onto an Fe atom, core
level electrons are excited which then tunnel to the detector tip via
overlapping atomic/molecular orbitals, and thus provide information on the Fe
atom and its chemical state. By comparing the chemical states of an Fe atom and
a Tb atom inside respective molecular hosts, they find that the Tb atom, is
rather isolated and does not change its chemical state while the Fe atom
strongly interacts with its surrounding. fields.
For more information:
Nature, May 31 (2023) page 69; Phys.org, May 31 (2023); Physicsworld, July 3
(2023).
WEEK OF MAY 29, 2023 [No. 1525]
Density wave created in an atomic gas by far photon interaction:
researchers at EPFL in Lausanne and Universität Innsbruck, Innsbruck, have
originated density-wave (DW) ordering in a unitary Fermi gas made of a thin gas
of Li atoms cooled to cryogenic temperatures in an optical cavity with
photon-mediated far interactions. They use a Fermi gas with both strong, tunable
contact interactions and photon-mediated, spatially structured long-range
interactions in a transversely driven high-finesse optical cavity. By loading
atoms inside a high-finesse cavity and driving them with a transverse pump beam
in the far-detuned, dispersive regime, an effective interaction between the
atoms is produced. A photon-mediated density–density interaction leads to the
self-organization into a DW phase. In weakly interacting BECs, the DW
self-ordering is a manifestation of the Dicke superradiant phase transition. The
researchers here realize a doubly tunable Fermi gas combining simultaneously and
independently the control over contact and photon-mediated long-range
interactions. They explore the regime where both interactions are strong, the
latter leading to DW ordering. Above a critical long-range interaction strength,
DW order is stabilized in the system, which they identify via its superradiant
light-scattering properties. They quantitatively measure the variation of the
onset of DW order as the contact interaction is varied across the BCS superfluid
and BEC crossover, in qualitative agreement with a mean-field theory. The atomic
DW susceptibility varies over an order of magnitude upon tuning the strength and
the sign of the long-range interactions below the self-ordering threshold,
demonstrating independent and simultaneous control over the contact and
long-range interactions. The researchers used the cavity to cause the particles
in the Fermi gas to interact at long distance: a first atom would emit a photon
that bounces onto the mirrors, which is then reabsorbed by second atom of the
gas, regardless how far it is from the first. The cavity has a combination of
atoms colliding directly with each other in the Fermi gas, while simultaneously
exchanging photons over long distance. When enough photons are emitted and
reabsorbed (tunable in the experimental set up here) the atoms collectively
organize into a density wave pattern. For fermionic particles, the Pauli
principle restricts the effects of interactions to the Fermi surface; thus, the
resonant s-wave contact interactions yield Cooper pairing at low temperatures.
By contrast, the photon-mediated interaction couples particle-hole excitations
on the Fermi surface at discrete wave vectors k± =
kc ± kp, imposed by the pump-cavity geometry. In the 3D
system here, the low-energy physics is associated with scattering processes with
the wave vector k−, which is smaller than the Fermi wave vector kF, leading to a
broad particle-hole spectrum. This is described by the Lindhard function for
free fermions, which is maximum at zero frequency for low momenta close to k−.
This contrasts with large momenta, where the Pauli principle does not restrict
the available phase space unless the Fermi surface is deformed. The researchers
find that even in the presence of strong contact interactions, photon-mediated
interactions modify the zero-frequency particle-hole susceptibility and lead to
the spontaneous formation of a DW pattern above a critical strength in the
attractive case. They operate with atoms in the deeply degenerate regime with
temperatures on the order of T = 0.08 TFn, with
TFn the Fermi temperature calculated for a harmonic trap,
where for all interaction strengths, the system is superfluid in the absence of
the photon-mediated interactions. For a wide range of the short-range
interaction strength, the system enters the DW-ordered phase upon increasing the
photon-mediated interaction strength and returns to the superfluid phase when
the long-range interaction is ramped back to zero, with limited heating. This
leaves open the question of whether the system remains paired and superfluid in
the presence of strong long-range interactions and in the DW-ordered state.
Compared with condensed-matter systems showing an interplay of charge DW and
superfluidity, the system here has a fully controllable microscopic Hamiltonian.
The photon-induced DW order shares similarities with type II charge-DW
compounds, with cavity photons playing the role of phonons in real materials. In
this context, the real-time weakly destructive measurement channel through the
cavity field opens the possibility of gaining insight into the interplay of
structural effects and strong interactions in complex quantum materials.
For more information:
Nature, May 24 (2023) page 716; Phys.org, May 24 (2023).
Cross section of accreting neutron star 34Ar(α,p) 37K
reaction measured:
an international group lead by ORNL scientists has used the JENSA very pure
high
density He gas jet target system (focus spot ~ 2 mm) with the high-energy-angle
resolution SECAR charge particle detector at FBIR in Michigan State University,
to produce a signature nuclear reaction powering thermonuclear explosion
processes that propagate across the surface of a neutron star in a binary
system. The researchers here present a direct measurement constraining the
34Ar(α,p)37K reaction cross section, using the JENSA
system. The combined cross section for the
34Ar,Cl(α,p)37K,Ar reaction is found to agree well with
Hauser-Feshbach predictions. The 34Ar(α,2p) 36K,Ar cross
section, which can be exclusively attributed to the 34Ar beam
component, also agrees to within the typical uncertainties quoted for
statistical models. This indicates the applicability of the statistical model
for predicting astrophysical (α,p) reaction rates in this part of the αp
process, in contrast to earlier findings from indirect reaction studies
indicating orders-of-magnitude discrepancies; and removes uncertainty in models
of H and He burning on accreting neutron stars. The αp-process is a sequence of
(α, p)(p, γ) reactions important to the nuclear trajectory to higher masses in
type I XR bursts. The αp-process is schematically pure He-burning, and thus
unlike pure H-burning processes, does not require slow β+ decays. Explosive He
burning is responsible for the observed short rise-times of XR bursts but
ultimately gives way to the rp-process as the Coulomb barrier increases. The
stellar reaction rates of (α, p) reactions are not well known over the relevant
astrophysical energies. The rate of the final step in the astrophysical αp
process, the 34Ar(α,p) 37K reaction, has large
uncertainties due to a lack of experimental data until now; this reaction rate
is important on the observable light curves of XR bursts and the composition of
the ashes of H and He burning on accreting neutron stars. The scientists here
struck a target of α particles with a beam of 34Ar producing excited
38Ca that ejected p and ended up as 37K nuclei. Accounting for the conservation
of energy and momentum, the researchers back-calculated to analyze the dynamics
of the reaction calculating reactions rates, elements formed, and their energy
levels. The researchers analyzed the reaction measured with JENSA using the
statistical theoretical model Hauser-Feshbach formalism, which assumes that a
continuum of excited energy levels of a nucleus can participate in a reaction.
so effects over each individual level are averaged out. For nuclei like
22Mg and 34Ar, there is an expectation that the nucleus
does not have enough levels for this averaging approach to be valid and the
researchers wanted to test that. Previous work explored the 22Mg
nucleus, and found the model incorrect by almost a factor of 10. The results
obtained here have shown that the statistical model is valid for this
particular
reaction and that the model correctly predicted reaction rates. Somewhere
between A = 20 and A = 30, the used statistical model accuracy breaks down.
For more information:
Phys.org, May 23 (2023); Phys. Rev. Lett., May 22 (2023) page 212701.
WEEK OF MAY 22, 2023 [No. 1524]
Electronic-lattice interaction observed in HTS:
researchers at the BNL in Upton, NY have used a spectroscopic imaging scanning
tunneling microscope (SI-STM) to study the charge density wave (CDW)associated
to atomic distortions in a copper-oxide superconductor. The researchers
introduce a STM-based technique to visualize the local bond-length variations
obtained from topographic imaging with pm precision. Application of this
technique to the HTS Bi2Sr2CaCu2O8+x
reveals a high-fidelity local lattice distortion of the BiO lattice as large as
2%. In addition, analysis of local breaking of rotational symmetry associated
with the bond lengths reveals modulations around four-unit-cell periodicity in
both B1 and E representations in the C4v group of the lattice, which coincides
with the unidirectional d-symmetry CDW previously identified within the
CuO2 planes, thus providing direct evidence of electron-lattice
coupling in the pseudogap state and a link between the XR scattering and STM
measurements. The researchers have observed periodic atomic displacements and
visualized the electron-lattice interaction in the material. Traditionally, XR
scattering techniques have been used to detect the breaking of the structural
symmetry of the lattice, which accompanies a periodic displacement of the atoms
associated with CDW formation in the cuprate pseudogap states. Similarly, SI-STM
has visualized the short-range CDW. However, local coupling of electrons to the
lattice in the form of a short-range CDW and, thus, a link between these
measurements has been missing. Normally, the atoms in the crystal lattice can
vibrate side-to-side but when cooled to the point where the ladder-like CDW
appears, the atomic positions shift along the rungs and the vibrations cease,
locking the atoms in place. These charge-ordered states are part of the
interactions that trigger superconductivity at lower temperatures. This is
observed in a phase that coexists with the superconducting phase of the sample.
The anomaly was a mysterious disappearance of vibrational energy from the atoms
that make up the material's crystal lattice. XR show that the atoms vibrate in
particular ways. But as the material is cooled, the XR studies showed, one mode
of the vibrations stops. By scanning the surface of the layered material with pm
precision, the researchers could map the atoms and measure the distances
between them while simultaneously measuring the electric charge at each
atomic-scale location. The measurements were sensitive enough to pick up the
average positions of the atoms when they were vibrating and showed how those
positions shifted and became locked in place when the vibrations stopped. They
also showed that the anomalous vibrational disappearance was directly linked to
the emergence of a CDW. The electrons that make up the CDW are localized and
separate from the more mobile electrons that eventually carry the current in the
superconducting phase, The localized electrons form a repeating pattern of
higher and lower densities that can be visualized as ladders laying
side-by-side. It is the appearance of this pattern that distorts the normal
vibrations of the atoms and shifts their positions along the direction of the
rungs. As the temperature goes down and the CDW emerges, the vibrational energy
goes down. By measuring both charge distribution and atomic structure
simultaneously, one can see how the emergence of the CDW locks the atoms in
place. This implies that, as the atoms vibrate, the charge density wave
interacts with the lattice and quenches the lattice. It stops the vibrations and
distorts the lattice. The results here suggest that the periodic lattice
displacements in E representations correspond to a locally frozen version of the
soft phonons identified by the XR scattering measurements, and a fluctuation of
the bond length is reflected by the fluctuation of the d-symmetry CDW formation
near the quantum critical point.
For more information:
Phys.org, May 17 (2023); Phys. Rev. X, May 17 (2023) page 021025
Spontaneous topological Hall effect driven by antiferromagnetic ordering:
a group lead by researchers from the University of Tokyo has observed
spontaneous topological Hall effect induced by non-coplanar antiferromagnetic
order in intercalated van der Waals materials, The researchers carried out
experiments to test theoretical studies suggesting that a non- coplanar
antiferromagnetic order with finite scalar spin chirality (a solid angle spanned
by neighboring spins) can induce a large spontaneous topological Hall effect,
without net magnetization or external magnetic field. They used the van der
Waals materials, CoTa3S6 and
CoNb3S6, both with a triangular lattice and a small net
magnetization. These compounds are reported to host unconventionally large
spontaneous Hall effects despite their vanishingly small net magnetization. The
analysis here reveals that that can be explained in terms of the topological
Hall effect that originates from the fictitious magnetic field associated with
scalar spin chirality. Hall effect in ferromagnets (electric currents induce a
transverse voltage proportional to the internal magnetization.), is not
predicted to spontaneously occur in antiferromagnets, materials in which
adjacent ions behave like magnets, aligning in regular patterns with neighboring
spins facing opposite directions. Recent theoretically studies, suggest that a
non- coplanar antiferromagnetic spin order could induce a large spontaneous Hall
effect in materials, without applying an external magnetic field, because
conduction electrons feel a fictitious magnetic field in proportion to a solid
angle spanned by neighboring spins. The researchers' findings, confirmed that a
specific non-coplanar antiferromagnetic order characterized by scalar spin
chirality could elicit a large and spontaneous topological Hall effect in these
compounds. Since a fictitious magnetic field plays a similar role as
magnetization, non-coplanar antiferromagnets are expected to host similar
functional responses as ferromagnets. In such systems, the fictitious magnetic
field is predicted to cause topological Hall effect and other phenomena like
topological magneto-optical effect and topological Nernst effect, whose
amplitude can be comparable to those in ferromagnets.
For more information:
Phys.org, May 17 (2023); Nat. Phys., April 20 (2023) page 961.
WEEK OF MAY 15, 2023 [No. 1523]
MATBG spin structure probed:
an international group lead by researchers at Brown University in Providence, RI
has measured electron spin in MATBG showing direct interaction between
electrons spinning in a 2D material and photons coming from microwave radiation.
The researchers here observe low-energy collective excitations in twisted
bilayer graphene near the magic angle, using a resistively detected electron
spin resonance (ESR) technique. Two independent observations show that the
generation and detection of microwave resonance relies on the strong
correlations within the flat moire energy band. The onset of the resonance
response coincides with the spontaneous flavor polarization at moire
half-filling, but is absent in the isospin unpolarized density range. The
researchers perform the same measurement on various systems that do not have
flat bands and observe no indication of a resonance response in these samples.
They conclude that the resonance response near the magic angle originates from
Dirac revivals and the resulting isospin order. Collective excitations contain
key information regarding the electronic order of the ground state of strongly
correlated systems. Various collective modes in the spin and valley isospin
channels of magic-angle graphene moire bands have been alluded to by a series of
recent experiments. However, a direct observation of collective excitations has
been impossible due to the lack of a spin probe. The absorption of microwave
photons by electrons establishes a technique for directly studying the
properties of how electrons spin in 2D quantum materials. Conventionally, NMR is
used to measure the spin of electrons, by exciting the nuclear magnetic
properties in a sample material using microwave radiation and then reading the
different signatures this radiation causes to measure spin. In 2D materials the
magnetic signature of electrons in response to the microwave excitation is too
small to detect. The research here, instead of directly detecting the
magnetization of the electrons, measured subtle changes in electronic
resistance, which were caused by the changes in magnetization from the
radiation. These small variations in the flow of the electronic currents allowed
the researchers to detect that the electrons were absorbing the photons from the
microwave radiation. The researchers observed that interactions between the
photons and electrons made electrons in certain sections of the system behave as
they would in an anti-ferromagnetic system (the magnetism of some atoms was
canceled out by a set of magnetic atoms that are aligned in a reverse
direction).
For more information:
Phys.org, May 11 (2023); Nat. Phys., May 11 (2023).
Double quantum dots in bilayer graphene:
researchers at Aachen University in Aachen have used a bilayer graphene to
create a nearly perfectly electron-hole symmetric double quantum dot, with the
electron and the hole being in different layers. The double quantum dot is made
of two opposing quantum dots, each housing an electron and a hole whose spin
properties mirror each other almost perfectly. This potential semiconductor spin
qubit structure allows for a robust read-out mechanism. The researchers here
have shown that particle–hole symmetry protects spin-valley blockade in graphene
quantum dots. They show that bilayer graphene allows the realization of
electron–hole double quantum dots that exhibit near-perfect particle–hole
symmetry, in which transport occurs via the creation and annihilation of single
electron–hole pairs with opposite quantum numbers. They show that particle–hole
symmetric spin and valley textures lead to a protected single-particle
spin-valley blockade that will allow robust spin-to-charge and valley-to-charge
conversion, as needed for the operation of spin and valley qubits. In the
low-energy limit, graphene is a prime example of a gapless particle–hole
symmetric system described by an effective Dirac equation in which topological
phases can be understood by studying ways to open a gap by preserving (or
breaking) symmetries. An important example is the intrinsic Kane–Mele spin-orbit
gap of graphene, which leads to a lifting of the spin-valley degeneracy and
renders graphene a topological insulator in a quantum spin Hall phase while
preserving particle–hole symmetry. Bilayer graphene has a bandgap that can be
tuned by an external electric field from 0 to 120 meV. The band gap can be used
to confine charge carriers in quantum dots. Depending on the applied voltage,
these can trap a single electron or a hole. The possibility of using the same
gate structure to trap both electrons and holes is a feature that has no counter
part in conventional semiconductors. The symmetry here is almost perfectly
preserved even when electrons and holes are spatially separated in different
quantum dots. This mechanism can be used to couple qubits to other qubits over a
longer distance. The symmetry results in a very robust blockade mechanism which
could be used to read out the spin state of the dot with high fidelity. This can
not be done in conventional semiconductors or any other 2D electron systems.
For more information:
Nature, May 3 (2023 page 51; Phys.org, May 8 (2023).
WEEK OF MAY 8, 2023 [No. 1522]
Superfluid response measured in 2D superconductor:
researchers at Cornell University in Ithaca, NY have characterized the
superfluid response of a van der Waals superconductor and found that it
significantly deviates from simple BCS-like behavior. They use a local magnetic
probe to directly measure on the tunable, gate induced superconducting state in
MoS2. They report the characterization of the superfluid response of
the atomically thin van der Waals superconductor using a local probe that
provides sufficient sensitivity to the small sample volume typical in this
material family. They find that the backgate changes the transition temperature
non-monotonically whereas the superfluid stiffness at low temperature and the
normal state conductivity monotonically increase. In some devices, they find
direct signatures in agreement with a Berezinskii-Kosterlitz-Thouless (BKT)
transition, whereas in others they find a broadened onset of the superfluid
response. They show that the observed behavior is consistent with disorder
playing an important role in determining the properties of superconducting
MoS2. Due to typically small sample size of these materials, only a
few measurements beyond electronic transport which directly probe the
superconducting state below Tc are available, and no
characterization of the magnetic response has been reported for any atomically
thin vdW superconductor. The researchers here realized a magnetic measurement of
ionic gated MoS2 over a 20µm van der Waals flake of MoS2
on a SiO2/Si substrate patterned into a disk. The researchers
modified the conventional SQUID measurement technique by spin coating it with a
micron-thick layer of ionic liquid and positioning it as close as possible to
the fragile sample without damaging it. The utilized SQUID pickup loop with
concentric field coil was approached to the cryogenically cooled sample. The
current in the field coil producing a magnetic field, which results in an
opposing screening current in the superconductor. The strength of the screening
current is magnetically detected by the pickup loop. The SQUID revealed the
material was expelling the device's magnetic field. The expulsion measurement
provided new information about the electrical transport in van der Waals
materials. How well the superconductor expels magnetic field gives information
about how many electrons are participating. The researchers found that many
electrons were not participating in the superconducting state probably because
there is electronic disorder in these samples. The findings show how 2D
superconductors differ from 3D superconductors. In some devices, the researchers
observed signatures of a BKT phase transition, which is specific to 2D
materials, whereas in others they found an expanded superfluid response. The
researchers report direct measurements of the magnetic response of the
gate-induced superconducting state in few-layer MoS2. Although
MoS2 is a semiconductor when undoped, ionic liquid gating can induce
an electron accumulation layer at the surface of a MoS2 flake which
exhibits superconductivity at carrier densities exceeding 0.5 ×
1014 cm−2 . Tc changes non-monotonically with
the carrier density with a maximum of ~ 10 K. Superconductivity is retained in
the monolayer limit, but is always in the 2D limit regardless of the flake
thickness because the accumulation layer is approximately confined to the
topmost layer. Spin-valley locking in the electronic bandstructure of
MoS2 gives rise to Ising protection of the superconducting state
leading to an in-plane critical field exceeding the Pauli limit. Recently,
tunneling measurements have suggested that the order parameter is not fully
gapped, a possible signature of an unconventional superconducting state. The
researchers find that the superfluid stiffness monotonically increases at low
temperatures as the backgate is tuned, even when the critical temperature
decreases. Their analysis suggests that their devices are in the dirty limit of
superconductivity in which the superfluid stiffness responds to changes in
device resistivity. This demonstrates that disorder plays an important role even
in crystalline 2D superconductors. They observe direct signatures of a BKT
transition in one device, whereas, in another, the universal jump is replaced by
a broad region of suppressed superfluid response close to Tc.
This indicates that a clear BKT transition is not ubiquitous in these systems,
but can be substantially obscured by disorder.
For more information:
Phys.org, May 1 (2023); Nat. Comm., April 12 (2023).
Proton's tensor charge measured:
the JAM Collaboration at the Jefferson Lab in Newport News, VA has made the most
precise empirical determination of the proton's tensor charge. The researchers
report an updated QCD global analysis of single transverse-spin asymmetries, the
extracting of H˜, and the role of the Soffer bound and lattice QCD. The only
way to obtain the tensor charge (net transverse spin of quarks in a proton with
transverse spin) from experimental data is using QCD to extract the transversity
function. This universal function encodes the difference between the number of
quarks with their spin aligned and anti-aligned to the proton's spin when it is
in a transverse direction. The difficulty is to connect the theory of quark
interactions (QCD) to experimental measurements of high-energy collisions
involving hadrons. The researchers here analyzed data from a wide range of
experiments where protons and/or quarks were transversely polarized. The results
were compared to computations of the proton's tensor charge by lattice QCD.
After about a decade of results showing disagreement between empirical methods
and lattice QCD for the proton's tensor charge, the researchers here have found
agreement between the two. A crucial part of the analysis was the utilization of
data from e−- e+, e−- p, and p - p scattering.
Thus, the researchers here present an update to the QCD global analysis of
single transverse-spin asymmetries of 2020. The previous report simultaneously
included transverse momentum dependent and collinear twist-3 observables, both
of which are sensitive to quark-gluon-quark correlations in hadrons. They
extract the twist-3 chiral odd fragmentation function H˜ by incorporating the
sinϕs modulation data from semi-inclusive deep-inelastic scattering along with
its contribution to the single transverse-spin asymmetry in π production from p
- p collisions. They also explore the impact of lattice QCD tensor charge
calculations and the Soffer bound on their global analysis. They find that both
constraints can be accommodated within their results, with H˜ playing a key role
in maintaining agreement with the data from p - p collisions.
For more information:
Phys.org, May 1 (2023); Phys. Rev. D, August 12 (2022) page 034014.
WEEK OF MAY 1, 2023 [No. 1521]
Collectively induced transparency observed:
researchers at Caltech in Pasadena, CA have observed the phenomenon of
collectively induced transparency (CIT) that causes groups of atoms to abruptly
stop reflecting light at specific frequencies. They studied how a large,
inhomogeneously broadened ensemble of solid-state emitters coupled with high
cooperativity to a nanophotonic resonator behaves under strong excitation. The
researchers discovered a sharp CIT in the cavity reflection spectrum, resulting
from quantum interference and collective response induced by the interplay
between driven inhomogeneous emitters and cavity photons. Coherent excitation
within the CIT window leads to highly nonlinear optical emission, spanning from
fast superradiance to slow subradiance. CIT was discovered by confining Yb
atoms inside an optical cavity (an ensemble of atoms strongly coupled to the
same optical field ) and blasting them with a laser. The optical resonator
measures 20 µm in length and includes features < 1 µm. Although the laser's
light will bounce off the atoms up to a point, as the frequency of the light is
adjusted, a transparency window appears in which the light simply passes through
the cavity unimpeded. An analysis of the transparency window points to it being
the result of interactions in the cavity between groups of atoms and light. This
phenomenon is akin to destructive interference, in which waves from two or more
sources can cancel one another out. The groups of atoms continually absorb and
re-emit light, which generally results in the reflection of the laser's light.
However, at the CIT frequency, there is a balance created by the re-emitted
light from each of the atoms in a group, resulting in a drop in reflection. The
researchers used conventional quantum optics measurement techniques. They
observed that the collection of atoms can absorb and emit light from the laser
either much faster or much slower compared to a single atom depending on the
intensity of the laser. These processes (superradiance and subradiance), are not
understood because of the large number of interacting quantum particles.
For more information:
Nature, April 26 (2023) page 271; Phys.org, April 27 (2023).
Photon bound states observed:
an international group at the University of Basel in Basel has observed the
photon bound state dynamics from a single artificial atom. The researchers here
report the direct observation of a photon-number-dependent time delay in the
scattering off a single artificial atom (a semiconductor quantum dot coupled to
an one-sided optical cavity). By scattering a weak coherent pulse off the
cavity–QED system and measuring the time-dependent output power and correlation
functions, they show that single photons and two- and three-photon bound states
incur different time delays, becoming shorter for higher photon numbers. This
reduced time delay is a fingerprint of stimulated emission, where the arrival of
two photons within the lifetime of an emitter causes one photon to stimulate the
emission of another. In the limit where the optical nonlinearity is expressive
on the scale of a few photons, one can observe quantum nonlinear phenomena. One
manifestation of the nonlinearity is the presence of two- and higher-order
photon bound states. Photons in these bound states are strongly correlated, such
that the likelihood of observing a photon at any one time is fixed, but once one
photon is detected, the arrival of another is much more likely than at a random
time. Photon bound states are distinct from bunched photon states, as photon
bound states are quasiparticles that have their own dispersion relation and are
eigenstates of the underlying Hamiltonian that describes the nonlinear medium.
It has been predicted theoretically that the photon-number-dependent propagation
velocity of photon bound states can lead to the formation of highly entangled,
ordered states of light. Photon bound states have been predicted to exist in a
number of systems, such as unidirectional waveguide QED and strongly correlated
Rydberg gases. In the latter case, experimental observations consistent with
their presence have been reported. To observe directly the dynamics of photon
bound states, the researchers here examine the unidirectional propagation of
few-photon wavepackets strongly interacting with a single atom. The nonlinearity
provided by the atom in the interaction between photons and a single two-level
atom leads to a strong dependence of the light–matter interface on the number of
photons interacting with the two-level system within its emission lifetime. This
nonlinearity unveils strongly correlated quasiparticles (photon bound states),
giving rise to processes such as stimulated emission and soliton propagation.
Although signatures consistent with the existence of photon bound states have
been measured in strongly interacting Rydberg gases, their hallmark
excitation-number-dependent dispersion and propagation velocity had not yet been
observed. To induce strong photon interactions the researchers here guided
pulses of very dim laser light (low number of photons) via a circulator into a
quantum-dot cavity system. Weak coherent gaussian pulses of light were guided
via a circulator to the one-sided quantum-dot–cavity system. The light is
back-scattered and redirected by the circulator towards a Hanbury Brown–Twiss
(HBT) set-up equipped with single-photon detectors that record the time of
arrival of individual photons. The pulses can be approximated as having zero,
one or two photons in them but the probability of having one photon is much
larger than two photons. When they measure the intensity of the pulse packet,
that measurement is dominated by the one-photon part of the pulse because the
two-photon part is much smaller in magnitude. They overcome this problem by
measuring the second-order correlation function of light, which allows them to
measure the likelihood of two photons arriving within a very short time
difference at the detectors. The measurement technique is insensitive to single
photons so it only records the two-photon part of the pulse. By comparing these
two measurements, the researchers observed that the two-photon state was delayed
less than one-photon state. They were able to see the difference between one
photon interacting with their system compared with two because the device that
they built induces such strong interactions between the photons. With this very
strong photon–photon interaction, the two photons form the two-photon bound
state. To measure single photons, they measure its time of arrival at just one
of the detectors in their setup. To measure the correlation between two photons,
they measure the time of arrival of two photons at two different detectors. If
there is only one photon, only one of the two detectors clicks, and the other
one not, so the correlation between the two detectors is null. This is why this
measurement is insensitive to single-photons: they make use of two detectors.
Being able to see one photon and two photons interacting differently with a
quantum dot (which behaves like a single two-level artificial atom) basically
means they are doing nonlinear optics with just two photons.
For more information:
Physicsworld, April 25 (2023); Nat. Phys., March 20 (2023) page 857.
WEEK OF APRIL 24, 2023 [No. 1520]
Observation of Λ hyperon production by semi-inclusive deep inelastic scattering:
CLAS Collaboration researchers at the Jefferson Lab in Newport News, VA have
used the Continuous Electron Beam Accelerator Facility (CEBAF) to make
observations of how lambda particles are produced by semi-inclusive deep
inelastic scattering (SIDIS).
The researchers studied how these particles of strange matter (with a strange
quark) form from collisions of ordinary matter. It is difficult to use this
method to study Λ particles, because the particle decays quickly so they can not
be measured directly. This class of measurement had only been performed on
protons before, and on lighter, more stable particles. The experiment that
collected this data, EG2, used the CEBAF Large Acceptance Spectrometer (CLAS)
detector in Jefferson Lab's Experimental Hall B. The dataset was originally
collected in 2004. These data hint that quarks and gluons, are capable of
marching through the atomic nucleus in pairs (diquarks), at least part of the
time. The researchers shot CEBAF's electron beam at different targets, including
C, Fe, and Pb targets. When a high-energy electron from CEBAF reaches one of
these targets, it breaks apart a proton or neutron inside one of the target's
nuclei. Because the proton or neutron is totally broken apart, there is little
doubt that the electron interacts with the quark inside. After the electron
interacts with a quark or quarks via an exchanged virtual photon, the struck
quark(s) begins moving as a free particle in the medium, typically joining up
with other quark(s) it encounters to form a new composite particle as they
propagate through the nucleus. And some of the time, this composite particle
will be a Λ which is short-lived (it decays into a pion and either a proton or
neutron). In the experiment, they tracked what happened when electrons from
CEBAF scatter off the target nucleus and probed the confined quarks inside
protons and neutrons making observations for the Λ baryon in the forward and
backward fragmentation regions. A finding from this analysis changes the way to
understand how Λ form in the wake of particle collisions. In similar studies
that have used SIDIS to study other particles, the particles of interest usually
form after a single quark is struck by the virtual photon exchanged between the
electron beam and the target nucleus. But the signal left by Λ in the CLAS
detector suggests a more complex process. The authors' analysis showed that when
forming a Λ, the virtual photon has been absorbed part of the time by a pair of
quarks (diquark not predicted by present QCD) instead of just one quark. After
being struck, this diquark went on to find a strange quark and to form a Λ. Data
for EG2 were collected with 5.014 GeV electron beams from the CEBAF's 6 GeV
accelerator. The results here represent the first measurements of the Λ
multiplicity ratio and transverse momentum broadening as a function of the
energy fraction (z) in the current and target fragmentation regions. The
multiplicity ratio exhibits a strong suppression at high z and an enhancement at
low z. The measured transverse momentum broadening is an order of magnitude
greater than that seen for light mesons. This indicates that the propagating
entity interacts very strongly with the nuclear medium, which suggests that
propagation of diquark configurations in the nuclear medium takes place at least
part of the time, even at high z. The trends of these results are qualitatively
described by the Giessen Boltzmann-Uehling-Uhlenbeck transport model,
particularly for the multiplicity ratios. Planned future experiments will use
electron beams from the updated CEBAF, which now extend up to 11 GeV for
Experimental Hall B, as well as an updated CLAS detector (CLAS12).
For more information:
Phys.org, April 18 (2023); Phys. Rev. Lett., April 4 (2023) page 142301.
On-chip entangled light source built:
an international group lead by researchers at Leibniz University Hannover,
Hannover has built a chip-integrated quantum light source for the generation of
entangled photons. Until now, the major technical challenge preventing making a
turnkey quantum light system has been the integration of a stable and tunable
laser with a filter (crucial to eliminate the noise that suppresses quantum
phenomena) and a nonlinear parametric source of entangled photons (creating
signal and idler photons through spontaneous parametric effects). The absence of
a unique material platform that suitably provides all quantum photonic
functionalities (low-loss guiding, filtering, efficient parametric generation of
entangled photon pairs and their active manipulation) while at the same time
providing lasing gain is the main factor impeding monolithic integration.
Low-loss indirect or high-bandgap materials with a high refractive index (Si,
SiO2 , Si3N4 , ....) are most commonly
preferred for light guiding, signal processing and efficient spontaneous
parametric effects (and thus efficient entangled photon-pair generation),
whereas direct-bandgap III–V semiconductors (InAs, GaAs, InP, ...) are suitable
for optical gain and lasing. Unfortunately, the fabrication techniques/process
flows for each group of materials differ and are often not compatible with one
another. Thus, the hybrid integration of an excitation laser with a parametric
photon source into a combined photonic circuit, drawing the best advantages of
the different materials while avoiding their deficiencies, is considered the key
step towards achieving a fully integrated on-chip quantum device. In addition to
parametric spontaneous sources, semiconductor quantum dots (QDs), excited either
optically or electrically, can emit single and entangled photons in compact
forms. However, it is difficult to integrate QDs with a fully operational
quantum photonic circuit due to the random growth of QDs on wafers, the presence
of charge noise, the lack of a precise emission wavelength and the need for
cryogenic cooling to ensure an indistinguishable, narrowband (~ MHz) and
deterministic single-photon output. The QDs are generally connected with the
rest of the circuit by optical fibers, which makes the device bulky, or by
wafer-bonding, where loss of the single-photon signal due to misalignment and
refractive-index mismatch at the circuit interface is unavoidable. The emission
from InP quantum wells covers the entire C band, making InP a perfect candidate
for a semiconductor optical amplifier (gain medium) and to realize lasers that
will drive on-chip quantum light sources. However, the intrinsic waveguide
losses in such amplifiers and in InP passive waveguides are high. This imposes a
short photon lifetime on the laser cavity, causing a high spectral linewidth. A
solution to this is to extend the photon lifetime with hybrid integration of
waveguide feedback circuits based on Si3N4 waveguides;
this offers a broad transparency window covering the entire C band, with low
loss. Si3N4 has a high nonlinear refractive index (n2 ≈
2.4 × 10−19 m2 W−1), moderately high mode
confinement, low material dispersion, near-to-zero Raman scattering and, due to
the large bandgap, an absence of two-photon absorption (TPA) in the telecom
wavelength range. This absence of TPA in Si3N4 permits
operation in a high-power regime, effectively reducing the intrinsic laser
linewidth to the sub-kHz range. The researchers here reduced the size of the
light source by x 1,000 by combining an InP laser, a filter and a
Si3N4 cavity on a single chip that can emit
frequency-entangled qubit states with good quality and efficiency. On the chip,
in a spontaneous nonlinear process, two photons are created from a laser field.
Each photon spans a range of colors simultaneously (superposition) and the
colors of both photons are correlated. The researchers demonstrate a fully
integrated quantum light source through the integration of a laser cavity, a
highly efficient tunable noise suppression filter (>55 dB) exploiting the
Vernier effect, and a nonlinear microring for entangled photon-pair generation
through spontaneous four-wave mixing. The hybrid quantum source employs an
electrically pumped InP gain section and a Si3N4 low-loss
microring filter system, and demonstrates high performance parameters: pair
emission over four resonant modes in the telecom band (bandwidth of ~ 1 THz) and
a pair detection rate ~ 620 Hz at a coincidence-to-accidental ratio ~ 80. The
source directly creates high-dimensional frequency-bin entangled quantum states
(qubits/qudits), as verified by quantum interference measurements with
visibilities up to 96% (violating Bell’s inequality) and by density matrix
reconstruction through state tomography, showing fidelities of up to 99%.
For more information:
Phys.org, April 17 (2023); Nat. Phot., April 17 (2023).
WEEK OF APRIL 17, 2023 [No. 1519]
Hybrid Andreev reflection - Klein tunneling electronic transport observed:
an international group lead by researchers at the SUNY Polytechnic Institute in
Albany, NY has discovered a quantum transport mechanism in a HTS - graphene
electronic junction. They have demonstrated that the electronic transport
between graphene and the HTS was dominated by a transport process arising from
the combination of graphene's Klein tunneling and the HTS's Andreev reflection.
The observed transport process is consistent with the theoretical predictions
concerning hybrid Andreev-Klein electronic transport. While Andreev reflection
involving HTS and metals is well understood, doing the same with graphene
electrons and HTS had not been demonstrated before. Scattering processes in
quantum materials emerge as resonances in electronic transport, including
confined modes, Andreev states, and Yu-Shiba-Rusinov states. However, in most
instances, these resonances are driven by a single scattering mechanism. The
junction here shows the appearance of resonances due to the combination of two
simultaneous scattering mechanisms, one from superconductivity and the other
from graphene p−n junctions. These resonances stem from Andreev reflection and
Klein tunneling that occur at two different interfaces of a hole-doped region of
graphene formed at the boundary with superconducting graphene due to proximity
effects from the HTS BSCCO (in this case
Bi2Sr2Ca1Cu2O8+x). The
resonances persist with gating from p+-p and p-n configurations. The suppression
of the oscillation amplitude above the bias energy which is comparable to the
induced superconducting gap indicates the contribution from Andreev reflection.
The experimental measurements are supported by quantum transport calculations in
the interfaces, leading to analogous resonances. The results here demonstrate
the existence of a hybrid scattering mechanism in graphene–HTS heterojunctions
For more information:
Phys.org, April 13 (2023); Phys. Rev. Lett., April 12 (2023) page 156201.
2D spin-polarized electron source - channel demonstrated at RT:
a group lead by researchers at the Chalmers University of Technology in Göteborg
has demonstrated robust room-temperature lateral spin-valve device operations
using van der Waals metallic ferromagnet Fe5GeTe2/graphene
heterostructures with the spin-polarized electrons injected from the 2D magnet
into the graphene channel. The 2D magnet act as a source for spin-polarized
electrons and the graphene channel serves for spin transport and communication.
The RT spintronic properties of Fe5GeTe2 are measured at
the interface with graphene with a negative spin polarization. Lateral
spin-valve and spin-precession measurements have been realized by probing the
Fe5GeTe2/graphene interface spintronic properties via
spin-dynamics measurements, revealing multidirectional spin polarization.
Density functional theory calculations in conjunction with Monte Carlo
simulations reveal significantly tilted Fe magnetic moments in
Fe5GeTe2 along with the presence of negative spin
polarization at the Fe5GeTe2/graphene interface. Todate
operation of active spintronic devices with van der Waals ferromagnets is
limited to cryogenic temperatures. RT magnetism and proximity effects have been
reported using van der Waals magnets without active spintronic device operation.
The researchers here demonstrate robust RT lateral spin-valve device operations
using Fe5GeTe2/graphene heterostructures. Spin transport
and precession experiments show basic building blocks such as efficient spin
injection, transport, detection, and dynamic functionalities. They have
characterized the magnetic properties of Fe5GeTe2 with
multidirectional spin polarization in Fe5GeTe2/graphene
heterostructures. The experimental results are well supported by density
functional theory calculations, where the electronic and magnetic properties of
Fe5GeTe2/graphene heterostructures were investigated
including: magnetic moments, interatomic magnetic exchange interactions, and
magnetic anisotropy energies.
For more information:
Phys.org, April 13 (2023); Ad. Matt., January 15 (2023).
WEEK OF APRIL 10, 2023 [No. 1518]
IR time interference in double slit demonstrated:
an international group lead by researchers at College London has shown it is
possible to achieve the equivalent effect of interference between light waves
sent through and diffracted in a pair of narrow slits using double time slits.
The temporal counterpart of Young’s double-slit experiment is a wave interacting
with a double temporal modulation of an interface. The researchers here
considered a time-domain version: using an IR beam twice gated in time to
produce an interference in the frequency spectrum. The time slits, narrow enough
to produce diffraction at IR frequencies, are generated from the IR excitation
of a thin film of ITO near its epsilon-near-zero point. The separation between
time slits determines the period of oscillations in the frequency spectrum,
whereas the decay of fringe amplitude in frequency reveals the shape of the time
slits. Many more oscillations are visible than expected from existing theory,
implying a rise time that approaches an IR cycle.
The researchers turned the reflectivity of a semiconductor mirror on and off
twice in quick succession and recorded interference fringes along the frequency
spectrum of light bounced off the mirror. When a light wave impinges on a
barrier containing two narrow slits separated in space, its frequency remains
unaltered but its momentum changes as it diffracts outwards. This means that the
distribution of the light’s electric field on the screen is roughly equal to the
Fourier transform of the mathematical function that describes the shape of the
slits in space (space diffraction). Its temporal analogue involves fixed
momentum and changing frequency. A material in which two slits rapidly and
sequentially appear and disappear should cause incoming waves to maintain their
path in space but spread out in frequency (time diffraction). The frequency
spectrum would be the Fourier transform of the function describing the slits in
time, with interference between waves at different frequencies (rather than
different spatial positions) generating the fringes. Showing temporal
interference with any kind of wave requires generating an abrupt and pronounced
change in the medium’s properties across a sufficiently large volume. Until now,
a similar result had been achieved only with water waves. Last month the
researchers here demonstrated it for electromagnetic waves, by propagating
microwave signals through electronic circuits. They have now observed such
fringes by firing sets of three IR laser pulses at a layer of ITO
(In2-xSnxO3) just 40 nm thick that is
sandwiched between glass and Au (they have shown a large and abrupt change over
only a thin surface for now). The two shortest pulses acted as the slits (second
pulse ~200 fs excited electrons in the material affecting its optical
properties), each briefly transforming the layer from a transparent
semiconductor to a reflective metal (reflection in ~ 1 fs was used as it is
easier to carry out than transmission). The researchers positioned a light
sensor along the reflected beam. When they shot two ultrashort pulses separated
by a few 10's fs (turning the ITO mirror on twice in rapid succession) they saw
that the waveform of the twice-reflected light changed in response. It went from
a simple, monochromatic wave to a more complex one. The third pulse acted as the
probe, having its frequency spectrum broadened as it underwent the double
reflection. Measuring the spectrum of the reflected probe pulses, they found
that the pulses’ initial bandwidth was stretched by about a factor of ten. That
spectrum contained a series of peaks that became progressively smaller further
from the pulse’s central carrier frequency and those peaks got further apart the
shorter the delay between the pump pulses. The results are what would be
expected of temporal diffraction. The peaks are the fringes generated by
interference between light at different frequencies. And just as the fringes in
a space double-slit experiment become more spread out in space when the slits
are closer together, so too in this experiment they got further away in
frequency terms when the slits were nearer to one another in time. While the
size of the fringes closely matched theoretical predictions, their staying power
came as a surprise with the peaks further from the central frequency being more
pronounced than expected. This slow decay indicates that the ITO responds more
quickly than expected to the leading edge of the slit pulses (< 10 fs ~
length of one optical cycle of the used IR radiation). The results showed that
the ITO took less than 10 fs to get excited, much faster than expected
theoretically or from previous measurements using a different technique (limited
to 50–100 fs to measure the response time). The interference disappears if the
mirror is turned on only once. This is analogous to what happened in the classic
Young experiment, where the interference patterns vanished if the light was
shone through one slit rather than two.
For more information:
Physicsworld, April 6 (2023); Nat. Phys., April 3 (2023).
Light amplification by a 2D photonic time crystal demonstrated:
researchers at Aalto University have made 2D metasurface photonic time crystals
(2-mm thick time-based versions of optical materials). The photons in the
crystal are synchronized and coherent with a pattern that repeats over time that
can lead to constructive interference and amplification of the light. The
researchers have created photonic time crystals that operate at MW frequencies,
and have shown that the crystals can amplify incident light with high gain. They
demonstrate that time-varying metasurfaces not only preserve key physical
properties of volumetric photonic time crystals (despite their simpler topology)
but also host common momentum bandgaps shared by both surface and free-space
electromagnetic waves. On the basis of a MW metasurface design, they
experimentally confirmed the exponential wave amplification inside a momentum
bandgap and the possibility to probe bandgap physics by external (free-space)
excitations. Photonic time crystals are not natural time crystals. Their
fundamental commonality is that both crystals have structural patterns over
time, with time crystals being quantum materials with atoms suspended in quantum
states, and photonic time crystals being artificial materials not necessarily
suspended in quantum states. The researchers created the photonic time crystal
optical structures that refract light changes periodically by modulating the
electromagnetic property of the metasurface over time. They modified the
electromagnetic properties of the metasurface to create photonic crystals that
are good at amplifying light waves (in photonic time crystals, energy is not
conserved; hence the states residing in the momentum gap can have exponentially
increasing amplitudes).
For more information:
Science Advances, April 5 (2023); Gizmodo, April 6 (2023); Phys.org, April 5
(2023).
WEEK OF APRIL 3, 2023 [No. 1517]
Proton mass radius measured by J/ψ decay in p,γ scattering:
a group lead by researchers at Temple University in Philadelphia, PA has used
the accelerator in the Jefferson Lab in Newport News, VA to determine the
gluonic gravitational form factors of the proton. This measurement may have
finally shed some light on the mass that is generated by the proton's gluons by
pinpointing the location of the matter generated by these gluons The researchers
here investigated the gravitational density of gluons using a small color
dipole, and the threshold photoproduction of the J/ψ particle. . The radial
origin of this core of matter was found to reside at the center of the proton.
After using a variety of models, they determined in all cases, that the proton
mass radius is smaller than the electric charge radius. In some, but not all
cases, depending on the model, the obtained mass radius agrees well with
first-principle predictions from lattice QCD. Little is known about the inner
mass density of the proton, which is dominated by the energy carried by gluons.
Gluons are hard to access using electron scattering because they do not carry an
electromagnetic charge. The experiment here was performed in Experimental Hall C
in Jefferson Lab's Continuous Electron Beam Accelerator Facility. In the
experiment, energetic 10.6 GeV electrons from the CEBAF accelerator were sent
into a small block of Cu. The electrons were slowed down or deflected by the
block, causing them to emit bremsstrahlung radiation. This beam of photons then
struck the protons inside a LH target. Detectors measured the remnants of these
interactions as e+, e- . The experimenters were interested
in those interactions that produced J/Ψ particles (that quickly decay into an
e+,e- pair) amongst the H's proton nuclei. Of the billions
of interactions, the experimenters found ~ 2,000 J/ψ particles in their cross
section measurements of these interactions by confirming the concomitant
e+,e- pairs. Earlier research used electron elastic
scattering experiments on the proton to determine the proton's spin and charge
distribution. Here the researchers only considered the photoproduction of J/Ψ in
photon- proton scattering, thus getting the proton's gluon distribution instead
of its charge distribution. Then they inserted these cross section measurements
into theoretical models that describe the gluonic gravitational form factors of
the proton. The gluonic form factors detail the mechanical characteristics of
the proton, such as its mass and pressure. They used: the generalized parton
distributions model and the holographic QCD model. and compared the results from
each of these models with lattice QCD calculations. The researches determined
the gluonic mass radius, a larger radius of attractive scalar gluons that
extends beyond the moving quarks and confine them. In one of the theoretical
model approaches, the data hints to a scalar gluon cloud distribution that
extends well beyond the electromagnetic proton radius.
For more information:
Nature, March 29 (2023 page 813; Phys.org, March 29 (2023).
Quantum memory created in a trapped-ion quantum network node:
researchers at the University of Oxford have built a robust quantum memory that
stores information in a trapped-ion quantum network by using an objective lens
to collect single photons emitted from a single atomic ion to be entangled with
a 88Sr+ ion trapped inside a vacuum chamber. The researchers
integrate a long-lived memory qubit into a mixed-species trapped-ion quantum
network node. Ion-photon entanglement first generated with a network qubit in
88Sr+ is transferred to 43Ca+ with 0.977(7) fidelity, and
mapped to a robust memory qubit. Then they entangle the network qubit with a
second photon, without affecting the memory qubit. They perform quantum state
tomography to show that the fidelity of ion-photon entanglement decays ~ 70
times slower on the memory qubit. Dynamical decoupling further extends the
storage duration; they measure an ion-photon entanglement fidelity of 0.81(4)
after 10 s. The group had previously implemented a reliable way of interfacing
88Sr+ ions and photons, and used this to generate high-quality remote
entanglement between two distant network nodes. It had also developed
high-fidelity quantum logic and long-lasting memories for 43Ca+ ions.
In the experiment here, they combine these capabilities to show that it is
possible to create high-quality entanglement between a 88Sr+ ion and
a photon and store this entanglement in a nearby 43Ca+ ion. A robust
quantum memory can store information for long periods of time against concurrent
network activity and the quantum information stored in the memory does not
degrade while a network link is established. It requires extreme isolation
between the memory and the network, but at the same time, it needs to be a fast
and reliable mechanism to couple the memory to the network when needed. To
create their quantum memory the researchers here used two different atomic
species to allow them to minimize crosstalk while establishing a network link.
The limited crosstalk in this mixed-species architecture also allowed them to
detect errors in real-time and to utilize in-sequence cooling. Mixed-species
entangling gates provided the missing connection between the network and the
memory. One of the technical error sources that are faced with trapped-ion
qubits is dephasing due to magnetic field noise. 43Ca+ features
transitions that are insensitive to magnetic fields, eliminating this error,
hence boosting their coherence time. 88Sr+ is well suited for
generating photons for networking but it is sensitive to magnetic field noise.
Although 88Sr+ is sensitive to magnetic field noise, the researchers
were able to preserve entanglement between their memory ion and a photon for a
longer time, by transferring quantum information from the 88Sr+ to
43Ca+ in the system. Specifically, they could preserve the
entanglement between a trapped ion and photon for > 10 s (over 1000 times
longer than they observed between a bare 88Sr+ ion and a photon). The
88Sr+ ion can be reused to generate further entangled photons, and
the researchers show that this process does not affect the fidelity of
entanglement between the memory and the previous photon, hence achieving
robustness to network activity in distributed quantum information processing
within scalable quantum computing systems. Using this design, individual quantum
computational nodes can be loaded with a given number of processing qubits
(43Ca+), while the network qubit (88Sr+) can then be used
to create quantum links between distant modules. Using small modules that can
process quantum information and interconnecting them with other modules
circumvents the need for large and complex ion traps. In the case of entangled
atomic clocks, the long entanglement storage durations achieved in the
experiment here can lead to an order-of-magnitude improvement in the precision
of frequency comparison between distant clocks.
For more information:
Phys.org, March 27 (2023); Phys. Rev. Lett., March 3 (2023) page 090803.
WEEK OF MARCH 27, 2023 [No. 1516]
Ferromagnetic topological insulator created by antisite disorder:
an international group lead by researchers at Universität Würzburg and IFW
Dresden has designed the ferromagnetic topological insulator (FM TI)
MnBi6Te10 and discovered intermixing-driven surface and
bulk ferromagnetism in its quantum anomalous Hall antisite disorder. The recent
realizations of the quantum anomalous Hall effect (QAHE) in
MnBi2Te4 and MnBi4Te7 benchmark the
(MnBi2Te4)(Bi2Te3)n
family as a promising hotbed for further QAHE improvements. The family owes its
potential to its ferromagnetically ordered MnBi2Te4
septuple layers (SLs). However, the QAHE realization is complicated in
MnBi2Te4 and MnBi4Te7 due to the
substantial antiferromagnetic (AFM) coupling between the SLs. An FM state,
advantageous for the QAHE, can be stabilized by interlacing the SLs with an
increasing number n of Bi2Te3 quintuple layers (QLs). However, the mechanisms
driving the FM state and the number of necessary QLs are not understood, and the
surface magnetism remains obscure. The researchers here demonstrate robust FM
properties in MnBi6Te10 (n = 2) with Tc ≈
12 K and establish their origin in the Mn/Bi intermixing phenomenon by a
combined experimental and theoretical study. The measurements reveal a
magnetically intact surface with a large magnetic moment, and with FM properties
similar to the bulk. Theory provides a venue toward quantum effects such as the
QAHE by inducing a long-range FM order in topological insulators (TI). Yet, the
QAHE has only been demonstrated in the sub-K range. The experimental realization
of the QAHE is complicated by several simultaneous requirements to a candidate
system: The Dirac point (DP) of the parent TI should be well within its bulk
band gap; the chemical potential has to be tuned to the DP; the introduced
magnetic subsystem should lead to a substantial surface ferromagnetism to open a
large exchange gap at the DP; and the material's bulk should remain insulating.
The first AFM TI, MnBi2Te4, was fabricated in 2019. This
material has its own internal magnetic field, allowing for electronic components
that can store information magnetically and transport it on the surface without
any resistance. Based on the previously discovered
MnBi2Te4, the researchers here have engineered a FM TI. In
FM materials, the individual Mn atoms are magnetically aligned in parallel. By
contrast, in its AFM predecessor, MnBi2Te4, only the
magnetic moments within a single layer of the material are aligned in this way.
The slight change in the crystal's chemical composition makes the FM TI
MnBi6Te10 exhibit a stronger and more robust magnetic
field than its AFM predecessor. The quantum material
MnBi6Te10 becomes FM at 70 K and lossless current
conduction starts at 1.5 K. The material's surface exhibits FM properties,
enabling it to conduct current without any loss, whereas its interior does not
share this characteristic. When the researchers figured out how to produce the
crystalline material, they discovered that some atoms needed to be repositioned
from their original atomic layer. The distribution of Mn atoms across all
crystal layers causes the surrounding Mn atoms to rotate their magnetic moment
in the same direction (like if the magnetic order propagates). This atomic
antisite disorder in the crystal, is usually considered disruptive and ordered
atomic structures are easier to calculate and better understood. However here
this very disorder is the critical mechanism that enables
MnBi6Te10 to become FM.
For more information:
Phys.org, March 21 (2023); Ad. Sci., February 17 (2023).
BEC created in a twisted-bilayer optical lattice:
researchers at Shanxi University in Taiyuan have used lasers to simulate
superconductivity in twisted bilayer lattices. The researchers have established
a twistronics research system with ultracold atoms in highly controllable
optical lattices that can be applied to different lattice geometries in both
boson and fermion systems. They have demonstrated a quantum simulation of a
superfluid to Mott insulator transition in a twisted-bilayer square lattice
based on atomic Bose–Einstein condensates loaded into spin-dependent optical
lattices. The lattices are made of two sets of laser beams that independently
address atoms in different spin states, and form the synthetic dimension
accommodating the experiment's two layers. The interlayer coupling is
controllable by a microwave field, enabling a low flat band and correlated
phases in the strong coupling limit. The researchers can directly observe the
spatial moire pattern and the momentum diffraction, confirming the presence of
two superfluid forms and showing a modified superfluid-to-insulator transition
in a twisted-bilayer lattice. They cooled Rb atoms, organized the atoms into two
superimposed lattices, and rotated the lattices using lasers. They applied
microwaves to induce lattice interaction with the interaction strength
controlled by the applied microwave field). The experimental set up displayed
superfluidity including a form that raised by adjusting the twist angle of the
lattices. The experimental set up here can be extended to multilayer
simulations.
For more information:
Phys.org, March 21 (2023); Nature, February 22 (2023) page 231.
WEEK OF MARCH 20, 2023 [No. 1515]
Electromagnetic time reflection observed:
researchers at CUNY in New York, NY have observed photonic time reflection and
associated broadband frequency translation of microwave signals in a switched
transmission-line metamaterial whose effective capacitance is homogeneously and
abruptly changed via a synchronized array of switches. A pair of temporal
interfaces are combined to demonstrate time-reflection-induced wave
interference, realizing the temporal counterpart of a Fabry–Perot cavity. Time
reflections arise when the entire medium in which the wave is traveling suddenly
and abruptly changes its properties across all of space. At such an event, a
portion of the wave is time reversed, and its frequency is converted to a new
frequency. Time reflection is a uniform inversion of the temporal evolution of a
signal, which arises when an abrupt change in the properties of the host
material occurs uniformly in space. At such a time interface, a portion of the
input signal is time reversed, and its frequency spectrum is homogeneously
translated as its momentum is conserved, forming the temporal counterpart of a
spatial interface. To date, this phenomenon had never been observed for
electromagnetic waves. Temporal reflections have only observed in water waves
and seeing them in electromagnetic radiation is complicated by the high
frequency of the waves. The trick is being able to switch a material’s
refractive index uniformly at a high enough speed (taking much less time than
the wave period) and with a great enough contrast so as to generate a measurable
effect. The optical properties of a material cannot be easily changed at a speed
and magnitude that induces time reflections. The researchers here have used a
metamaterial design, to realize the conditions to change the material's
properties in time abruptly and with large contrast. This has caused a
significant portion of the broadband signals traveling in the metamaterial to be
instantaneously time reversed and frequency converted. The effect forms a
strange echo in which the last part of the signal is reflected first. The
researchers demonstrate that the duration of the time-reflected signals was
stretched in time due to broadband frequency conversion. As a result, if the
light signals were visible, all their colors would be abruptly transformed. The
used set up consists of a 6 m-long strip of metal serving as a microwave
waveguide that snakes back and forth 20 times to form a device with ~ 30
cm2. Thirty capacitive circuits are positioned at regular intervals
along the length of the strip, but separated from it by switches. The idea is to
inject a train of microwave pulses and then switch all the circuits on or off at
the same time while the pulses are in transit along the strip, causing a sudden
change in the metamaterial’s effective refractive index and impedance that
temporally reflects the microwave signal. The researchers injected broadband
signals into the meandered strip of metal printed on a board and loaded with a
dense array of electronic switches connected to reservoir capacitors. All the
switches were then triggered at the same time, suddenly and uniformly doubling
the impedance along the line. This quick and large change in electromagnetic
properties produced the desired temporal interface, and the measured signals
carried a time-reversed copy of the incoming signals. A control signal is used
to uniformly activate the set of switches distributed along the metal stripline.
Upon closing/opening the switches, the electromagnetic impedance of the
metamaterial is abruptly decreased/increased, causing a broadband
forward-propagating signal to be partially time-reflected, with all its
frequencies converted. The key issue that prevented time reflections in previous
studies was the belief that it would require large amounts of energy to create a
temporal interface. It is very difficult to change the properties of a medium
quick enough, uniformly, and with enough contrast to time reflect
electromagnetic signals because of their fast oscillations. The idea here was to
avoid changing the properties of the host material, and instead create a
metamaterial in which additional elements can be abruptly added or subtracted
through fast switches. The researchers were able to double (or halve) the
refractive index in far less time than it took the wave to complete a single
oscillation, thanks to their switching circuitry taking a short cut across the
snaking waveguide. Injecting a signal consisting of two unequally strong peaks
and then connecting the capacitive circuits, the researchers found that a part
of the signal arrived back at the input port with the peaks in reverse order and
stretched out in time, just as would be expected for a time-reflected wave. The
rest of the signal instead returned to the port with the two peaks in their
original order, having spatially reflected off the far end of the metamaterial.
The experiment here has demonstrated that it is possible to realize a time
interface, producing efficient time reversal and frequency transformation of
broadband electromagnetic waves. The electromagnetic properties of metamaterials
have so far been engineered by combining many spatial interfaces. The experiment
here shows that it is possible to add time interfaces into the mix, extending
the degrees of freedom to manipulate waves. The researchers are now working on a
chip-scale version that would operate at much higher frequencies than here (10's
GHz, rather than 100's MHz; for THz they would have to use laser pulses rather
than electrical switches).
For more information:
Phys.org, March 13 (2023); Physicsworld, March 17 (2023); Nat. Phys., March 13
(2023).
Coherence emergence observed in secondary photoemission:
researchers at Northeastern University in Boston, MA and Westlake University in
Hangzhou, Zhejiang have observed unusual photoemission properties that go beyond
the existing theoretical descriptions in a reconstructed surface of potential
photocathode candidate perovskite oxide SrTiO3(100) single crystals
prepared by simple vacuum annealing. Unlike other positive-electron-affinity
(PEA) photocathodes, the PEA SrTiO3 surface produces discrete
secondary photoemission spectra at room temperature, a characteristic of
efficient negative-electron-affinity photocathode materials. Secondary electron
emission refers to when the primary electrons dislodged have suffered an energy
loss as a result of collisions within the material prior to ejection. At low
temperatures here, the photoemission peak intensity is enhanced substantially,
and the electron beam obtained upon non-threshold excitations displays
longitudinal and transverse coherence over record values (by over an order of
magnitude). Using several photon energies in the 10 eV range, researchers were
able to produce a very intense coherent secondary photoemission, stronger than
anything seen before. There is not known mechanism that can produce such an
effect. The observed emergence of coherence in secondary photoemission points to
the development of an underlying novel in the current theoretical photoemission
framework. SrTiO3 represents a new class of photocathode quantum
material, opening prospects for applications that require intense coherent
electron beams without the need for monochromatic excitations, electron
filtering or beam acceleration.
For more information:
Phys.org, March 14 (2023); Nature, March 8 (2023).
WEEK OF MARCH 13, 2023 [No. 1514]
Phason mass acquisition demonstrated in a charge-density wave insulator:
researchers at the University of Illinois at Urbana-Champaign in Urbana, IL have
observed a massive phason in a charge density wave material settling the
long-standing question of whether a charge density wave phason acquires mass by
coupling to long-range Coulomb interactions. This is a direct measurement of the
Anderson-Higgs mechanism of mass acquisition and the demonstration of the
existence of a massive phason in a charge density wave material. The massless
phason is a collective modulation of the phase of the charge-density-wave order
parameter. However, long-range Coulomb interactions should push the phason
energy up to the plasma energy of the charge-density-wave condensate, resulting
in a massive phason and fully gapped spectrum. Using time-domain THz emission
spectroscopy, the researchers investigate this issue in
(TaSe4)2I, a quasi-1D charge-density-wave insulator. On
transient photoexcitation at low temperatures, the material emits coherent,
narrowband THz radiation. The frequency, polarization and temperature
dependences of the emitted radiation imply the existence of a phason that
acquires mass by coupling to long-range Coulomb interactions. The observations
here underscore the role of long-range interactions in determining the nature of
collective excitations in materials with modulated charge or spin order.
Considering the electronic charge density wave in certain metals as frozen in
space, if the wave is disturbed, its collective excitations are generated; they
can by either amplitude change or by phase shifting. The collective excitation
in the phase shifting of the charge density wave (phason) has negligible mass
(like sound wave excitations). More than 40 years ago, researchers predicted
that if the phason interacts strongly with the background lattice of ions over
long-distances through long-range Coulomb interactions, then it will try to drag
the heavy ions as it moves. As a result, the phason will require a lot more
energy to get it to move, the phason is said to acquire mass. Direct observation
of this mass acquisition has remained elusive, primarily because long-range
Coulomb interactions do not exist in most charge density wave materials. The
material used here (TaSe4)2I is a very good insulator at
low temperatures and one of the best insulators for observing charge density
waves. Because of that, long-range Coulomb interactions were thought here to be
likely present in the system with those interactions giving mass to the
otherwise massless excitation. In theory, if the material is heated, it would
become less insulating, the Coulomb interactions would weaken, and the massive
phason should become massless. To analyze the charge density wave phason the
researchers here developed a nonlinear optical technique, time-domain THz
emission spectroscopy at low temperatures (< 10 K). A 150 fs IR pulse was
shined on the sample, generating the collective excitations of the system. What
they detected was the massive phason radiating in the THz region, with a very
narrow bandwidth. When they heated the material, the massive phason became
massless and stopped radiating, as expected in long-standing theoretical
predictions. While (TaSe4)2I is conducive to hosting a
massive phason, it is a difficult material to work with because it grows as very
thin needles making sample alignment very difficult. The researchers here
managed to grow (TaSe4)2I crystals with substantially
large width, which enabled the application of THz emission spectroscopy on this
material. Generating narrowband radiation in the THz region can be very
difficult as well. However, due to the strikingly narrow bandwidth THz radiation
resulting from the massive phason in (TaSe4)2I, there
seems to be a good possibility of developing it (and other similar materials) as
a THz emitter. The frequency and intensity of this THz emission can potentially
be controlled by varying sample properties, applying either external magnetic
fields or strain.
For more information:
Phys.org, March 9 (2023); Nat. Mater., March 9 (2023).
Giant orbital magnetic moments observed in graphene quantum dots:
an international group lead by researchers at the University of California Santa
Cruz in Santa Cruz, CA has used an STM to create and probe single and coupled
electrostatically defined graphene quantum dots, and thus, unravel the
magnetic-field responses of artificial relativistic nanostructures. Graphene
electrons electrostatic confined in a quantum dot make ultrarelativistic
circular loops around the edge of the dot. These current loops create magnetic
moments that are very sensitive to external magnetic fields so this set up can
generate a quantum sensor that can detect magnetic fields at the nano scale with
high spatial resolution. They looked at quantum dots in both monolayer graphene
and twisted bilayer graphene. The graphene here rests on an insulating layer of
h-BN, and a voltage applied with the STM tip creates charges in the h-BN that
serve to electrostatically confine electrons in the graphene. The researchers
here observe a giant orbital Zeeman splitting and orbital magnetic moment up to
~70 meV T–1 and ~ 600μB respectively, in single graphene
quantum dots. For coupled graphene quantum dots, Aharonov–Bohm oscillations and
a strong Van Vleck paramagnetic shift of ~ 20 meV T–2 are observed.
The results here indicate that graphene quantum dots can host a giant persistent
current in a small magnetic field.
For more information:
Phys.org, March 6 (2023); Nat. Nanot., March 6 (2023) page 250.
WEEK OF MARCH 6, 2023 [No. 1513]
QGP first order transition observed:
the STAR Collaboration in BNL at Upton, NY has analyzed the beam energy
dependence of fifth- and sixth-order net-proton number fluctuations in Au + Au
collisions at the Relativistic Heavy Ion Collider (RHIC) and found evidence that
quark-gluon plasma production turns off' at low energy. The researchers were
using the data in collisions of Au nuclei at the RHIC to search for signs of
transitions between different phases of nuclear matter. By varying the collision
energies they altered the temperature and baryon density (pressure) of the
matter produced in the collisions. The QGP off signal shows up as a sign change
(negative to positive) in data that describe higher order characteristics of the
distribution of protons produced in these collisions. They report the beam
energy and collision centrality dependence of fifth and sixth order cumulants
(C5, C6) and factorial cumulants (κ5,
κ6) of net-proton and proton number distributions, from
center-of-mass (CM) energy 3Â GeV to 200Â GeV Au + Au collisions. Cumulant ratios
of net-proton (taken as proxy for net-baryon) distributions generally follow the
hierarchy expected from QCD thermodynamics, except for the case of collisions at
3 GeV. The measured values of C6/C2 for 0%–40% centrality
collisions show progressively negative trend with decreasing energy, while it is
positive for the lowest energy studied. These observed negative signs are
consistent with QCD calculations (for baryon chemical potential, μB
≤ 110  MeV) which contains the crossover transition range. In addition, for
energies > 7.7 GeV, the measured proton κn, within uncertainties,
does not support the two-component (Poisson+binomial) shape of proton number
distributions that would be expected from a first-order phase transition. Taken
in combination, the hyperorder proton number fluctuations suggest that the
structure of QCD matter at high baryon density, μB ~ 750  MeV at 3
GeV is sharply different from those at vanishing μB ~ 24  MeV at 200
GeV and higher collision energies. Most 200 GeV collisions set free, for an
instant, the quarks and gluons that make up nucleons. QGP exists down to 19.6
GeV. The analysis here used data collected by RHIC's STAR detector during the
first phase of the RHIC Beam Energy Scan (BES I) to systematically search for
the energy at which production of this thermalized state of quarks and gluons is
turned off. They analyzed 10 collision energies, from CM energy 200 GeV (RHIC's
highest collision energy between two Au beams) down to 3 GeV (Au beam colliding
with stationary Au target). These data provide the widest available coverage of
the nuclear phase diagram. To determine whether a QGP was created at each
collision energy the researchers looked at the distribution of protons produced
in each collision event. They measured, event by event, the number of protons
minus the number of antiprotons produced, and the distribution of that
net-proton production. The team analyzed data on a variety of characteristics
of the distribution, including the mean value, the variance, how skewed the data
were, up to 5th and 6th order characteristics. Then, they compared their
observations with predictions calculated using 4D space-time lattice-QCD about
the behavior of higher order characteristics of conserved charge distributions
in QCD (that have the formation of a thermalized QGP built in). If the data
maTch the predictions, it is evidence that QGP is present. The QCD
calculations for the creation of thermalized QGP matter predict a hierarchical
ordering of the net-proton distribution characteristics and that some
relationships among these characteristics should all have negative values. The
results here indicate that these thermodynamic patterns generally persist at all
but the lowest collision energy. The measured higher order characteristics
(from 3rd order to 6th order) measured in central Au nuclei collisions represent
the fluctuation of the number of protons minus the number of antiprotons
produced, over the collisions of Au nuclei. The data at 200 GeV, 62.4 GeV, 54.4
GeV, 39, 27 GeV, 19.6 GeV, .... , 3 GeV CM energies are all consistent with a
thermalized QGP. Below 19.6 GeV, the data continued to maTch the
predictions, though the range of uncertainty about those measurements was large.
At the lowest energy, 3 GeV, the researchers observed an abrupt shift. The order
of the hierarchy among the analyzed characteristics and the sign of the key
relationships flipped. This sign change from negative to positive is a robust
indication (by first-principles calculations), that the formation of a QGP is
turned off at RHIC's lowest collision energy. The researchers will be using date
from RHIC's Beam Energy Scan II (BES II) to narrow the uncertainty of all these
results, especially for the energies below 19.6 GeV. From a thermalized system,
they now observe a smooth pattern from 200 GeV to 19.6 GeV. Then the
observation gets irregular down to 3 GeV. An earlier analysis of fluctuations in
net proton production suggested that those irregularities could be an indication
of a particular combination of temperature and pressure where the way the QGP is
formed from ordinary nuclear matter changes. The results presented here and the
addition of data from BES II will help to narrow the search for the critical
point.
For more information:
Phys.org, February 27 (2023); Phys. Rev. Letts., February 24 (2023) page 082301.
Molecular quantum tunneling measured:
researchers at Universität Innsbruck in Innsbruck have traced the quantum
mechanical tunnel effect in a simple chemical reaction and measured it in a very
slow ion-molecule reaction. Since the tunnel effect makes the reaction unlikely
and thus slow, its experimental observation was difficult. Hydrogenic systems
allow for accurate first-principles calculations. They introduced D into an ion
trap, cooled it down and then filled the trap with H2 gas. The rate
of the gas-phase proton-transfer tunneling reaction of H2 molecules
with D anions, H2 + D− → H− + HD, has been
calculated theoretically, but has so far lacked experimental verification. The
researchers here present high-sensitivity measurements of the reaction rate
carried out in a cryogenic 22-pole ion trap. They observe an extremely low rate
constant of (5.2 ±â€‰1.6) × 10−20 cm3
s−1. Because of the very low temperatures, the negatively charged D
ions lack the energy to react with H2 molecules in the conventional
way. In very rare cases, however, a reaction does occur when the two collide.
due to the tunnel effect. In their experiment, the researchers give possible
reactions in the trap about 15 m and then determine the amount of H ions formed.
From their number, they can deduce how often a reaction has occurred. In 2018,
it was theoretically calculated that in this system quantum tunneling occurs in
10-11 collisions. This corresponds very closely with the results measured here,
confirming the theoretical model for the tunneling effect in a chemical
reaction. A deviation of the reaction rate from linear scaling, which is
observed at high H2 densities, can be traced back to previously
unobserved heating dynamics in RF ion traps.
For more information:
Nature, March 1 (2023); Phys.org, March 1 (2023).
Collective rotation determines nuclear shape transition:
a group of international researchers at ORNL at Oak Ridge, TN has probed the
shape and motion of the Cd-106 nucleus using Coulomb excitation. They found
experimental evidence that the conventional vibrational description fails for
this isotope's nucleus and support that nuclear rotational motion describes it
more than the classical nuclear vibrational motion. The researchers have
measured the E2 rotational invariants of states for the 106Cd:
nucleus and observed the emergence of collective rotation. This work follows in
a long and currently active quest to answer the fundamental question of the
origin of nuclear collectivity and deformation and understand the transition
between spherical and deformed nuclei. This transition often includes
vibrational motion as an intermediate step. The finding here is counter to the
expected results. Spherical nuclei are often described by the motion of a small
fraction of the protons and neutrons, while deformed (ellipsoidal) nuclei tend
to rotate as a collective whole. In nuclear vibration (proposed since the 50's)
, atomic nuclei fluctuate about an average shape. The behavior that takes place
during the spherical - deformed transition is not yet known, but the evidence
here points to a description based on rotational motion of a nucleus together
with a reorganization of its outermost protons and neutrons. The
106Cd nucleus is a prime example of emergent collectivity that
possesses a simple structure (it is free of complexity caused by shape
coexistence and has a small, but collectively active number of valence
nucleons). The researchers used the Argonne Tandem Linac Accelerator System
(ATLAS), at ANL, to accelerate a beam of 106Cd nuclei to ~ 0.09c and
direct it onto a 1-µ thick 208Pb target foil. During the collision,
GR from the 106Cd nuclei were emitted and detected by the Gamma-Ray
Energy Tracking In-beam Nuclear Array (GRETINA), and the recoiling Pb and Cd
nuclei were detected by the Compact Heavy Ion Counter 2 (CHICO2). The collective
structure of 106Cd was elucidated by multi-step Coulomb excitation of
a 3.849 MeV/A beam of 106Cd on a 1.1 mg/cm2
208Pb target using GRETINA-CHICO2 at ATLAS. The intensities of the GR
provided a measure of the probability of exciting 106Cd nuclei via
the electromagnetic interaction, from which the electromagnetic properties of
106Cd were established. The researchers integrated these properties
into a model-independent measure of the nuclear shape and compared the result to
expectations from several nuclear theories. The results indicate that at low-
energies, 106Cd is not vibrational but instead more in line with the
rotation of a slightly deformed triaxial rotor (deflated ellipsoid). The results
are discussed in terms of phenomenological models, the shell model, and
Kumar-Cline sums of E2 matrix elements (fourteen E2 matrix elements were derived
from measurements.). The xxx matrix element is determined here, providing a
total, converged measure of the electric quadrupole strength of the
first-excited level relative to the ground state. It does not show an initially
expected increase of harmonic and anharmonic vibrations. Strong evidence for
triaxial shapes in weakly collective nuclei is implied while collective
vibrations are excluded. This is contrary to the only other Cd result of this
kind obtained with 114Cd (1988) (that complicated by low-lying shape
coexistence near midshell).
For more information:
Phys.org, February 27 (2023); Phys. Lett. B, November 10 (2022) 036601.
WEEK OF FEBRUARY 27, 2023 [No. 1512]
Quantum twisting microscope demonstrated:
researchers at the Weizman Institute of Science in Rehovot, Israel, have
developed a quantum twisting microscope (QTM) that can create quantum materials
and observe their quantum electronic waves. The QTM is capable of performing
local interference experiments at its tip. The QTM is based on a unique van der
Waals tip, allowing the creation of 2D junctions, which provide a multitude of
coherently interfering paths for an electron to tunnel into a sample. With the
addition of a continuously scanned twist angle between the tip and sample, this
microscope probes electrons along a line in momentum space similar to how a STM
probes electrons along a line in real space. The QTM involves the twisting of
two atomically-thin layers of material with respect to one another. The twist
angle between graphene layers is the most critical parameter for controlling the
behavior of electrons. The trick for observing quantum waves is to spot the same
electron in different locations at the same time. The measurement here is
conceptually similar to the two-slit experiment. The only difference is that the
researchers here perform that experiment at the tip of a STM. They replaced the
atomically sharp tip of the STM with a tip that contains a flat layer of
graphene. When this layer is brought into contact with the surface of the sample
of interest, it forms a 2D interface across which electrons can tunnel at many
different locations. Quantum mechanically, the electrons tunnel in all locations
simultaneously, and the tunneling events at different locations interfere with
each other. This interference allows an electron to tunnel only if its
wavefunctions on both sides of the interface match exactly.
Generally, the electronic waves in the tip and the sample propagate in different
directions and therefore do not match. The QTM uses its twisting
capability to find the angle at which matching occurs. By
continuously twisting the tip with respect to the sample, the tool causes their
corresponding wavefunctions to also twist with respect to one another. Once
these wavefunctions match on both sides of the interface, tunneling
can occur. The twisting allows the QTM to map the electronic wavefunction
dependence on momentum, similarly to the way lateral translations of the tip
enable the mapping of its dependence on position. Knowing at which angles
electrons cross the interface supplies much information about the probed
material. including the collective organization of electrons within the sample,
their speed, energy distribution, patterns of interference and the interactions
of different waves with one another. The researchers: demonstrate RT quantum
coherence at the tip; study the twist angle evolution of twisted bilayer
graphene; image the energy bands of monolayer and twisted bilayer graphene
directly; and, visualize the gradual flattening of the low-energy band of
twisted bilayer graphene while applying large local pressures.
For more information:
Nature, February 22 (2023) page 682; Phys.org, February 22 (2023).
Electron magnetic moment measurement accuracy increased:
researchers at the Northwestern University in Evanston, IL have determined the
value of the electron’s magnetic moment as −μ / μB = g/2 = 1.001  159
652  180 59 (13) [0.13 ppt] (more than 3000 times smaller precision than
achieved for the muon magnetic moment and larger than the previous best estimate
for the electron magnetic moment obtained with 0.28-ppt accuracy at Harvard
University in 2008). The electron magnetic moment is used to test the standard
model by studying interactions between electrons and virtual particles that come
into existence inside of a vacuum chamber. The result here is the most precisely
determined property of an elementary particle and tests the most precise
prediction of the standard model (SM) to 1 ppt. The study involves measuring the
affect of collisions on both the magnetic moment and its g factor and then
comparing the results to what is described by the SM. The experimental system
includes a dilution cryorefrigerator supporting a 50 mK electron cylindrical
Penning trap with Ag electrodes and a microwave inlet that is placed within a
4.2K cooled NbTi solenoid providing a very stable field to the inside of the
trap. The key idea for a high-precision magnetic-moment determination is to
obtain it from the measurement of the ratio of two frequencies. The magnetic
moment of the electron is proportional to its spin and g factor. In a constant
magnetic field, the deviation of g from 2 (g-2), is given by ν a/ ν
c, where vc is the cyclotron frequency (at which the electron spins
around the field), and ν a = ν c − ν s (where ν
s is the electron spin frequency). One advantage of this approach is
that both ν a and ν c are to a first approximation
proportional to the magnetic field, so the field dependence cancels out
(assuming the field is stable over the measurement time). This cancellation
makes the experiment less sensitive to slow field drifts. In addition, since ν
a and ν c differ by only 1 part in 103, an
accuracy of 1 part in 1010 on the measurement of both frequencies
results in a precision of 1 part in 1013 in their ratio, and thus in
g. A single electron is kept in the Penning trap under a constant, 5-T magnetic
field and cooled to temperatures at which the electron’s cyclotron motion is
quantized, with the electron initially sitting in the ground state. Next, ν
a and ν c are determined by observing quantum jumps of the
electron between the lowest-energy levels. With the addition of a small
magnetic-field gradient, the setup allows the researchers to perform quantum
nondemolition detection, a measurement that detects the quantum jumps without
affecting the electron quantum state, which is key to reducing the measurement
uncertainty. The achieved measurement accuracy comes from several technical
improvements on the experimental set up. It boosts the stability and homogeneity
of the magnetic field through its suspension and cooling schemes. The design of
the trapping cavity allows for precise control of the electron’s axial motion
and strongly inhibits spontaneous-emission transitions between the electron
quantum levels, which would broaden the transition line shapes and thus reduce
the precision of frequency determination. The researchers also reduced
systematic biases due to shifts of the cyclotron frequency induced by the
coupling of the cyclotron motion to resonant modes of the trapping cavity. By
characterizing the frequencies and quality factors of each of 72 contributing
cavity modes, the researchers could account for such shifts. The test accuracy
here would improve an order of magnitude if the uncertainty from discrepant
measurements of the fine structure constant α is eliminated (the SM prediction
is a function of α). The measurement here and the SM theory together predict
α−1 = 137.035 999 166 (15) [0.11 ppb] with an uncertainty 10 times
smaller than the current disagreement between measured α values. There is a
5.5-σ discrepancy between the two most accurate measurements of α, performed by
groups at the University of California, Berkeley, CA and Sorbonne University,
France. The new setup has much potential for improvement so it is expected that
the electron g-2 measurements will reach comparable sensitivity to that of the
muon g-2 measurements that have revealed the 4.2-σ inconsistency with SM.
For more information:
Physics, February 20 (2023); Phys.org, February 20 (2023); Phys. Rev. Lett.,
February 13 (2023) page 071801.
Increasing critical temperature in BLG:
researchers at CalTech in Pasadena, CA have placed monolayer WSr2 on
top of untwisted bylayer graphene (BLG) to enhance its superconductivity
critical temperature by an order of magnitude Also by applying electric fields,
the researchers can add or remove electrons from the BLG as well as push them
toward and away from the WSr2 monolayer. In the presence of a large
perpendicular electric field, Bernal-stacked bilayer graphene features several
broken-symmetry metallic phases as well as magnetic-field-induced
superconductivity. The superconducting state is quite fragile, however,
appearing only in a narrow window of density and with a maximum critical
temperature Tc ≈ 30 mK. The researchers here show that placing
monolayer WSr2 on BLG promotes much Cooper pairing and thus,
superconductivity: appears at zero magnetic field; exhibits an order of
magnitude enhancement in Tc ; and occurs over a density range that is
increased by a factor of eight. By mapping quantum oscillations in
BLG–WSr2 as a function of electric field and doping, they establish
that superconductivity emerges throughout a region for which the normal state is
polarized, with two out of four spin-valley flavors predominantly populated.
In-plane magnetic field measurements reveal that superconductivity in
BLG–WSr2 can exhibit dependence of the critical field on doping, with
the Chandrasekhar–Clogston (Pauli) limit roughly obeyed on one end of the
superconducting dome and sharply violated on the other. The superconductivity
arises only for perpendicular electric fields that push BLG hole wavefunctions
towards WSr2, indicating that proximity-induced (Ising) spin–orbit
coupling plays an important role in stabilizing the pairing.
For more information:
Phys.org, February 22 (2023); Nature, January 11 (2023) page 268.
WEEK OF FEBRUARY 20, 2023 [No. 1511]
Majoranas observation in minimal Kitaev chain:
researchers at Delft University of Technology in Delft have created Majorana
particles and measured their properties with great control using a minimal
Kitaev chain in coupled quantum dots. These Majoranas are based on two quantum
dots in a nanowire, which could be scaled up to a larger chain of quantum dots
with more resilient Majorana behavior. A model proposed by Kitaev shows that
Majorana bound states can arise at the ends of a spinless p-wave superconducting
chain. Practical proposals for its realization require coupling neighboring
quantum dots (QDs) in a chain through both electron tunneling and crossed
Andreev reflection. Although both processes have been observed in semiconducting
nanowires and carbon nanotubes, crossed-Andreev interaction was neither easily
tunable nor strong enough to induce coherent hybridization of dot states. The
researchers here demonstrate the simultaneous presence of all necessary
ingredients for an artificial Kitaev chain made up of two spin-polarized QDs in
an InSb nanowire strongly coupled by both elastic co-tunneling and crossed
Andreev reflection. They fine-tune this system to a sweet spot where a pair of
Majorana states is predicted to appear. At this sweet spot, the transport
characteristics satisfy the theoretical predictions for such a system, including
pairwise correlation, zero charge, and stability against local perturbations.
Unlike regular qubits, Majoranas always appear in pairs and each pair forms a
delocalized electron. Since one part of the Majorana particle can reside on one
end of a nanowire and the second part on the other end, if one part is affected
by noise, the other half will remain unscathed. The researchers start by
producing two quantum dots close to each other, separated by a short
semiconductor/superconductor nanowire. The quantum dots are electrically
connected to each other by electrons hopping between both dots and by pairs of
electrons that simultaneously enter and leave the semiconductor/superconductor
nanowire. The researchers have developed a method to precisely control the two
processes critical to the formation of the Majorana particles. They will seek
to have more dots in their system so that the electron halves are more widely
separated and thus have better qubit protection against noise. The researchers
main objectives next are to create a full topological Majorana system and to
use these Majoranas to create qubits using multiple copies of the system.
For more information:
Nature, February 15 (2023 page 445; Phys.org, February 15 (2023).
Quantum geometry effect assessed in MATBG superconductivity:
a group lead by researchers at the Ohio State University in Columbus, OH has
observed the key role that quantum geometry plays in allowing magic angle
twisted bilayer graphene to become a superconductor. The researchers here
explore the effect of vanishingly small velocity in a superconducting Dirac flat
band system. Using Schwinger-limited non-linear transport studies , they
demonstrate an extremely slow normal state drift velocity ≈ 1,000 m
s–1 for filling fraction between −1/2 and −3/4 of the moire
superlattice. In the superconducting state, the same velocity limit constitutes
a new limiting mechanism for the critical current, analogous to a relativistic
superfluid. The measurement of superfluid stiffness, which controls the
superconductor’s electrodynamic response, shows that it is not dominated by the
kinetic energy but instead by the interaction-driven superconducting gap,
consistent with recent theories on a quantum geometric contribution. The
researchers find evidence for small Cooper pairs, characteristic of the BCS -
BEC crossover, with an unprecedented ratio of the superconducting transition
temperature to the Fermi temperature exceeding unity and discuss how this arises
for ultra-strong coupling superconductivity in ultra-flat Dirac bands. Electrons
move very slowly in the MATBG flat band electronic structure (their speed
approaches zero when angle = 1.08°) though a small charge carrier group velocity
in the conventional BCS theory implies vanishing coherence length, superfluid
stiffness and critical current. The researchers set up an experiment with a
device at cryogenic temperatures so close to 1.08° that the electrons were
nearly stopped by usual condensed matter physics standards. The sample
nevertheless showed superconductivity. The researchers demonstrated the slow
speeds of the electrons and gave more precise measurements of electron movement
than had been previously available. They concluded that the geometry of the
quantum wavefunctions in flat bands, together with the interaction between
electrons, leads to the flow of electrical current without dissipation in MATBG.
They found that conventional equations could explain ~ 10% of the observed
superconductivity signal. The experimental measurements here suggest that
quantum geometry is ~ 90% of what makes this system a superconductor.
For more information:
Nature, February 15 (2023) page 440; Phys.org, February 15 (2023).
WEEK OF EBRUARY 13, 2023 [No. ]
Broad bandwidth quantum squeezing demonstrated in parametric amplifier:
researchers at MIT in Cambridge, MA have demonstrated broadband squeezed
microwaves production and amplification with a Josephson traveling-wave
parametric amplifier. This superconducting parametric amplifier can achieve
quantum squeezing over much broader bandwidths than other designs. The
researchers here develop a dual-pump, broadband Josephson traveling-wave
parametric amplifier that combines a phase-sensitive extinction ratio of 56 dB
with single-mode squeezing on par with the best resonator-based squeezers. They
demonstrate two-mode squeezing at microwave frequencies with bandwidth in the
GHz range much wider than in contemporary resonator-based squeezers. This
amplifier is capable of simultaneously creating entangled microwave photon pairs
with large frequency separation. A certain amount of noise is inherent in any
quantum system. It is possible to effectively get around this limitation by
using parametric amplification to squeeze the noise. While the total amount of
noise remains the same, it is effectively redistributed from the variable of
interest to its conjugate. Thus, more accurate measurements are possible by
looking only at the lower-noise variable. The researchers here have developed a
superconducting parametric amplifier that operates with the gain of previous
narrowband squeezers while achieving quantum squeezing over much larger
bandwidths. They demonstrate squeezing over a broad frequency bandwidth of over
1.75 GHz (previous microwave parametric amplifiers generally achieved bandwidths
< 100 MHz) while maintaining a high degree of squeezing. In superconducting
circuits, the resonator-based Josephson-junction parametric amplifiers,
conventionally used to generate squeezed microwaves, are constrained by a narrow
bandwidth and low dynamic range. A conventional Josephson parametric amplifier
is resonator-based (an echo chamber with a superconducting nonlinear Josephson
junction in the middle). Photons enter the echo chamber and bounce around to
interact with the Josephson junction multiple times. In here, the system's
nonlinearity (realized by the Josephson junction) is enhanced and leads to
parametric amplification and squeezing. Since the photons traverse the same
Josephson junction many times before exiting, the junction is stressed. As a
result, both the bandwidth and the maximum signal that a resonator-based
amplifier can accommodate are limited. Instead of embedding a single or a few
Josephson junctions inside a resonator, the researchers here chained more than
3,000 junctions together (a Josephson traveling-wave parametric amplifier).
Photons interact with each other as they travel from junction to junction,
resulting in noise squeezing without stressing any single junction. This
traveling-wave system can tolerate much higher-power signals than conventional
resonator-based Josephson amplifiers without the bandwidth constraint of the
resonator, leading to broadband amplification and high levels of squeezing.
Researchers can tune the frequency of photons coming from each pump to generate
squeezing at the desired signal frequency. To squeeze a 6-GHz signal, they would
adjust the pumps to send photons at 5 and 7 GHz, respectively. When the pump
photons interact inside the device, they combine to produce an amplified signal
with a frequency right in the middle of the two pumps. Squeezing of the noise
results from a two-photon quantum interference effect that arises during the
parametric process. This architecture enabled a reduction of the noise power by
a factor 10 below the fundamental quantum limit while operating with 3.5 GHz of
amplification bandwidth. The researchers demonstrated broadband generation of
entangled photon pairs. The materials used to fabricate the amplifier introduce
some microwave loss, which can reduce performance. They plan to look to
different fabrication methods that could improve the insertion loss.
For more information:
Phys.org, February 9 (2023); Nat. Phys., February 9 (2023).
Chiral superconductivity observed in Si:
an international group lead by researchers at the University of Tennessee in
Knoxville, TN has found evidence for chiral superconductivity on a Si surface.
Sn adatoms on a Si(111) substrate with a one-third monolayer coverage form a 2D
triangular lattice with one unpaired electron per site. These electrons order
into an antiferromagnetic Mott-insulating state, but doping the Sn layer with
holes creates a 2D conductor that becomes superconducting at low temperatures.
Although the pairing symmetry of the superconducting state is currently unknown,
the combination of repulsive interactions and frustration inherent in the
triangular adatom lattice opens up the possibility of a chiral order parameter.
The researchers here study the superconducting state of Sn/Si(111) using
scanning tunneling spectroscopy and quasiparticle interference imaging. They
find evidence for a doping-dependent superconducting critical temperature with a
fully gapped order parameter, the presence of time-reversal symmetry breaking
and a strong enhancement in zero-bias conductance near the edges of the
superconducting domains. The researchers replicated cuprate-like physics by
growing one-third of a monolayer of Sn atoms on a substrate of Si. The system is
engineered such that the repulsion between the Sn electrons is so strong that
they can not move and thus, they will not superconduct. They worked around this
by implanting B atoms in the Si layer's diamond-like crystal structure. The B
atoms take electrons from the Sn layer (~ 10 %) giving the remaining Sn
electrons the freedom to conduct. The Sn layer thus became metallic and even
superconducting at a critical temperature exceeding that of nearly all elemental
superconductors. The phenomenon scaled with the number of B atoms (behavior
reminiscent of the cuprate superconductors). The researchers found evidence that
this Sn-Si material hosts chiral superconductivity. The superconducting
wavefunction in the Sn layer turns out to be clockwise in parts of the sample
and counterclockwise in other parts. (time-reversal symmetry is broken). The
system has two 1D conduction channels along the perimeter of the sample
material. These channels host topologically protected Majorana particles.
Although each individual piece of observed evidence could have a more trivial
explanation, the combined results obtained here suggest the possibility that
Sn/Si(111) is an unconventional chiral d-wave superconductor.
For more information:
Phys.org, February 9 (2023); Nat. Phys., January 30 (2023).
WEEK OF FEBRUARY 6, 2023 [No. 1509]
Anti-neutrino charged-current elastic scattering on free proton analyzed:
the MINERvA collaboration lead by University of Rochester, Rochester, NY
researchers has used the NuMI neutrino beamline at Fermilab in Batavia, IL, and
the plastic scintillator target of the MINERvA experiment, to measure the
νμ¯p→μ-n cross-section (from the H atom)
with high-statistics, then extracting the nucleon transition axial form factor
(FA) from free proton targets and measuring the nucleon axial charge
radius, rA, to be 0.73 ±0.17 fm. The νμ¯– H
scattering presented here can provide FA without the need for nuclear
theory corrections, and enables direct comparisons with the increasingly precise
lattice QCD computations. The charged-current elastic (CCE) process on H was
measured with the MINERvA detector in a νμ¯ beam produced
at the NuMI neutrino beamline with average energy 5.4 GeV.
νμ¯ interactions were selected by requiring μ-
and n signatures in the MINERvA detector. Scattering of high energy particles
from nucleons probes their structure (as it was done in the experiments that
established the non-zero size of the proton using electron beams). The use of
charged leptons as scattering probes enables measuring the distribution of
electric charges, which is encoded in the vector form factors of the nucleon.
Scattering weakly interacting neutrinos gives the opportunity to measure both
vector and axial vector form factors of the nucleon, providing an additional,
complementary probe of their structure. The FA can be measured from
neutrino scattering from free nucleons, νμn →μ−p and
νμ¯p→μ-n, as a function of the negative
four-momentum transfer squared, Q2. Up to now,
FA(Q2) has been extracted from the bound nucleons in
νμ–D scattering , which requires uncertain nuclear corrections. Form
factors measured in scattering processes describe the structure of composite
objects. They have been thought to be the Fourier transform of charge
distributions in the non-relativistic limit of low negative Q2. The
slopes of the form factors at Q2 = 0 provide a measure of the
mean-squared radius < r2A > for the particle in the
charge species described. Nucleon electric (GNE) and magnetic (GNM) form factors
are precisely measured in electron–nucleon elastic scattering experiments,
enabling the radius of the nucleon to be inferred. Neutrino scattering
measurements yield the analogous FA, which characterizes the weak
charge distribution. FA is also a key input to neutrino oscillation
experiments to precisely measure the neutrino oscillation parameters, including
CP violation, and to establish mass hierarchy. Previous measurements of
FA in neutrino scattering were performed by measuring
dσ/dQ2 in the reaction νμD →μ−pp in D
bubble chambers. Even in D nuclei, theoretical assumptions about the Fermi
motion of the bound nucleons, the application of the Pauli exclusion principle
in the proton-proton (pp) final state, and the nuclear wave function are
required to extract FA from these measurements. Previous extractions
assumed FA to follow the dipole form factor. Although there are many
efforts to calculate FA for Q2 > 0 from lattice QCD
with increasing precision, calculations in the Q2 region above 1
GeV2 remain imprecise. In the CCE reaction on the free nucleon
νμ¯H→ μ-n, a νμ¯
elastically scatters off the free p from the H atom, turning the
νμ¯ into the more massive μ- and the p into a
n. This reaction is free from the nuclear theory corrections in scattering from
D and provides a direct measurement of FA. It is also a two-body
reaction with a nucleon at rest; therefore, the neutrino direction and the
final-state μ- momentum fully specify the interacting system. MINERvA
is a segmented scintillator detector with hexagonal planes constructed from
strips of triangular cross-section assembled into planes perpendicular to the
νμ¯ beam. This analysis reconstructs neutrino interactions
in the active tracker region of the detector (the scintillator). The
scintillator strips point in either the vertical (X) or one of the ±60°
(U,V) directions. This region is fully active, consisting of 128 tracker planes
stacked in alternating patterns of XUXV. The alternating orientation enables
extraction of a 3D position from the strips when charged particles traverse two
or more consecutive planes. Muons produced from charged-current neutrino
interactions in the MINERvA detector may exit from the rear and enter the MINOS
near detector (ND), which is located immediately downstream of the MINERvA
detector. The MINOS ND is a fully magnetized scintillator and steel detector
that determines the μ’s charge and momentum by measuring its curvature and
range. Only μ in the energy range 1.5 GeV < Eμ < 20 GeV with an opening
angle θμ < 20° with respect to the neutrino direction are selected
because they can be efficiently measured by the MINOS ND. The vertex is defined
to be the beginning of the muon track. Energy deposits from other charged
particles, such as p and Ï€±, can be reconstructed into tracks if they
span at least four planes. Photon pairs from π0 can be reconstructed from their
electromagnetic showers. Although n are not directly observable from ionization
as charged particles, they produce secondary particles with observable energy
deposits when they elastically, quasi-elastically or inelastically scatter in
the detector. The dominant interactions produce low-energy p, which can be
observed. Neutrons also scatter undetectably, for example by inelastically
knocking out n from C nuclei or elastically scattering from C, in ways that
change the n direction and energy. Monte Carlo simulation studies of single-n
transport in the MINERvA detector show that the angle between the reconstructed
and true n directions follows the sum of two exponential distributions, with 68%
of the candidates within 12°. In the statistically significant measurement of
the νμ¯ CCE scattering on the free p here, the researchers
observe 5,580(180) signal events over the estimated background of 12,500 events.
The measured cross-section is given, as a ratio to the cross-section prediction
assuming a dipole FA. The analysis is dominated by statistical
uncertainties at all Q2. Systematic uncertainties arise from the
small remaining differences, due in part to the regularization, between the
post-fit background prediction in each systematic variation of the input model.
The dominant systematic uncertainties in this measurement are the n secondary
interaction in the detector (4.8%), the normalization in the CCQE cross-section
(4.5%), the μ energy scale (4.2% from MINOS and 3.1% from MINERvA), the flux
(3.9%), n FSI (~ 3%), and the 2p2h process (2.3%).
For more information:
For more information: Nature, February 1 (2023); Phys.org, February 1 (2023).
Electrical switching of superconductivity:
researchers at MIT in Cambridge, MA have demonstrated how to switch
superconductivity on and off using a short current pulse in a bistable moire
superconductor by properly stacking and aligning the graphene layers between two
offset BN layers. The angle and alignment of each layer in the magic-angle
twisted bilayer graphene (MATBG), enables the researchers to turn
superconductivity on and off in graphene with a short electric pulse (rather
than a continuous electric field as previously demonstrated before). Recently,
gate hysteresis and resultant bistability in Bernal-stacked bilayer graphene
aligned to its insulating hexagonal BN gate dielectrics have been discovered.
The researchers here observe this same hysteresis in MATBG with aligned BN
layers. This bistable behavior coexists alongside the strongly correlated
electron system of MATBG without disrupting its correlated insulator or
superconducting states. This all-van der Waals platform enables configurable
switching between different electronic states of this system. They demonstrate
reproducible bistable switching between the superconducting, metallic and
correlated insulator states of MATBG using gate voltage or electric displacement
field. In 2019, a team at Stanford University discovered that MATBG could be
coerced into a ferromagnetic state. The researchers found that MATBG could
exhibit ferromagnetic properties in a way that could be tuned on and off. This
happened when the graphene sheets were layered between two sheets of BN such
that the crystal structure of the graphene was aligned to one of the BN layers.
In the setup here the researchers used two graphene sheets (the top rotated
slightly at 1.1° with respect to the bottom sheet) with an above layer of BN,
exactly aligned with the top graphene sheet. And a second layer of BN below the
entire structure and offset it by 30° with respect to the top layer of BN. The
team then measured the electrical resistance of the graphene layers as they
applied a gate voltage. They found, as others have, that the MATBG switched
electronic states, changing between insulating, conducting, and superconducting
states at certain known voltages. What the researchers did not expect was that
each electronic state persisted rather than immediately disappearing once the
voltage was removed (bistability). They found that, at a particular voltage, the
graphene layers turned into a superconductor, and remained superconducting, even
as the researchers removed this voltage. This bistable effect suggests that
superconductivity can be turned on and off with short electric pulses rather
than a continuous electric field. It is not clear what enables this switchable
superconductivity, though the researchers suspect it has something to do with
the special alignment of the twisted graphene to both BN layers, which enables a
ferroelectric-like response of the system (ferroelectric materials display
bistability in their electric properties).
For more information:
Phys.org, January 30 (2023); Nat. Nanot., January 30 (2023).
WEEK OF JANUARY 30, 2023 [No. 1508]
Si electron spins coherently manipulated:
researchers at the University of Rochester in Rochester, NY have developed a
method to coherently manipulate either single or multiple electron spins in Si
quantum dots based on coherent spin–valley oscillations. For qubits based on
single spins, electron spin resonance with real or effective time-varying
magnetic fields is the standard method for universal quantum control. The
researchers here show that spin–valley coupling in Si, which drives transitions
between states with different spin and valley quantum numbers, enables coherent
control of single- and multi-electron spin states without oscillating
electromagnetic fields. They demonstrate Rabi oscillations between effective
single-spin states in a Si/SiGe double quantum dot that are driven by
spin–valley coupling. Together with the exchange coupling between neighboring
electrons, spin–valley coupling also enables universal control of effective
two-spin states, driving singlet–triplet and triplet–triplet oscillations that
feature coherence times ~ µs. Electron spins in Si quantum dots are good qubits
because they have long coherence times and high gate fidelities and are
compatible with advanced semiconductor manufacturing techniques. Electrons in Si
experience a coupling between their spin and valley states. In the presence of a
DC voltage an electron can undergo coherent spin-valley oscillation. Electrons
in Si quantum dots have both spin and valley quantum numbers. Their spin state
can be up or down, while their valley state can be + or −. At a certain
magnetic field, the energy of the up/+ state can be almost equal in energy to
the down/- state. Because the energy difference between the + and − states
depends on electric fields, the researchers can use a voltage pulse to then
bring up/+ exactly into resonance with down/−. When this happens, an electron
initially prepared in an up /+ state will coherently oscillate to down/− and
back and forth in spin-valley oscillations. Spin-valley coupling (interaction
between an electron's spin and valley states) was initially theoretically
identified to directly mediate coherent transitions between different spin
states. The strategy for controlling electron spins in Si proposed here takes
advantage of that coupling. To date, the standard method to manipulate electron
spins in Si quantum dots entailed the use of time-varying magnetic fields. The
researchers here have showed that their strategy enables the coherent
manipulation of electron spins without the need to use oscillating
electromagnetic fields. Oscillating magnetic fields can be especially difficult
to generate at cryogenic temperatures, and spin-valley coupling eliminates this
need. A problem yet to be solved on this method is that the magnetic field needs
to be tuned separately for each qubit. The valley degree of freedom in Si has
often been disregarded rather than a useful feature of Si qubits. The results
here show otherwise and prove spin–valley coupling as a good candidate mechanism
for coherent control of qubits based on electron spins in semiconductor quantum
dots. The researchers plan to get a better understanding of what characteristics
of the growth, fabrication, and tuning of quantum dots can impact spin-valley
coupling.
For more information:
Phys.org, January 26 (2023); Nat. Phys., January 9 (2023).
Weyl photocurrent flow observed:
researchers at Boston College have imaged bulk and edge photocurrent flow in
anisotropic Weyl semimetals and discovered that the photocurrent flows in along
one crystal axis and flows out along its perpendicular axis. In Weyl semimetals
that break inversion symmetry, bulk photocurrents may arise due to nonlinear
optical processes that are enhanced near the Weyl nodes. However, the
photoresponse of these materials is commonly studied by scanning photocurrent
microscopy, which convolves the effects of photocurrent generation and
collection. The researchers here directly image the photocurrent flow inside the
type-II Weyl semimetals WTe2 and TaIrTe4 using
high-sensitivity quantum magnetometry with NVC spins. They elucidate a mechanism
for bulk photocurrent generation (anisotropic photothermoelectric effect,
differences in how heat is converted to current along the different in-plane
directions of the Weyl semimetal), where unequal thermopowers along different
crystal axes drive intricate circulations of photocurrent around the
photoexcitation spot. Using overlapping scanning photocurrent microscopy and
magnetic imaging at the interior and edges of the sample, they visualize how the
anisotropic photothermoelectric effect stimulates the long-range photocurrent
collected in the WTe2 and TaIrTe4 devices through the
Shockley–Ramo mechanism. The researchers have developed a technique (sort of
photocurrent flow microscope), using quantum magnetic field sensors (diamond
NVC's) to image the local magnetic field produced by the photocurrents and
reconstruct the full streamlines of the photocurrent flow. With most materials,
shining a light onto their surface will not generate any electricity because
there is no preferred direction for the electricity to flow. Most
photoelectrical devices require two different materials to create an asymmetry
in space. The researchers here showed that the spatial asymmetry (in its
thermoelectric transport properties) within a single material can give rise to
spontaneous photocurrents. They set out to understand why Weyl semimetals are
efficient at converting light into electricity. Researchers have suspected that
the studied materials would be good candidates for photocurrent generation
because their crystal structure is inherently inversion asymmetric. Previous
measurements could only determine the amount of electricity coming out of a
device. To better understand the origin of the photocurrents, they sought to
visualize the flow of electricity within the device. The researchers found the
electrical current flowed in a four-fold vortex pattern around where the light
shined on the material. They visualized how the circulating flow pattern is
modified by the edges of the material and revealed that the precise angle of the
edge determines whether the total photocurrent flowing out of the device is
positive, negative, or zero. The flow images allowed them to explain that the
photocurrent generation mechanism is due to an anisotropic photothermoelectric
effect. The appearance of anisotropic thermopower is not necessarily related to
the inversion asymmetry displayed by Weyl semimetals, and thus, it might be
present in other classes of materials.
For more information:
Phys.org, January 26 (2023); Nat. Phys., January 23 (2023).
WEEK OF JANUARY 23, 2023 [No. 1507]
Spin alignment preference of φ0 mesons observed in heavy nuclei
collisions:
the STAR Collaboration in BNL at Upton, NY has obtained RHIC data showing that
local fluctuations in the nuclear strong force may influence the spin
orientation of φ0 mesons. The researchers show here that, in
relativistic heavy-ion collisions, in which quarks and gluons are set free over
an extended volume, two species of produced vector (spin-1) mesons
(φ0 and K*0) emerge with an unexpected pattern of global
spin alignment. The global spin alignment for φ0 is unexpectedly
large, whereas that for K*0is consistent with a zero value. The
observed spin-alignment pattern and magnitude for φ0 cannot be
explained by conventional mechanisms, whereas a model with a connection to
strong force fields (an effective proxy description within QCD), accommodates
the current data. Conventional mechanisms such as the magnetic field strength or
the vorticity of the matter generated in the particle collisions, cannot explain
the data but the model that includes local fluctuations in the nuclear strong
force can do it. If this explanation proves to be correct, these measurements
can provide a way to gauge how large the local fluctuations in the strong force
are. The head-on collisions at RHIC melt the boundaries of individual nucleons,
setting free the quarks and gluons, normally confined within, to create a QGP.
Earlier measurements from STAR revealed that when Au nuclei collide in a
somewhat off-center way, the glancing impact sets the QGP spinning. Scientists
then measured the vorticity of the swirling QGP by tracking its influence on
the spins of Λ0 s emerging from the collisions. The degree to which
their spin axes align with the angular momentum generated in each off-center
collision is a direct proxy for measuring the QGP's vorticity. The researchers
look for spin alignment preferences among particles emerging from the QGP. More
recently STAR analyses sought to measure the spin alignment of various types of
particles, including the φ0 and K*0mesons (three
possible spin orientations). As in previous studies, the researchers measured
the spin alignment of these particles by tracking the distribution of their
decay products relative to the direction perpendicular to the reaction plane of
the colliding nuclei. For the φ0 and K*0mesons, they
translate those measurements into a probability that the parent particle was in
one of the three spin states. For K*0mesons the probability of each
of the three states equals one-third, so there is no preference for them to be
in any one of the three spin alignment states. For φ0 mesons, there
was a strong signal that one state was preferred over the other two states.
Tracking pairs of K+− (decay emission of φ0
mesons) revealed that φ0 mesons appear to have that preference.
Describing the global spin alignment of the φ0 meson using only the
conventional mechanisms results in a value lower than the one measured. A
theoretical model considering local fluctuations in the strong nuclear force
within the QGP as the driving seems to explain the φ0 mesons'
apparent spin alignment preference. When strong- force effects add up in a
direction they influence same-flavor particles like φ0 (strange
quarks ) in the same direction. In K*0 with mixed flavors (strange
and up-down quarks), the strong force is pointing in different directions, so
its influence would not show up as much. To test this idea, the researchers plan
to study the global spin alignment of J/ψ mesons (charm quarks). Finding a
global spin alignment preference for J/ψ mesons would add support to the local
fluctuations strong-force explanation. It would also validate the approach of
using these particles' global spin alignment as a way to study local
strong-force fluctuations in QGP.
For more information:
Nature, January 18 (2023); Phys.org, January 18 (2023).
Cherenkov radiation observed in 2D space:
researchers at the Technion-IIT in Haifa have observed Cherenkov radiation
confined in 2D and have demonstrated that in the 2D space, radiation behaves in
a completely different manner than in 3D. A quantum description of light,
revealing the quantum-photonic nature of electron radiation, is required to
explain the experiment's results. The researchers forced the electrons to travel
in proximity to a photonic-plasmonic surface and observed the spontaneous
quantum nature of radiation emission. The researchers here present the
observation of Cherenkov surface waves, where free electrons emit
narrow-bandwidth photonic quasiparticles propagating in 2D. The low
dimensionality and narrow bandwidth of the effect enabled the researchers to
identify quantized emission events through electron energy loss spectroscopy.
Their results support the recent theoretical prediction that free electrons do
not always emit classical light and can instead become entangled with the
photons they emit. The 2D Cherenkov interaction achieves quantum coupling
strengths over 2 orders of magnitude larger than before reported for 3D
Cherenkov radiation , reaching the single-electron–single-photon interaction
regime with free electrons. The electron velocity was accurately set to obtain a
large coupling strength, greater than in previous 3D Cherenkov radiation
experiments. The obtained electron radiation emission in this experiment was
very high. Whereas the most advanced previous 3D Cherenkov radiation experiments
achieved a regime in which ~ 0.01 of the electron population emitted radiation,
the researchers here achieved an interaction regime in which every electron
emitted radiation (interaction efficiency or coupling strength ~ 1). It was
observed here that free electrons can become entangled with the photons they
emit.
For more information:
Phys.org, January 18 (2023); Phys. Rev. X, January 6 (2023) page 011002.
WEEK OF JANUARY 16, 2023 [No. 1506]
P-wave interaction between spin-polarized fermions observed:
esearchers at the University of Toronto in Toronto, ON and JILA in Boulder, CO
have isolated two spin-polarized fermion 39K atoms, observed their
p-wave interaction strength, and found that the result confirms a longstanding
prediction. The researchers here create isolated pairs of spin-polarized
fermionic atoms in a multiorbital 3D optical lattice. They isolated pairs of
atoms within a 3D optical lattice created at the intersection of three laser
beams. The intersecting beams generated stationary nodes of high intensity which
trapped the pairs. They spectroscopically measure elastic p-wave interaction
energies of strongly interacting pairs of atoms near a magnetic Feshbach
resonance. The interaction strengths are widely tunable by the magnetic field
and confinement strength, and yet collapse onto a universal curve when rescaled
by the harmonic energy and length scales of a single lattice site. All
observations here are compared both to an exact solution using a p-wave
pseudopotential and to numerical solutions using an ab initio interaction
potential. The absence of three-body processes enables the observation of
elastic unitary p-wave interactions, as well as coherent oscillations between
free-atom and interacting-pair states. P-wave and other antisymmetric
interactions are weak in naturally occurring systems but it was predicted that
they have a much higher maximum theoretical limit; their enhancement via
Feshbach resonances in ultracold systems has been limited by three-body loss.
The researchers here have confirmed that the p-wave force between these
39K atoms reached the predicted maximum value.
For more information:
Nature, January 11 (2023) page 262; Phys.org, January 11 (2023).
Heat generation from a quantum phase slip measured in a Josepheson junction:
an international group lead by researchers at the University of Grenoble Alpes
in Grenoble has realized the calorimetry of a phase slip in a hysteretic
Josephson junction and observed heat generation in that elementary quantum
process. Irreversibility in Josephson junctions arises from abrupt slips of the
quantum phase difference across the junction. This quantum phase slip can be
visualized as the tunneling of a flux quantum in the transverse direction to the
superconducting weak link, which produces dissipation. The researchers here
detect the instantaneous heat release caused by a phase slip in a Josephson
junction, signaled by an abrupt increase in the local electronic temperature in
the weak link and a subsequent relaxation back to equilibrium. At the
instability point of the Φx(φ) relation, the phase drop φ and the
screening current abruptly relax to smaller values, as a quantum of flux tunnels
perpendicular to the Josephson junction, releasing heat. A local energy minimum
in the potential energy as a function of φ can become unstable as the externally
applied flux is changed. By macroscopic quantum tunneling of the phase, a lower
energy valley is reached, releasing energy. To measure the heat generated by the
superconducting quantum system, the researchers developed a method that can
measure and display the temperature curve to 1-µs accuracy throughout the
process of reading one qubit. They measured the electron temperature based on
the conductivity of the contacts. The issue is how quickly one can take the
measurements as changes to a quantum state take ~ 1 µs. The trick here was to
have the resistor measuring the temperature inside a resonator that produces a
strong response at a certain frequency. This resonator oscillates at 600 MHz and
can be read out fast as required here.
For more information:
Phys.org, January 10 (2023); Nat. Phys., January 5 (2023)
WEEK OF JANUARY 9, 2023 [No. 1505]
2D polarized photon-gluon tomography of ultrarelativistic nuclei demonstrated:
the STAR collaboration has used the RHIC accelerator at BNL in Upton, NY to map
out the arrangement of gluons within the nucleus with higher precision than
before and observe quantum interference between dissimilar particles that makes
their measurements possible. When two relativistic heavy nuclei pass one another
at a distance of a few nuclear radii, the photon around one nucleus may interact
through a virtual quark-antiquark pair with gluons from the other nucleus,
forming a short-lived Ï0 vector meson. In the experiments here, the
photon polarization is used in diffractive photoproduction process measured in
197Au + 197Au and 238U + 238U
collisions to observe a unique spin interference pattern in the angular
distribution of Ï0 → Ï€+π− decays. Thus,
the transverse linear polarization of photons in ultra-peripheral collisions is
used as an interferometry tool to explore the structure of heavy nuclei. The
observed interference is a result of an overlap of two wave functions at a
distance an order of magnitude larger than the Ï0 travel distance
within its lifetime. The strong-interaction nuclear radii were extracted from
these diffractive interactions and found to be 6.53 ± 0.06 fm
(197Au) and 7.29 ± 0.08 fm (238U), larger than the
nuclear charge radii. The observable is demonstrated to be sensitive to the
nuclear geometry and quantum interference of nonidentical particles. The method
here relies on photons that surround 197Au nuclei as they speed
around the collider and a quantum entanglement type not observed before. If two
197Au nuclei pass one another very closely without colliding, the
photons surrounding one nucleus can probe the internal structure of the other.
While photons do not interact directly with gluons (they do not carry color),
interactions can still occur when the photon temporarily fluctuates into a
quark-antiquark pair that, in turn, interacts with the gluons inside the
nucleus. The researchers had demonstrated before that the photons are polarized,
with their electric field radiating outward from the center of the
197Au nucleus. Then, they use that polarized light from one
near-miss high energy nucleus, to effectively image the other near-miss high
energy nucleus. The near-miss 197Au nucleus - 197Au
nucleus interaction here induces photon-gluon interaction that produces a
Ï0 meson that quickly decays (< 4 ys lifetime) into
π+and π- pair. By measuring the velocities and angles at
which these particles strike the RHIC's STAR detector, the researchers can
backtrack to get information about the photon, and then about the gluons. They
measured two outgoing particles and observe clearly that their charges are
different but they see interference patterns that indicate these particles are
entangled even though they are distinguishable particles. The measured
π+and π- particles experience a new type of quantum
entanglement. The closer the angle between either Ï€ and the Ï0 's
trajectory is to 90° , the clearer the obtained gluon distribution. When the
near-miss nuclei pass one another, it is as if two Ï0 are generated
(one in each nucleus) at a distance of 20 fm. As each Ï0 decays, the
wavefunctions of the Ï€- from each Ï0 decay interfere and
reinforce one another, while the wavefunctions of the π+from each
decay do the same, resulting in one π+and one π-
wavefunction striking the detector. These reinforcing patterns would not be
possible if the π+and π- were not entangled. The quantum
interference observed between the π+'s and the π- 's
makes it possible to measure the photons polarization direction very precisely.
And from that obtain the gluon distribution both along the direction of the
photons motion and perpendicular to it (2D imaging). All past measurements,
where the polarization direction was unknown, measured the density of gluons as
an average (as a function of the distance from the center of the nucleus). The
previous 1D imaging came out making the nucleus look too big when compared with
what was predicted by theoretical models and measurements of the distribution of
charge in the nucleus. The 2D imaging here shows that the momentum and energy of
the photons gets convoluted with that of the gluons. Measuring just along the
photons direction (or not knowing what that direction is) results in imaging
distorted by these photon effects. Additionally measuring in the transverse
direction prevents photon-induced imaging blurring. 2D imaging results allow to
distinguish the density of gluons at a given angle and radius. The images are so
precise that researchers can see the difference between where the protons are
and where the neutrons are laid out inside the large nuclei studied here. The
results maTch up qualitatively with the theoretical predictions of
nuclear radius based on gluon distribution, as well as measurements of nuclear
charge distribution. The sum of the momenta of π+π− gives
the momentum of the parent Ï0 particle and information about the
gluon distribution and the photon blurring effect. To only obtain the gluon
distribution, the researchers measure the angle between the path of either
Ï€+or Ï€- and the Ï0's trajectory. The closer
that angle is to 90°, the less blurring from the photon probe. By tracking
Ï€+π− 's that decay from Ï0 particles moving at
a range of angles and energies, the researchers can map out the gluon
distribution across the entire nucleus. The measurements are made possible
because the π+and π- particles striking the STAR detector
result from interference patterns produced by the new identified entanglement of
these two dissimilar oppositely charged particles. When the photons surrounding
two near-miss high energy nuclei interact with gluons inside the nuclei, it is
as if those interactions actually generate two Ï0 particles, one in
each nucleus. As each Ï0 decays into a Ï€+and
Ï€-, the wavefunction of Ï€- from one Ï0 decay
interferes with the wavefunction of the π- from the other. When the
reinforced wavefunction strikes the STAR detector, the detector sees one
Ï€-. The same thing happens with the wavefunctions of the two
π+, and then the detector sees one π+. The interference
is between two wavefunctions of the identical particles, but without the
entanglement between the two dissimilar particles (the π+and the
Ï€-) this interference would not materialize. The Ï0s could
not be entangled. The Ï0 particle wavefunctions originate at a
distance 20 times the distance they could travel within their short lifetime, so
they cannot interact with each other before they decay to π+and
π-. But the wavefunctions of the π+and π- from
each Ï0 decay retain the quantum information of their parent
particles; their crests and troughs are in phase, despite striking the detector
meters apart. If the π+and π- were not entangled, the two
wavefunctions would have a random phase, without any detectable interference
effect. And, thus the researchers would not see any orientation related to the
photon polarization and make these precision measurements.
For more information:
Science Advances, January 4 (2023); Phys.org, January 4 (2023).
2D ferroelectricity - superconductivity coupling observed:
an international group lead by researchers at Columbia University in New York,
NY has demonstrated the coexistence of superconductivity and ferroelectricity
(two properties thought to be incompatible with one another) in bilayer
Td-MoTe2 (non-centrosymmetric temperature-driven phase
transition [Td phase] of the semiconductor MoTe2). Recent
investigations have revealed superconductivity tunable by electrostatic doping
in twisted graphene heterostructures and in 2D semimetals such as
WTe2. Some of these systems have a polar crystal structure that gives
rise to ferroelectricity, in which the interlayer polarization exhibits
bistability driven by external electric fields. The researchers here show that
bilayer Td-MoTe2 (this material is extremely air-sensitive
and needs encapsulation in an inert environment to probe its properties in the
ambient) simultaneously exhibits ferroelectric switching and
superconductivity. They have observed a tunable switch between
ferroelectricity and superconductivity with both properties intrinsic to the
material and created a superconducting switch controlled by an
external voltage. The researchers found initial evidence of superconductivity in
single layers of MoTe2. Then, they found superconductivity to be much
easier to control in a two-layer system (not symmetric). Atoms between the
layers do not line up, which should inherently create an internal electric field
(ferroelectricity). Whereas superconductivity is typically characteristic of
electricity-conducting metals, ferroelectricity was long thought to only occur
only within insulating materials. In 2018, they found initial evidence of a
ferroelectric metal in WTe2, with the same crystal structure as
MoTe2. Having already confirmed that superconductivity occurs in
MoTe2, they wondered if ferroelectricity might also turn up. When
they applied an electric field, they observed it. The team performed additional
measurements across temperatures, magnetic fields, and electrostatic doping and
found a manipulatable transition between superconductive and ferroelectric
states within MoTe2. Since the MoTe2 is an extremely
air-and water-sensitive material, the layers had to be carefully prepared and
combined in the confines of a glovebox and then sealed to prevent exposure to
the ambient environment as they were transported to different instruments for
testing. The superconducting state itself is surprising, particularly in the
way it behaves as the number of charge carriers is changed by gating.
Theoretical modeling suggest that superconductivity is not of the conventional
type with phonon induced pairing. It requires two types of charge carriers
(electron-like and hole-like carriers). A field-driven, first-order
superconductor-to-normal transition is observed at its ferroelectric transition.
Bilayer Td-MoTe2 also has a maximum in its superconducting
transition temperature (Tc) as a function of carrier density and
temperature, allowing independent control of the superconducting state as a
function of both doping and polarization. They find that the maximum
Tc is concomitant with compensated electron and hole carrier
densities and vanishes when one of the Fermi pockets disappears with doping. The
researchers argue that this unusual polarization-sensitive 2D superconductor is
driven by an interband pairing interaction associated with nearly nested
electron and hole Fermi pockets.
For more information:
Nature, January 4 (2023) page 48; Phys.org, January 5 (2023).
WEEK OF JANUARY 2, 2023 [No. 1504]
Topological junction metasurface for efficient beam emission:
researchers in Hanyang University in Seoul have developed a topological beam
emitter submicron structure with high beam emission efficiency, small angular
divergence, and adaptable beam shaping capacity. The researchers studied the
far-field optical properties associated with non-Hermitian topological photonics
and showed how a topological junction metasurface of two distinct thin-film
subwavelength guided-mode resonance gratings directly adjacent to each other in
the absence of an aperture can serve as efficient emitters. In these structures,
the leaky Jackiw-Rebbi (JR) state at the junction, emits a narrow beam of light.
In leaky photonic systems such as guided-mode resonance gratings, thin-film
photonic crystals, and metasurfaces, the non-Hermiticity naturally arises
because of their inherent radiative decay processes (towards the radiation
continuum). These decay channels in the optical far field can be efficiently
used to coherently control or probe certain desired topological states for their
spectral intensity, phase, and polarization properties. The leaky JR state leads
to in-plane optical confinement with funnel-like energy flow and enhanced
emission probability, resulting in highly efficient optical beam emission. In
addition, the structure allows adaptable beam shaping for any desired profiles
by Dirac mass distribution control, which is possible with moderate fill factor
variations allowed in chip integrated optic architectures and it can be directly
encoded in lattice geometry parameters. The process here is driven by cavity-QED
coupling and electromagnetic funneling effects. The researchers report
theoretical and numerical analyses of experimental structures operating in VIS.
They have explored the leaky JR state localized at a photonic topological
junction metasurface, where the structure maintained a high-index film. Under
specific conditions, the first-order diffraction from the JR state leads to beam
leakage radiation emission. They used FEA to calculate the radiation pattern,
which showed a narrow beam emitted in the optical far-field. Then, they designed
a structure, where two grating regions have identical Dirac mass to achieve
ideal symmetry of the emitted beam. During these experiments, the narrow beam
emission from the isotropic light sources followed the exact diffraction
properties of radiation leakage from the JR state. For an optimized design, the
beam divergence approaches the uncertainty principle minimum value and the
emission rate increases by an order of magnitude. The topological beaming effect
allows regulation of the beam shape directly from the source. The researchers
described the Dirac mass distribution required to generate the expected beam
profile. A flat top beam is generated by extending a zero Dirac mass region
across the desired width and around the junction of the device.
For more information:
Phys.org, December 30 (2022); ScienceAdvances, December 9 (2022).
NOTE: previous Research News (since WEEK OF MARCH 1, 1994 [No. 1],
around the time when the Quantum Cascade Lasers were demonstrated at
AT&T Bell Laboratories in Murray Hill, N.J., as promising MIR
solid-state room temperature sources that would enable laser
spectroscopy in the spectral region where fundamental
rotational-vibrational transitions of most molecules take place) not posted.
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