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Rodolfo Carrera, Editor
WEEK OF MARCH 25, 2024 [No. 1568]
Valley-coherent QAH state observed in semiconductor twisted
bilayer:
researchers at Cornell University in Ithaca, NY have
observed aligned spin polarization in semiconductor
MoTe2/WSe2 twisted bilayers. They have shown that
the recently discovered QAH state in AB-stacked
MoTe2/WSe2 moiré bilayers is not valley polarized
but valley coherent. Their layer- and helicity-resolved optical
spectroscopy measurements reveals that the QAH ground state possesses
spontaneous spin (valley) polarization aligned (antialigned) in two TMD
layers. Saturation of the out-of-plane spin polarization in both layers
occurs only under high magnetic fields, supporting a canted spin
texture. They found that, when this material exhibits quantum anomalous
Hall effect (Hall resistance quantized under zero magnetic field, QAHE),
electronic states are hybridized from different valleys (K and K′) of
the two layers, contrary to theoretical expectations. In TMD moiré
semiconductors, electrons can be labeled by their valley degree of
freedom, denoting which of the two valleys in momentum space the
electrons reside in. The researchers report that in one such
semiconductor the electrons are in a quantum superposition of the two
valleys (valley coherent), contrary to what has been thought up to now.
With the two layers’ atomic lattices in momentum space, the K and K′
valleys are coherent, leading to an aligned spin polarization. In 2021,
QAHE was observed in a twisted bilayer formed of MoTe2 and
WSe2 monolayers. This effect emerges when the material’s
electronic structure is engineered to yield topologically protected
states. The researchers here have used optical spectroscopy to study the
interaction between these two monolayers when they are in the QAH state.
They show that theoretical predictions about this interaction are not
correct. Their measurements are layer resolved as well as handedness
resolved (distinguish which valley the electrons are in). This
resolution reveals that the QAH ground state possesses spontaneous
valley coherence. The QAHE has been observed in graphene and twisted
graphene bilayers. It is thought to arise from the interaction-driven
valley polarization of the narrow moiré bands. Although, the origin of
the QAHE in twisted graphene multilayers is well established, it was
surprising to observe the phenomenon in twisted bilayers of TMDs such as
MoTe2 and WSe2, where the bilayer’s electronic
structure was thought to be described by a simple, existing theoretical
model. The QAHE’s origin in these moiré materials is still not
understood. The two momentum space valleys in the conduction band K and
K ′ have a spin polarization of up or down in materials with an
asymmetric crystal structure, such as MoTe2 and
WSe2. The K valley has a spin polarization of up in
MoTe2 and of down in WSe2, and the K ′ valley has
the opposite spin polarization. Such spin–valley coupling implies that
twisted MoTe2/WSe2 bilayers can exist in one of
two regimes. In the valley-coherent regime, the two layers have an
aligned spin polarization, and electronic states are hybridized from the
K valley of one layer and the K ′ valley of the other. In the
valley-polarized regime, the two layers have an antialigned spin
polarization, and states are hybridized from the same valley of the
different layers. Twisted graphene multilayers showing QAHE are valley
polarized, and the theoretical expectation was that twisted
MoTe2/WSe2 bilayers exhibiting the phenomenon
would be as well. To test this prediction, the researchers here used
helicity-resolved reflectance-contrast spectroscopy ( circularly
polarized light is used with no external magnetic field). If the
material is spin polarized, its optical response will be different for
the two possible helicities of the circularly polarized light (magnetic
circular dichroism, MCD). For twisted MoTe2/WSe2
bilayers, the two layers have identical optical responses for the same
spin polarization but are optically excited at different energies. This
feature allowed the researchers to determine the spin polarization of
each layer by measuring the bilayer’s optical response as a function of
energy, magnetic-field strength, and light helicity. The MCD
measurements revealed that, in the QAH state, the two layers had an
aligned spin polarization, corresponding to the valley-coherent regime
rather than the expected valley-polarized regime. They noted a
dependence of the MCD on the strength of the applied magnetic field with
the magnitude of the MCD at 0 T being about half the value at > 6 T.
The MCD probes the out-of-plane spin polarization, so this reduced value
at zero field suggests that the bilayer’s spin polarization has an
unanticipated in-plane component. These unexpected results indicate that
the current theory of the QAH state in this moiré material is deficient.
They also highlight the difference between TMD bilayers and graphene
multilayers.
For more information:
Physics, March 19 (2024); Phys. Rev. X, January
10 (2024) page 011004.
Weak fluctuations in superconductivity analyzed:
researchers at the Tokyo Institute of Technology in Tokyo have discovered a
quantum critical point (QCP) in 2D superconductors and a broadened
quantum critical ground state in a disordered superconducting thin film.
They realized Nernst effect measurements with high sensitivity to the
fluctuations of the superconducting order parameter. From a mapping of
the Nernst effect in the B-T plane, they found a thermal-to-quantum
crossover line of the superconducting fluctuations, a temperature line
associated with the quantum phase transition and a field line associated
with a thermal transition. The QCP is identified as the T = 0 intercept
of the temperature line inside the anomalous metallic (AM) state
(confirming that the AM state is a broadened critical state of the
magnetic field-induced superconductor-to-insulator transition, SIT,
caused by quantum fluctuations). They measured the thermoelectric effect
in superconductors and analyze the fluctuations in superconductivity
over a wide magnetic field range and over a wide temperature range (from
over Tc ≈ 2.4 K to T ≈ 0.1 K). They demonstrate that the
origin of the AM state in a magnetic field is the existence of a QCP
(where quantum fluctuations are strongest). In a magnetic field of
moderate magnitude, magnetic flux lines penetrate in the form of defects
accompanied by vortices of superconducting currents. For samples with
relatively weak localization effects, an AM state appears in the
intermediate magnetic field region where the electrical resistance is
several orders of magnitude lower than the normal state. This is thought
to be produced by a liquid-like state in which magnetic flux lines that
penetrate into the superconductor move around due to quantum
fluctuations. However, this has not been demonstrated because most
experiments on 2D superconductors have used electrical resistivity
measurements that examine the voltage response to current, and it is
difficult to distinguish between voltage signals originating from the
motion of magnetic flux lines and those originating from the scattering
of normally-conducting electrons. Quantum motion of magnetic flux lines
occurring in an AM state was reported in 2020 by using the
thermoelectric effect with the motion of magnetic flux lines and the
superconducting fluctuations generate a voltage perpendicular to the
heat flow. The researchers here clarify the transition mechanism of the
SIT. They used Mox Ge1-x , 10-nm thin films with
amorphous structure. When a temperature difference is applied in the
longitudinal direction of the sample, the superconducting fluctuations
and the motion of the magnetic flux lines generate a voltage in the
transverse direction. In contrast, normal electron motion generates
voltage primarily in the longitudinal direction. In samples such as
amorphous materials, where electrons do not move easily, the voltage
generated by electrons in the transverse direction is negligible, so the
fluctuation contribution alone can be selectively detected by measuring
the transverse voltage. Thermoelectric effect measurements, can detect
superconductivity-amplitude fluctuations and superconductivity-phase
fluctuations (magnetic flux line motion). The superconducting
fluctuations survive in the liquid region of the magnetic flux (where
superconducting phase fluctuations are more pronounced), and over a wide
temperature-magnetic field region including where there is no
superconductivity. The researchers have determined the contour map of
the Nernst signal in the B-T plane, revealing an evolution of the
superconducting fluctuations in the disordered 2D superconductor
exhibiting the superconductor-anomalous metal-insulator transition. They
found the thermal-to-quantum crossover temperature line T*(B)
associated with the quantum phase transition, as well as the field line
B*(T) associated with a thermal transition. The T = 0 QCP
determined from T*(B → BQCP) → 0 is located inside
the AM state, indicating that the AM state is a broadened critical state
of the SIT.
For more information:
Phys.org, March 16 (2024); Nat. Comm., March 16
(2024) page 442.
WEEK OF WEEK OF MARCH 18, 2024 [No. 1567]
Cooperative Lamb shifts due to atom gas dipole-dipole
interactions measured:
researchers at the University of
Regensburg in Regensburg have studied the dynamics of atomically
localized energy levels and their interaction with the environment
enabling local control of discrete energy levels. They resolve in space,
time and energy the evolution of a spin–orbit-split energy level in an
isolated Se vacancy in a moiré-distorted WSe2 monolayer using
controlled excitation of lattice vibrations by lightwave-driven scanning
tunneling spectroscopy (LW-STS). By locally launching a phonon
oscillation and taking ultrafast energy-resolved snapshots of the
vacancy’s states faster than the vibration period, they directly measure
the impact of electron–phonon coupling in an isolated single-atom
defect. The researchers used scanning tunneling ultrafast, atom-scale
spectroscopy (combination of sub-ps temporal, Ã… spatial, and meV energy
resolutions) to observe and control how the energy of a single electron
is tuned by the vibrations of the surrounding atoms in a monolayer
crystal. A discrete energy level of an atomic vacancy in an atomically
thin material shifts upon excitation of a drum-like vibration. The
temporal evolution of the localized energy level is intricately linked
to the atomic excursions by the drum mode. They changed a discrete
energy level on a vacancy defect by triggering a drum-like vibration of
an atomically thin membrane: the atomic motion of the surrounding atoms
shifts and thus controls the energy level of the vacancy. The
researchers here introduce time-resolved LW-STS to demonstrate with
direct real-space access how atomic motion transiently modulates
spin–orbit-split energy levels of a single Se vacancy in a
moiré-distorted WSe2 monolayer on Au. Ultrafast snapshots of
electronic tunneling spectra reaching atomic spatial and 300 fs temporal
resolution reveal transient energy shifts of the lowest bound defect
state by up to 40 meV, and changing according to the amplitude and phase
of the locally excited coherent lattice vibration. The observed shift is
larger than thermal smearing at RT. The technique here allows to
directly correlate ultrafast electronic energy shifts with local atomic
displacement. The combination of atomic, sub-ps and 10-meV-scale
resolution in LW-STS allowed the researchers to observe how the energy
levels of a single Se vacancy evolve during dynamic atomic
displacements. They revealed how the excitation of an acoustic drum mode
adiabatically shifts the first defect level on timescales shorter than
the oscillation period. The unexpected unipolar and non-monotonic
behavior of the shift with the oscillation amplitude is caused by the
interplay of lattice distortions.
For more information:
Phys.org, March 14 (2024); Nat. Phot., March 14
(2024).
Penning micro-trap built:
researchers at ETH in
Zurich have trapped a cooled ion using static electric and magnetic
fields and performed quantum operations on it, including moving the
trapped ion in a 2D plane and illuminating it with a laser beam. They
constructed a Penning trap based on a superconducting 3 T magnet and a
microfabricated chip with several electrodes. They demonstrate full
quantum control of an ion in this setting, as well as the ability to
transport the ion arbitrarily in the trapping plane above the chip.
Using a system of cryogenically cooled mirrors, they channeled the laser
light through the magnet to the ions. A single trapped ion (that can
stay in the trap for several days), was moved arbitrarily on the chip
(including connecting points as the crow flies) by controlling the
various electrodes. Since no oscillating fields are needed for trapping,
many of those traps can be packed onto a single chip. The researchers
demonstrated that the qubit energy states of the trapped ion could also
be controlled while maintaining quantum mechanical superpositions.
Coherent control worked both with the electronic (internal) states of
the ion and the (external) quantized oscillation states as well as for
coupling the internal and external quantum states (a prerequisite for
creating entangled states). They plan to trap next two ions in
neighboring Penning traps on the same chip and demonstrate that quantum
operations with several qubits can also be performed. On a QCCD
architecture, the Penning QCCD can be envisioned as a scalable approach,
in which a micro-fabricated electrode structure enables the trapping of
ions at many individual trapping sites, which can be actively
reconfigured during an algorithm by changing the electric potential.
Beyond the static arrays considered in previous work, the researchers
here conceptualize that ions in separated sites are brought close to
each other to use the Coulomb interaction for two-qubit gate protocols
implemented through an applied laser, before being transported to
additional locations for further operations. The transport of ions can
be performed in 3D almost arbitrarily without the need for specialized
junctions, enabling flexible and deterministic reconfiguration of the
array with low spatial overhead. The researchers demonstrate here the
basic building block of such an array by trapping a single ion in a
cryogenic micro-fabricated surface-electrode Penning trap. They show
quantum control of spin and motional degrees of freedom and measure a
heating rate lower than in any comparably sized rf trap. They use this
system to demonstrate flexible 2D transport of ions above the electrode
plane with negligible heating of the motional state. The experimental
setup involves a single 9Be+ confined using a
static quadrupolar electric potential generated by applying voltages to
the electrodes of a surface-electrode trap. They use a radially
symmetric potential, centered at a position 152 μm above the chip
surface. The trap is embedded in a homogeneous magnetic field ≈ 3 T
aligned along the z-axis, supplied by a superconducting magnet. The trap
assembly is placed in a cryogenic, ultrahigh vacuum chamber that fits
inside the magnet bore, reducing background-gas collisions and motional
heating. Using a 235 nm laser, they loaded the trap by
resonance-enhanced multiphoton ionization of the neutral atoms produced
from either a resistively heated oven or an ablation source. They
regularly trapped single ions for more than a day, with the primary loss
mechanism being related to user interference.
For more information:
Nature, March 13 (2024) page 510; Phys.org,
March 13 (2024).
WEEK OF MARCH 11, 2024 [No. 1566]
Quark coalescence observed:
researchers of the LHCb
Collaboration. have used the LHC accelerator at CERN in Geneva to
observe enhanced production of Λb0 baryons in pp
collisions at COM energy √s = 13  TeV. The production rate of
Λb0 baryons relative to B0 mesons in pp
collisions was measured in the LHCb experiment. The ratio of
Λb0 to B0 production cross sections
shows a significant dependence on both the transverse momentum and the
measured charged-particle multiplicity. At low multiplicity, the ratio
measured at LHCb is consistent with the value measured in
e+e- collisions, and increases by a factor ≈ 2
with increasing multiplicity. At relatively low transverse momentum, the
ratio of Λb0 to B0 cross sections is
higher than what is measured in e+e- collisions,
but converges with the e+e- ratio as the
transverse momentum increases. These results imply that the evolution of
heavy b quarks into final-state hadrons is influenced by the density of
the hadronic environment produced in the collision. The researchers here
examined the production rates of Λb0 baryons and
B0 mesons. They monitored relative changes between these two
production rates for pp collisions that created charged particles in
different numbers and with different transverse momenta. The
collaboration found that the production rate of
Λb0 baryons increased relative to that of
B0 mesons as the number of charged particles increased or as
the transverse momentum of the particles decreased. These trends imply
that, as the particle environment becomes denser, bottom quarks are
increasingly more likely to be incorporated into baryons, as opposed to
mesons. The researchers conclude that this result disfavors conventional
models in which the consolidation of quarks is independent of the
particle environment. Instead, it fits with alternative models in which
baryon formation depends on the environment’s density. They have shown
that bottom quarks are increasingly more likely to exist in three-quark
states rather than two-quark ones as the density of their environment
increases. And thus, that quark coalescence (existing quarks with
overlapping wavefunctions combine when in a QGP rather than create new
quarks) plays a role in the evolution of quarks into hadrons following
proton collisions. It is most pronounced at low transverse momenta, and
gradually turns off as quarks escape rapidly from the collision point.
Coalescence in heavy ion collisions has been generally accepted to
explain the ratios of protons to pions produced in experiments. The
researchers here studied the production of b quarks in pp collisions.
 The production of b quarks is almost certain to produce either a
Λb0 baryon or a B0 meson, with both
containing a b quark. The production ratio between these two has been
studied in experiments in which the b quark is produced by e-p
collisions. This leads only to fragmentation and with only
fragmentation, this ratio should be universal. The researchers here
combed through several years’ data on pp collisions and studied the
decay products from collisions that had produced b quarks. For
collisions with high transverse momenta relative to the colliding beams
and few other outgoing particles detected at the same time, the
baryon-to-meson ratio was approximately equal to the ratio in e-p
experiments. However, as the transverse momenta dropped and as the
number of other particles detected simultaneously grew, the proportion
of baryons gradually increased relative to the proportion of mesons. The
researchers thought that another process more likely to produce baryons
was at work in these collisions. In this scenario the b quark is
surrounded by other quarks but became increasingly disfavored as the
produced quark was more separated from the other particles. The
researchers concluded that coalescence was required to explain that.
They systematically showed how the effect goes away and recovers the
electron-positron limit as a function of how many hadrons are observed,
which is an observable measuring how many quarks and antiquarks there
are to coalesce with. The underlying assumptions used to calculate the
fragmentation fractions involve the quarks being isolated, so the
results at low transverse momenta might be incorrect when this is not
the case.
For more information:
Physicsworld, March 6 (2024); Physics, February
20 (2024); Phys. Rev. Let., February 20 (2024) page 081901.
Electron charge fractionalization observed in metallic kagome
ferromagnet:
an international group of researchers at the Paul
Scherrer Institute in Villigen has spectroscopically observed the
fractionalization of electronic charge at low temperature in the
topological ferromagnetic metal Fe3Sn2. The
researchers describe spectroscopy revealing non-Fermi-liquid behavior
for the studied material. They discover three C3-symmetric
electron pockets at the Brillouin zone center, two of which are expected
from DFT. The sharpest third band emerges at low temperatures and at low
binding energies by means of fractionalization of one of the other two
bands. This seems to be due to enhanced e-e interactions owing to a flat
band predicted to lie just above the Fermi level. The researchers
promoted strong interactions and collective behavior. by creating a
lattice structure that reduces electron kinetic energies and allows
electron interaction. In rhombohedral Fe3Sn2 , a
ferromagnetic metal with kagome lattice, the building blocks are
bilayers of distorted, three-fold symmetric kagome layers of Fe atoms
alternating with single layers of stanene. It has high Curie temperature
(640 K), and undergoes a first-order spin-reorientation transition near
120 K. Atoms assemble in Fe3Sn2 according to the
triangular kagome pattern, with all corner sites removed on a 2 × 2
superlattice (pattern of corner-sharing triangles, trihexagonal tiling
lattices). Initially, the researchers were just interested in verifying
whether flat bands existed as predicted for this ferromagnetic material.
Using micro-focused laser angle-resolved photoemission spectroscopy
(µ-ARPES) to overcome averaging over crystallographic twins and increase
the surface sensitivity of conventional synchrotron-based ARPES, they
probe the local electronic structure of the material at high resolution.
The band structure in kagome Fe3Sn2 is different
depending on which ferromagnetic domain is probed. Their interest was to
detect inhomogeneities in the electronic structure correlated to domains
that had been undetected previously. Focusing on certain crystal
domains, they identified electron pockets of minimum electron energy
where electrons behave as quasiparticles. They observed a dispersive
band not continuous but extremely sharp with sudden cut off, that did
not match with DFT predictions. They observed a dispersive band
interacting with a flat band, possibly allowing a new phase of matter to
emerge. When the two bands meet, they hybridize to make a new band. The
original dispersive band is occupied. The flat band is unoccupied as it
lies above the Fermi level. When the new band is created, the charge is
split between the original dispersive band and the new band. The
combination of high-resolution spectroscopy with single domain selection
allowed examination of the quasiparticles in the strongly correlated
kagome metal. At low temperature, after elastic scattering contributions
were subtracted (the scattering rates for the α band, predicted by DFT),
quasiparticles scaled with their momenta, implying marginal rather than
ordinary Fermi liquid behavior and associated strong correlation
phenomena. However, the peculiarities of the sharpest band (β, not
accounted for by DFT), indicate that Fe3Sn2 is
hosting anomalous strong correlations. The β-band spectral weight comes
at the expense of diminishing γ weight, raising the possibility that, on
cooling, electrons are fractionalized between β and γ bands in ratio ≈
1:2 (corresponding to charge 1/3 quasiparticles). The flat band seen in
DFT just above EF may be a relevant feature. Hybridization of
γ with this band could enhance the e-e interactions beyond the already
large U/W ( effective Coulomb interaction / kinetic energy) ≈ 10
obtained from the ARPES/DFT work, thus accounting for the sharp and
displaced β band, which appears < 70 K together with other anomalous
magnetotransport properties.
For more information:
Nature, March 6 (2024) page 67; Phys.org, March 6
(2024)
WEEK OF MARCH 4, 2024 [No. 1565]
Focused high energy electron beam demonstrated in Si dielectric laser
accelerator:
researchers at Stanford
University in Stanford, CA have built a 0.5-mm-long Si subrelativistic,
alternating phase focusing dielectric laser accelerator (DLA) that
speeds up and confines electrons, creating a focused beam of high-energy
electrons. They demonstrate a Si-based electron accelerator that uses
laser optical near fields to both accelerate and confine electrons over
extended distances. They built the Si structure with a sub-µ channel
placed in a vacuum system, injected electrons into one end and
illuminated the structure from both sides with a shaped laser pulse that
delivered leaps of kinetic energy, gaining 23.7 keV of electron energy
(25% greater than their starting energy). Periodically, the laser fields
flipped between focusing and defocusing properties, which bunched the
electrons together, keeping them from swerving off track. Two DLA
designs were tested, each consisting of two arrays of Si pillars pumped
symmetrically by pulse front tilted laser beams, designed for average
acceleration gradients of 35 and 50  MeV/m, respectively. The DLAs are
designed to act as alternating phase focusing (APF) lattices, where
electrons, depending on the electron-laser interaction phase, will
alternate between opposing longitudinal and transverse focusing and
defocusing forces. By incorporating fractional period drift sections
that alter the synchronous phase between ± 60° off crest, electrons
captured in the designed acceleration bucket experience half the peak
gradient as average gradient while also experiencing strong confinement
forces that enable long interaction lengths. The researchers demonstrate
APF accelerators with interaction lengths up to 708  μm and energy gains
up to 23.7 ± 1.07  keV FWHM, a 25% increase from starting energy. Their
ability to steer electrons has been limited to 2D so far; 3D electron
confinement will be required to allow the accelerator to be long enough
for greater energy gains to occur.
For more information:
Phys.org, February 26 (2024); Phys. Rev. Lett., February 23 (2024) page 085001
H2+ molecular vibration
measured:
researchers at Heinrich-Heine-Universität Düsseldorf
in Düsseldorf have used laser spectroscopy to precisely measure a
rovibrational transition in the molecule
H2+, with results closely
matching theoretical predictions. They studied a first-overtone
electric-quadrupole transition and measured its two hyperfine
components. The determined spin-averaged vibrational transition
frequency has a fractional uncertainty = 1.2 × 10−8 and is in
agreement with the theoretically predicted value. They measured an
analogous electric-quadrupole transition in HD+ to estimate
systematic uncertainties. The researchers observed a much improved line
quality factor compared to previous electric-quadrupole spectroscopy of
molecular ions. H2+ although
being the simplest molecule, has remained relatively unexplored to date.
Due to the charge and mass symmetry of the two atomic nuclei, the
molecule absorbs and emits almost no VIS and IR radiation so it is
almost impossible to observe it with telescopes. The researchers here
have taken a direct look at how the
H2+ molecule can be made to
rotate and vibrate using laser light.
H2+ has no electric dipole moment
so they made use of the molecule's electric quadrupole moment although
their transition rate is substantially lower compared with electric
dipole moments. The researchers developed a laser system that proved
effective in exciting a transition between two vibrational states,
providing high power monochromatic laser radiation at 2.4 µm. They
confined the H2+ molecules in a
trap and used another laser to cool them near 0 K. The comparison of
precise predictions of the energy levels of
H2+ with measured vibrational
transition frequencies allows a direct determination of the
mp / me ratio
determined here with a relative uncertainty = 3×10−8. This
work demonstrates that first-overtone electric-quadrupole transitions
are suitable for precision spectroscopy of molecular ions and that
determining the mp /
me ratio with laser spectroscopy could
become competitive with mass spectrometry using Penning traps.
For more information:
Phys.org, February 27 (2024); Nat. Phys., January 12 (2024) page 383.
WEEK OF WEEK OF FEBRUARY 26, 2024 [No. 1564]
Non-Abelian anyons observed in a quantum processor
researchers at Quantinuum in Boulder, CO have produced non-Abelian
topological order and anyons on a trapped-ion processor. The researchers
started with a lattice of 27 trapped ions. They used partial, targeted
measurements to sequentially increase the complexity of their quantum
system, ending up with an engineered quantum wave function with the
exact properties and characteristics of the particles they were after.
They present the realization of non-Abelian topological order in the
wavefunction prepared in a quantum processor and demonstrate control of
its anyons. Using an adaptive circuit on Quantinuum’s H2 trapped-ion
quantum processor, they create the ground-state wavefunction of
D4 topological order on a kagome lattice of 27 qubits, with
fidelity per site > 98.4 %. By creating and moving anyons along
Borromean rings in spacetime, anyon interferometry detects an
intrinsically non-Abelian braiding process. Tunneling non-Abelian anyons
around a torus creates all 22 ground states, as well as an excited state
with a single anyon, a feature of non-Abelian topological order.
For more information:
Phys.org, February 21 (2024); Nature, February
14 (2024) page 505.
Fractional quantum Hall effect observed in graphene without
magnetic field:
researchers at MIT in Cambridge, MA have
observed fractional quantum anomalous Hall effect (FQAHE) with electrons
strongly interacting and passing through as fractions of their total
charge with no need for any external magnetic field in crystalline
pentalayer graphene / hexagonal boron nitride (hBN) moiré superlattices.
FQAHE is predicted to exist in topological flat bands under spontaneous
time-reversal-symmetry breaking.
Recently, researchers at the University of Washington observed FQAHE
in twisted MoTe2 at a moiré filling factor v > 1/2. The
researchers here prepared the material in a specific configuration
predicted to give the material an inherent magnetic field, enough to get
electrons to fractionalize without any external magnetic control.
Independently, the researchers here observed signs of anomalous
fractional charge in graphene. Then, they theoretically figured that
electrons might interact with each other even more strongly if the
pentalayer structure were aligned with hexagonal boron nitride (hBN). In
combination, the two materials should produce a moiré superlattice that
could slow electrons down in ways that mimic a magnetic field. The
researchers fabricated two samples of the hybrid graphene structure by
exfoliating graphene layers from a block of graphite and then using
optical tools to identify five-layered flakes in the steplike
configuration. They then stamped the graphene flake onto an hBN flake
and placed a second hBN flake over the graphene structure. They attached
electrodes to the structure and placed the sample in a dilution
refrigerator. As they applied a current to the sample and measured the
voltage output, they observed signatures of fractional charge with
voltage equaling the current multiplied by a fractional number. The
researchers report the observation of integer and fractional QAH effects
in a rhombohedral pentalayer graphene–hBN moiré superlattice. At zero
magnetic field, they observe plateaus of quantized Hall resistance at
v = 1, 2/3, 3/5, 4/7, 4/9, 3/7 and 2/5 of the moiré superlattice,
respectively, accompanied by clear dips in the longitudinal resistance
Rxx. Rxy equals at v = 1/2 and varies linearly
with v, similar to the composite Fermi liquid in the half-filled lowest
Landau level at high magnetic fields. By tuning the gate-displacement
field D and v, they observed phase transitions from composite Fermi
liquid and FQAH states to other correlated electron states.
For more information:
Nature, February 21 (2024) page 759; Phys.org,
February 21 (2024).
WEEK OF WEEK OF FEBRUARY 19, 2024 [No. 1563]
Quantum squeezing in an optomechanical system at RT:
researchers at EPFL in Lausanne have built an ultra- low noise
optomechanical cavity achieving optical squeezing in a well-isolated
from the environment nanopillar-loaded, 4mm-drum mechanical oscillator
sandwiched by two periodically segmented mirrors, allowing the laser
light to strongly interact with the drum quantum mechanically at RT. By
demonstrating optical squeezing at RT, they could effectively control
and observe quantum phenomena in a macroscopic system without the need
for extremely low temperatures. By using phononic-crystal-patterned
cavity mirrors, they reduce the cavity frequency noise by more than
700-fold. In this ultralow noise cavity, they insert a membrane
resonator with high thermal conductance and a quality factor Q = 1.8
x108, engineered using recently developed soft-clamping
techniques. These advances enable the operation of the system within a
factor of 2.5 of the Heisenberg limit for displacement sensing, leading
to the squeezing of the probe laser by 1.09(1) dB below the vacuum
fluctuations. The long thermal decoherence time of the membrane
oscillator (30 vibrational periods) enables the researchers to prepare
conditional displaced thermal states of motion with an occupation of
0.97(2) phonons using a multimode Kalman filter. In recent years,
quantum control has been extended to solid-state mechanical resonators,
both with radiation pressure optomechanical coupling and piezoelectric
coupling with superconducting qubits. Cavity optomechanics, in which the
mechanical oscillator is dispersively coupled to an optical cavity, has
enabled advances like ground state cooling, optomechanical light
squeezing and entanglement of separate mechanical oscillators, all
necessitating cryogenic cooling to reduce thermal fluctuations. To enter
the quantum regime of optomechanics, the product between the total force
noise (including environment thermal force as well as
measurement-induced backaction) and the displacement measurement
imprecision must approach the limit set by the Heisenberg uncertainty
principle. Using an ultralow noise cavity in conjunction with a phononic
crystal structured, density-modulated membrane, the researchers here
operate in the quantum regime of cavity optomechanics at RT. With a
single-detector homodyne scheme countering thermal-intermodulation
noise, they demonstrate optomechanical squeezing and conditional state
preparation of displaced thermal states with single-phonon occupation, a
prerequisite for real-time quantum control of the macroscopic mechanical
resonator. Their work extends the quantum control of solid-state
macroscopic oscillators to RT. With reasonable improvement of the
mechanical quality factor and a wider mechanical bandgap, together with
a real-time digital feedback using the studied optimal Kalman filters,
the researchers expect measurement-based feedback cooling to the ground
state to be feasible with continuous or gated feedback. Non-Gaussian
states at RT can also be prepared using nonlinear measurements (like
photon counting), inherently compatible with the nonlinear noise
cancellation scheme.
For more information:
Nature, February 14 (2024) page 512; Phys.org,
February 14 (2024).
Boson cooling in dimensional reduction shown:
researchers from the Universities of Innsbruck and Geneva in Innsbruck
and Geneva respectively have developed a new thermometry method to
measure temperatures for low-dimensional quantum gases and shown that a
strongly interacting quantum many-body system can experience cooling
when the dimensionality is reduced. They implement thermometry on
strongly interacting 2D and 1D Bose gases with high sensitivity in the
nK temperature range. Their method is aided by the fact that the decay
of the first-order correlation function is very sensitive to the
temperature when interactions are strong. They find that there may be a
substantial temperature variation when the 3D quantum gas is cut into 2D
slices or into 1D tubes. The temperature for the 1D case can be much
lower than the initial 3D temperature. The researchers find that the
temperature increases from 12.5 nK to 17 nK when compressing from 3D to
2D, and then decreases below 9 nK when further compressing to 1D. The
cooling occurs because of the interplay of the strong lateral
confinement in 1D and the strong interactions in the regime where the
bosons fermionize. The lowest temperatures for 2D and 1D systems have so
far been achieved by slicing low-entropy 3D samples such as essentially
pure atomic BECs into layers or tubes by means of lattice potentials.
This anomalous cooling effect needs very low initial temperatures,
strong interactions, and small 1D trapping frequencies.
For more information:
Science Advances, February 14 (2024); Phys.org,
February 15 (2024).
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.
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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