VALLEY RESEARCH

Research News is a digest of science and technology news items arising from research and development magazines, newspapers, trade magazines, newsletters, and other news sources that Valley Research processes daily for the benefit of its customers everywhere. It is provided freely to our customers who are free in turn to post or transmit it to other interested researchers provided only that credit to Valley Research is given. Research News is updated approximately once a week.

Rodolfo Carrera, Editor

WEEK OF OCTOBER 28, 2024 [No. 1599]

Non-reciprocal Hall effect observed:   researchers at Penn State in University Park, PA and MIT in Cambridge, MA have detected RT transverse non-reciprocal transport with a quadratic I-V characteristic and divergent non-reciprocity. This effect is observed in microscale Hall devices fabricated from Pt deposited by a focused ion beam on Si substrates. The transverse non-reciprocal Hall effect arises from the geometrically asymmetric scattering of textured Pt nanoparticles within the focused-ion-beam-deposited Pt structures. The non-reciprocal Hall effect generated in focused-ion-beam-deposited Pt electrodes can propagate to adjacent conductors such as Au and NbP through Hall current injection. This pronounced non-reciprocal Hall effect facilitates broadband frequency mixing. Electrons asymmetrically scatter from the textured nonsymmetrical Pt nanoparticles, inducing a voltage that runs perpendicular to the current applied to the Si semiconductor with V ~ I² and no magnetic field applied to the sample. The conventional Hall effect with V ~ I occurs only in electrical conductors or semiconductors in the presence of a magnetic field. Unlike the conventional Hall effect, which is driven by a force induced by the magnetic field, the nonreciprocal Hall effect arises from flowing conduction electrons interacting with the textured Pt nanoparticles. This process results in a violation of Ohm's law that sets the Hall voltage to be zero in the absence of a magnetic field.

For more information: Phys.org, October 24 (2024); Nat. Mat., October 21 (2024).

IR µmolar sensitivity:   researchers at MPI-Light in Erlangen have used femtosecond fieldoscopy (field-resolved detection of the impulsively excited molecules in the liquid phase) to measure micromolar level liquid quantities, with record sensitivity in the IR region. They demonstrate near-PHz electric field detection of a few fs pulses with 200 as temporal resolution and sub-fJ detection sensitivity; plus, temporal isolation of the response of the target molecules from those of the environment and the excitation pulse. In a proof-of-concept analysis of aqueous and liquid samples, they demonstrate field-sensitive detection of weak combination bands in H₂O and CH₃CH₂OH at 4.13 µmol concentrations. Stimulating the molecular composition of a sample with phase-coherent fs excitation pulses leads to temporal gating of the molecular response from the excitation pulses. This gating allows the detection of different phases of matter at weak resonance frequencies in a single measurement with high detection sensitivity, dynamic range and temporal resolution. The as temporal precision of the measurements allows precise access to the subcycle electric field of light and decomposition of the short-lived liquid molecular response from the long-lived ambient gas responses; and, it allows the real-time resolution of both molecular and electronic dynamics within matter. By using an ultrashort laser pulse, the molecule's signal is separated from the laser pulse itself, making it easier to detect the vibrational response in a background-free manner. Here an ultrashort pulse of light excites liquid molecules surrounded by water vapor at specific NIR wavelengths. The transmitted pulse captures the sample's combined response and the environment. A second ultrashort light pulse converts this pulse to higher optical frequencies, producing a time-dependent output in a crystal. This output reveals the initial pulse and the delayed responses from the liquid sample (lasting ps) and the surrounding water vapor (lasting 10's ps). The short-lived liquid and long-lasting gas responses can be separated by analyzing the data. The researchers generate high-power ultrashort light pulses using photonic crystal fibers filled with gas. These pulses, compressed to nearly a single cycle of a light wave, are combined with phase-stable NIR pulses for detection. They use electro-optic sampling (EOS), to measure these ultrafast pulses with near-PHz detection bandwidth, capturing fields with 200 as temporal resolution. Field-resolved detection allows the direct measurement of light–matter interactions with as precision in a subcycle regime, capturing both amplitude and phase information. For decades, as streaking was the sole method to probe the electric field of light with a bandwidth approaching the PHz range, in vacuum. Recently developed EOS has high detection sensitivity in the near-PHz field-resolved detection of light in ambient air. In EOS, a short probe pulse is employed to resolve the cycles of the electric field of light by up-converting its spectral bandwidth to higher frequencies, so Si detectors can be used for broadband NIR detection. The combination of bright ultrashort pulses and heterodyne detection allows higher detection SNR and higher detection sensitivity, leaving the shot noise of the probe pulse as the primary source of noise. The researchers have developed a laser source delivering broadband pulses with carrier-to-envelope phase (CEP) stability, which were intrinsically synchronized to near-single-cycle pulses at MHz repetition rates. This allowed direct electric-field detection of CEP-stable few-cycle pulses with high detection sensitivity and dynamic range via EOS with as temporal resolution. Few-fs phase-coherent pulses were utilized for both broadband molecular excitation and the near-PHz electric-field detection of their response. They conducted field-resolved detection of water vibration modes in both gas and liquid phases in the NIR region. Bright, intrinsically synchronized CEP-stable 15 fs pulses at 2 μm and 4.8 fs pulses at 1 μm were generated for impulsive excitation and probing of the molecular response via EOS. Using the MHz ultrashort laser pulses, they demonstrated the ambient air field-resolved detection of fs pulses with sub-fJ energy and a 10⁴ detection dynamic range in the electric field at near-PHz frequencies. The source’s MHz repetition rate augments both the SNR and detection sensitivity, while the signal up-conversion alleviates the bandwidth constraint inherent in Si-based detectors. Employing probe pulses with shorter temporal duration extends the detection bandwidth of EOS up to visible frequencies, constrained by the absorption of the gate pulses in ambient air. The high repetition rate of the source allows the tracking the sample’s dynamic temporal evolution on a time scale of 100's ms. Capturing spectrograms with exceptional temporal and spectral resolution provides information similar to that of multidimensional spectroscopy. This helps analysis of liquid samples that feature broad bandwidths and intricate spectral resonances.

For more information: Phys.org, October 22 (2024); Nat. Phot., October 21 (2024).

WEEK OF OCTOBER 21, 2024 [No. 1598]

Ferromagnetism emergence at onset of Kondo breakdown in moiré bilayers:   researchers in Cornell University at Ithaca, NY have used magneto-transport and magneto-optical spectroscopy to observe emergence of ferromagnetism at the onset of a density-tuned Kondo lattice regime breakdown in semiconductor angle-aligned MoTe₂/WSe₂ moiré bilayers. At a critical density, they observe a heavy Fermi liquid to insulator transition and a nearly concomitant emergence of ferromagnetic order. The observation is consistent with the emergence of a ferromagnetic Anderson insulator and suppression of the Kondo screening effect below the critical density. A moiré lattice emerges in the material studied here because of the 7% lattice mismatch between MoTe₂ and WSe₂ so no twist angle is required to create the moiré superlattice potential. The MoTe₂ and WSe₂ moiré Kondo lattice hosts local magnetic moments and a gas of itinerant carriers, respectively. The Kondo destruction transition (Kondo lattice break down, usually under varying external parameters like doping density, magnetic field and interaction strength) occurs at the critical density where ferromagnetic correlations and ordering are observed. The researchers here built on their previous realization of an electrically tunable moiré Kondo lattice system (with the fabrication of a tunable moiré artificial Kondo lattice using moiré semiconductors MoTe₂/WSe₂ moiré bilayers), and observation of gate tunable heavy fermions. They have studied now the fate of the heavy fermions by continuously tuning the density of the itinerant carriers in the system (which tunes the effective Kondo coupling strength). They observed a destruction of the heavy fermions and the simultaneous emergence of a ferromagnetic Anderson insulator, near a critical density. The researchers fabricated dual-gated Hall bar devices that allow independent control of the total doping density in the material as well as the relative partition of doping densities in each transition metal dichalcogenide layer. Thus, they prepared the sample in the Kondo lattice regime, which allowed the study of the Kondo destruction transition continuously as it occurred. They measured the anomalous Hall response and the spontaneous circular dichroism in the material to demonstrate the emergence of a ferromagnetic Anderson insulator. They examined the temperature and magnetic field dependent transport properties to show that the ferromagnetic Anderson insulator emerges near the Kondo destruction transition. As the material approached the density-tuned Kondo destruction transition, they observed a near simultaneous occurrence of a metal-to-insulator and a magnetic quantum phase transition. Both transitions involve different degrees of freedom (charge and spin, respectively), so the occurrence of the two transitions at nearly the same critical density was unexpected. When the sample approached the Kondo destruction transition, they observed ferromagnetic correlations contrary to normally observed antiferromagnetic magnetic correlations and ordering on other known Kondo destruction transitions. The researchers plan to push the Kondo destruction transition to occur at higher critical densities by engineering the material's twist angle. They expect much less impact from disorders at higher critical densities, thus allowing the study of the quantum phase transition in a more intrinsic manner with possible observations of quantum spin liquids and non-Fermi liquids.

For more information: Phys.org, October 17 (2024); Nat. Phys., September 9 (2024).

Large increase of magnetization lifetime in atomic array induced:   an international group lead by researchers at Delft University of Technology at Delft has shown that a short chain of Fe atoms can be coaxed into a junction point (JP), whose effect is to drastically lengthen the time the chain spends in one antiferromagnetic state or the other (magnetization lifetime x 10³). They demonstrate the manifestation of JPs through the spin dynamics of nanomagnets by assembling Fe atoms into chains on Cu₂N/Cu(100) using an STM operating at ~ 1.3  K. Adjusting chain length and interatomic spacing allows them to tailor the spin-spin interactions and engineer JPs in a controlled manner. When tuning the direction and strength of the external magnetic field near a JP, they observed an increase of magnetization lifetimes. Measuring magnetization switching in an Fe atomic chain under a tuned transverse magnetic field, they observe a nonmonotonic variation of magnetization lifetimes around a JP. Near JPs, local environment effects causing quantum tunneling of magnetization (QTM) are efficiently suppressed. Adjusting interatomic interactions further facilitates multiple JPs. The energy of an antiferromagnetic spin chain vs. the strength of the applied magnetic field has the shape of two inverted cones (the ground state is the upward-pointing cone and the first excited state is the downward-pointing cone) touching at the JP where the two states are degenerate. Scaling magnets down to where quantum size effects become prominent triggers QTM. In the vicinity of the JP, characterized by two metastable magnetization states separated by an energy barrier, QTM between these states is suppressed due to destructive interference among separate tunneling paths. Manipulation of magnetic atoms with a STM allows for the assembly of quantum nanomagnets with controllable energy barriers ranging from 100  μeV to 100 meV. The lifetime of magnetization states in these magnets can be determined by monitoring the spin-polarized current through one of the magnet’s atoms over time. When the magnetic anisotropy barrier exceeds the thermal energy, the lifetime is dominated by through-the-barrier transitions (QTM, resulting from hybridization between quantum states on either side of the barrier). The researchers positioned 5 Fe atoms on a CuN surface at cryogenic / UHV conditions. A magnetic field applied parallel to the surface had just the right value to create almost degenerate ground and first excited states. By measuring the spin of the middle atom using a STM, they found that, at the JP, the antiferromagnetic pattern flipped every ~ 10 s. By varying the parallel field and applying an additional perpendicular field, they mapped conditions away from the JP. At the greatest distance, the flipping time shrank by 3 orders of magnitude with a high sensitivity of the flipping time to the local magnetic field. The researchers demonstrated that the switching rate between the two lowest energy levels can be precisely controlled through tailored transverse magnetic fields, which suppresses QTM. This suppression emerges as a consequence of dehybridization of the lowest lying spin states near an avoided level crossing, which, in the case of Fe chains on Cu₂N, results in strong enhancement of lifetimes. While effects of JPs have been observed previously in single molecule magnets through ensemble measurements, the researches here have shown that JPs in quantum magnets can be manipulated and designed by tailoring the interaction of individual atomic spins on surfaces.

For more information: Physics, October 16 (2024); Phys. Rev. Lett., October 15 (2024) page 166703.

WEEK OF OCTOBER 14, 2024 [No. 1597]

Entangled optical clock demonstrated:   researchers at JILA in Boulder, CO have used some 100 entangled Sr atoms trapped in a lattice pattern to make an optical atomic clock beating the standard quantum limit (SQL) in certain conditions. They set and use a family of multi-qubit Rydberg gates to generate Schrödinger cat states of the Greenberger–Horne–Zeilinger (GHZ) type with up to 9 optical clock qubits in a programmable atom array. In an atom-laser comparison at sufficiently short dark times, they demonstrate a fractional frequency instability below the SQL using GHZ states of up to four qubits. However, because of their reduced dynamic range, GHZ states of a single size fail to improve the achievable clock precision at the optimal dark time compared with unentangled atoms. Thus, they simultaneously prepare a cascade of varying-size GHZ states to perform unambiguous phase estimation over an extended interval. The researchers generated the entangled state by exciting the electrons in the Sr atoms, thus making a strong interaction of electrons possible. The entangled pair of electrons increase their decay rates. They experimented with creating optical clocks that included a combination of individual atoms and entangled groups of two, four and eight atoms with four clocks ticking at four rates in one system. They found that, at least under certain conditions, entangled atoms have lower uncertainty in their decay emissions than the atoms in an unentangled optical atomic clock. They could only run the entangled optical clock effectively for ~ 3 ms before the entanglement between atoms faded away, causing the atomic decay rate to become chaotic.

For more information: Nature, October 9 (2024) page 315; Phys.org, October 9 (2024).

Continuous operation of 1,200-atom quantum register shown:   researchers at the MPI Quantenoptik in Garching have set up a register of 1,000 to 1,247 atoms in an optical lattice and kept it in continuous operation for 1 hr. The scheme relies on a continuously operated storage zone in an optical lattice, which is periodically replenished from a loading zone and a MOT. Using a bichromatic combination of loading and storage arrays, they achieve spatial control over the loading zone, strongly suppressing loading of sites in the storage register. Loading about 130 new atoms for each cycle, they grow and then continuously maintain an array of atoms in an optical lattice with more than 1,000 atoms. The researchers demonstrate a method to circumvent the scaling size problem of assembled neutral-atom arrays trapped in optical lattices (that preparation times increase with system size), by recycling atoms from one experimental run to the next, while continuously reloading and adding atoms to the array. Compared to directly assembling the large number of atoms thought possible using the technique here in a single experimental cycle, continuous loading moves only the newly loaded atoms for each cycle, thus reducing both move-induced losses as well as the resorting time overhead. The researchers generate densely packed arrays with > 1.000 atoms stored in an optical lattice, continuously refilled with a 3.5 s cycle time and about 130 atoms reloaded during each cycle. They continuously maintain such large arrays by reloading atoms that are lost from one cycle to the next without affecting the atomic array already present in the system. Until now, arrangements of this size have been difficult to maintain due to unavoidable atomic losses. The larger the neutral atom register, the more atoms are lost or heated, thus making the system more prone to errors over time. Until now, the entire register of atoms needed to be replenished regularly, which severely limits the size a system can attain. The researchers here have integrated a real-time reloading zone into their Sr experimental setup, They are now working on maintaining the coherence of qubits during the reloading step. An improvement of the experiment here would be to move the position of the MOT slightly away from the storage array. This would mitigate the need to empty the storage array from accidentally loaded atoms, allow parallel MOT and storage array operation, and reduce the small effect of unwanted off-resonant scattering from MOT light on the storage atoms.

For more information: Phys.org, October 9 (2024); Phys. Rev. Res., July 25 (2024) page 033104.

WEEK OF OCTOBER 7, 2024 [No. 1596]

Coherent coupling of Andreev pair qubits demonstrated:   an international group lead by researchers at the University of Basel in Basel haS performed photon-mediated long range coupling of two Andreev pair qubits each localized in a semiconducting nanowire. They demonstrate coherent remote coupling between two Andreev pair qubits mediated by a microwave photon in a superconducting cavity coupler. The coupler hosts two modes that are engineered to have very different coupling rates to an external port. The strongly coupled mode is used to perform a fast read-out of each qubit, while they use the weakly coupled mode to mediate the coupling between the qubits. When both qubits are tuned into resonance with the weakly coupled mode, they find excitation spectra with characteristic avoided crossings. They identify two-qubit states that are entangled over a distance of 6 mm. The researchers here coupled the two Andreev pair qubits (size ~ 100 nm) at a macroscopic distance (~ 6 mm) from one another at the two ends of a long, superconducting microwave resonator. This allows the exchange of microwave photons between the resonator and the qubits. They combine three quantum systems so that they can exchange photons between each other. The microwave resonator can be used in two different ways. In one mode, the qubits can be read out via the resonator, providing information on their quantum state. A second mode is used to couple the two qubits to each other, allowing them to communicate without losing microwave photons. The two qubits are then no longer independent of one another but rather share a new quantum state. The experimental results here agree with theoretical models.

For more information: Phys.org, October 3 (2024); Nat. Phys., October 3 (2024).

Microwave control of diamond SnVC spin qubit:   a group led by researchers at KIT in Karlsruhe has shown how Sn vacancy centers (SnVC, Sn atom on an interstitial site with two neighboring vacancies) in diamonds can be precisely controlled using microwaves. They demonstrate that the SnVC r electron spin can be very efficiently controlled, including optical readout and stable spectral properties of qubits and achieving long coherence times, with superconducting waveguides to direct microwave radiation to the defects without generating heat. They characterize the negatively charged SnVC regarding its magneto-optical properties and determine the relevant components, such as the orbital quenching factors from a full fit of the electronic spin Hamiltonian. They explain the properties of the ground and excited states under strain, the relevant qubit transitions, and the influence of an external magnetic field. They demonstrate coherent control of the electron spin for 10 ms by using standard dynamical decoupling sequences. Recent work has shown spin control of strained emitters using microwave lines that suffer from Ohmic losses, restricting coherence through heating. The researchers here utilize a superconducting coplanar waveguide to measure SnVCs subjected to strain. They analyze the SnVC electron spin Hamiltonian based on the angle-dependent splitting of the ground and excited states. They demonstrate coherent spin manipulation and obtain a Hahn echo coherence time of up to 430 μs. They explore how the spin properties behave under different magnetic field directions. And demonstrate that manipulating electron spins is easier in strained diamonds, as the electron spin is more responsive to an alternating magnetic field. The researchers precisely controlled the electron spins of SnVC qubits using microwaves. They used dynamical decoupling, that largely suppresses interference, to increase the coherence times of the diamond SnVC to 10 ms (a sixfold improvement over earlier works). Group-IV color centers in diamond are promising candidates for quantum networks due to their good optical qualities including symmetry-protected optical transitions that connect to coherent spin levels. Their inversion symmetry leads to a dominant zero-phonon line and reduced sensitivity against electrical noise. However, the presence of two orbital ground states opens a decoherence channel for the spin levels via phonon scattering. In this respect, the SnVC offers advantages due to its large ground-state splitting of more than 800 GHz, such that phonons at this frequency can be frozen out at comparably high temperatures greater than 1 K. Reported dephasing times for all-optical control range from 1.3  μs  to 5  μs at 2 K, with coherence times ranging from 0.3 ms using all-optical control to 1.6 ms for microwave control, respectively. However, direct spin control by microwave fields is hampered by orthogonal orbital contributions to the Zeeman states. Recently, it has been shown that by introducing strain, orbital mixing can be induced and high-fidelity microwave control becomes possible. The negatively charged SnVC possesses long electron spin lifetimes due to its large spin-orbit splitting. However, the magnetic dipole transitions required for microwave spin control are suppressed, and strain is necessary to enable these transitions. The researchers here demonstrate extended control over the optical and magnetic levels of the SnVC electron spin. They show how to incorporate strain into the diamond to allow for microwave control. They measure 2D rotation maps of the optical transitions under varying magnetic field orientations to determine the SnV axis with less than 1° uncertainty. They develop a fitting procedure for the optical and microwave transitions to the Hamiltonian of the electron spin, using the explicit doublets for each total angular momentum, allowing for a qualitative determination of the orbital quenching factors.

For more information: Phys.org, October 4 (2024); Phys. Rev. X, August 27 (2024) page 031036.

WEEK OF SEPTEMBER 30, 2024 [No. 1595]

Trapped ion qubit controlled with no neighboring qubits perturbed:   researchers at the University of Waterloo in Waterloo, ON have demonstrated how to mid-circuit measure and reset (in the middle of coherent dynamics) a trapped ion qubit to a known state without disturbing neighboring array qubits at µm distance (this inter-atomic separation is comparable to the optical resolution), that is, the preserving of a qubit during state-destroying operations on an adjacent qubit. They have demonstrated the feasibility of in-situ state-reset and state-measurement of trapped ions, achieving  probability of accidental quantum measurement P_AQM < 1 × 10^−3 of the asset qubit while resetting the process qubit placed at a distance of 6 μm, and P_AQM < 4 × 10^−3 while applying detection light on the process qubit for detection times of 11 μs. These low P_AQM correspond to the preservation of the quantum state of the asset qubit with fidelities > 99.9% (preserving an asset ion-qubit while a neighboring process qubit is reset) and 99.6% (preservation fidelity while applying a detection beam for 11 μs on the same neighbor at a distance of 6 μm) for the reset and measurement processes, respectively. Their measured low P_AQM is enabled by low relative intensity crosstalk of I_X < 1 × 10^−4 (I_X defined as the ratio of the intensity of the probe beam on the asset qubit I₁ to that on the process qubit I₂) that is maintained across a large spatial region of  > 400 μm, suitable to address  > 50 ions. This is achieved through precise wavefront control of addressing optical beams and using a single ion as both a quantum sensor for optical aberrations and an intensity probe with  > 50 dB dynamic range. The researchers used programmable holographic beam shaping technology to manipulate and destroy one qubit (in 2021). They have now destroyed any selected qubit while maintaining the quantum information in the other qubits around. Trapped ion qubits are measured with laser beams tuned to specific atomic transitions. The target ion scatters photons in all directions during this process. Even with perfect control over light, there is still a risk that these scattered photons could disturb the quantum states of nearby qubits. The crosstalk measurement scheme here employs temporal separation of probe light illumination and detection of an ion qubit and hence overcomes sensitivity limitations due to unwanted background scattering of resonant light from optics leaking onto photon detectors, and thus allows measurement of crosstalk over a large dynamic range. Ions are localized to < 100 nm at typical laser-cooling temperatures and trap frequencies, making it possible to characterize aberrations with the ion sensor for larger numerical aperture (NA) systems. With large NA, the beam waist w decreases, thus the ion separations can be decreased without increasing P_AQM to achieve higher qubit-qubit interaction strengths. The demonstrated high fidelity over a FOV ~ 450 μm corresponds to ~ 50 ions in a linear chain for typical harmonic trapping parameters (radial trap frequency ~.2π × 5  MHz and axial trap frequency ~ 2π × 30  kHz). The slight decay of fidelity away from the center of FOV can be compensated by recalibrating aberrations away from the center. However, even without extra calibrations, fidelity can be maintained over the entire chain, as inter-ion separation away from the center of an ion chain also increases in a harmonic trap (from 6 μm at the center becoming  15 μm near the edge for parameters above). For typical RF ion traps NA > 0.5 is accessible for photon collection simultaneously with NA ~ 0.3 (in a perpendicular direction) for optical addressing, allowing for independent optimization for photon collection and addressing. While high quantum efficiency detectors and negligible dark counts make  ~ 10 μs detection time possible, less-expensive PMT tubes can also allow ~ 20 μs detection time under otherwise identical conditions for maintaining high asset qubit preservation fidelities of  > 99.2%. While the asset qubit coherence is limited by intensity crosstalk, the inter-ion-scattering set P_AQM fundamental limit, P*_AQM for state detection may be suppressed even further with the proper choice of the local magnetic field. For ¹⁷¹Yb⁺ , it is possible to suppress the intensity of Ï€ light scattered from the process qubit in the direction of the asset qubit by aligning the magnetic field along the ion chain, maximizing fidelity. The optimal orientation of the magnetic field for state reset is perpendicular to the ion chain.

For more information: Phys.org, September 23 (2024); Nat. Comm., August 3 (2024).

Thermal effects in current-driven antiferromagnetic order switching measured:   researchers at the University of Urbana - Champaign in Urbana, IL have developed a technique to directly measure the thermal contribution to current-driven antiferromagnetic-order switching as related to next-generation spintronic devices. The currents required to do this are so large that device temperatures rise to the point where thermal effects impact spin structure in addition to electromagnetic effects. Although the switching mechanisms can be explained by spin dynamics induced by spin torques, demagnetization above the Néel temperature due to Joule heating might be critical for switching. The researchers have developed a systematic method to quantify the thermal contribution due to Joule heating in micro-electronic devices, focusing on current-driven octupole switching in the non-collinear polycrystalline antiferromagnet, Mn₃Sn. They focused their work on spin–orbit-torque-assisted octupole switching with an adjacent heavy-metal layer. The results consistently show that the critical temperature for switching remains relatively constant above the Néel temperature, while the threshold current density depends on the choice of substrate and the base temperature. They provide an analytical model to calculate the Joule-heating temperature, which quantitatively explains the experimental results. From numerical calculations, they explain the reconfiguration of magnetic order during cooling from a demagnetized state. In the experimental method here thermal effects are deduced from how a device heats substrates with different thermal conductivities. The researchers prepared antiferromagnetic samples on SiO2 substrates with different thicknesses. Antiferromagnets on thicker samples have higher temperatures when the same electric current is applied. If device heating is important for the spin structure changes, then there will be a difference across devices on different substrates. They found that heating had a significant effect. Since non-collinear antiferromagnets, such as Mn₃Sn and Mn₃Ge, have relatively low Néel temperatures T_N ≤ 430 K, Joule heating can play a critical role in switching. Although the electrical octupole switching might be primarily driven by the collective spin rotation induced by the spin–orbit torque and an in-plane magnetic field, heat might be critical as switching occurs around the Néel temperature. This is relevant in spintronics, where current densities are often close to device breakdown. The technique here allows to systematically compare the role of heating to that of electric current. The researchers varied the effective thermal resistance independently from the electrical resistance of the device, allowing to identify the role of thermal vs. electrical effects. Using W/Mn₃Sn films on Si/SiO₂ substrates with different SiO₂ layer thicknesses, they obtained the threshold current density for octupole switching which depends on both the SiO₂ thickness and the base temperature, T₀. They identified the heating temperature at the threshold current density, T_th, and consistently showed that T_th > T_N in all cases. Their analytical model for calculating the Joule-heating temperature quantitatively describes the experimental results. From numerical simulations, they show the reconfiguration of the magnetic octupoles from the demagnetization state. The results demonstrate the significant role of Joule-heating in current-driven octupole switching in Mn₃Sn. They show that the threshold current density for switching depends on both the substrate choice and T₀, while the switching temperature remains essentially constant above the Néel temperature.

For more information: Phys,org, September 24 (2024); APL Mat., August 14 (2024) page 081107.

WEEK OF SEPTEMBER 23, 2024 [No. 1594]

Cuprate disorder analyzed using 2D ARTHz spectroscopy:   an international group lead by researchers at MPSD in Hamburg has measured spatial disorder induced by interlayer tunneling in the opaque cuprate HTS La_1.83Sr_0.17CuO₄ using the isolated Josephson echo in angle-resolved 2D Terahertz spectroscopy (THz frequency is where collective modes of solids resonate). They measures Josephson plasmon echoes from an interlayer superconducting tunneling resonance in a near-optimally doped cuprate. The technique allows to study the multidimensional optical response of the interlayer Josephson coupling in the material and disentangle intrinsic lifetime broadening from extrinsic inhomogeneous broadening for interlayer superconducting tunneling. They find that inhomogeneous broadening persists up to a substantial fraction of T_c, above which this is overcome by the thermally increased lifetime broadening. Measurements of the electronic properties of cuprates using STM reveal disorder of the superconducting gap on nm length scales; the variations are correlated with the distribution of dopant atoms. The presence of co-existing and competing orders complicates the assignment of observables to superconductivity. Techniques based on tunneling from a superconducting tip can isolate the superconducting response limited to sampling surfaces amenable to tip-based techniques and temperatures below that of LHe. The researchers here probed the inhomogeneous cuprate superconductivity and followed the evolution in the transport properties of the cuprate's chemical-doping-induced disorder up to its superconducting T_c with good sensitivity. They sequentially excited the sample with multiple intense THz pulses including non-colinear pulses and isolated specific THz nonlinearities by their emission direction. They observed that superconducting transport in the cuprate was revived after excitation by the THz pulses and how these Josephson echoes revealed that the disorder in superconducting transport was lower than the corresponding disorder observed in the superconducting gap measured by spatially resolved techniques (like STM). Disorder near T_c remained stable up to ~ 0.7 T_c. Previous applications of 2D THz spectroscopy have all implemented a collinear geometry, in which the excitation fields and nonlinear signal emission all have identical wavevectors. A collinear geometry cannot isolate specific nonlinearities and precludes study of opaque materials. The researchers here implement a non-collinear excitation geometry with which different peaks in the 2D spectrum are emitted in unique phase-matched directions. By measuring the THz Josephson echo, the researchers observed an interlayer tunneling response that is largely immune to the underlying electronic disorder, which remains true even as temperature approaches the phase transition temperature.

For more information: Phys.org, September 16 (2024); Nat. Phys., September 16 (2024).

Laser-triggered ps ion source demonstrated:   researchers at TU Wien in Vienna have efficiently generated keV proton pulses with width < 500 ps from ultrafast electron-stimulated desorption (UESD). They accelerated fs photoelectron pulses emitted from focusing a UV laser onto a LaB₆ cathode to trigger UESD from adsorbates on a SS target under UHV conditions. The expelled ions are filtered and directed to form an ion pulse. The UESD technique here provides a ~ 20 cm long proton beam line for pump-probe experiments. The researchers have demonstrated the generation of ps ion pulses (400 ps time width, 10³ H+ / s at 100 kHz laser repetition rate) utilizing ultrashort electron pulses. While direct photoionization of atoms to form well-timed ion pulses can suffer from a large starting volume, in the method here a 2D starting solid plane of ions is defined with nm precision. The ion species produced in the setup here are determined by the residual gas adsorbed on the last drift tube of the utilized UEBIS (Ultrafast Electron Beam Ion Source ). The technique, demonstrated for proton ion pulses here, is extensible to the generation of pulses of other positive and negative ions and neutral atoms by switching the atoms adsorved into the SS film surface and the filters subsequently used. Sub-ps ion beam pulses could be similarly produced by using specially shaped alternating electromagnetic fields to slow down the initial ions in the pulse and accelerate the rest. The researchers use the ultrafast photoelectrons generated at low optical power densities of 10⁹–10¹⁰ w/cm² to drive a Franck-Condon transition upon impact on a surface. This transition can shift the ground state of adsorbed particles from the surface into repulsive excited states. As such, the excited particles are pushed away from the surface and can desorb. Distinct constituents, including positively and negatively charged ions, as well as neutral particles are created. To make use of short ion pulses for time-resolved surface studies (ions as pump, laser as probe), three major criteria must be fulfilled: (a) ion pulses must be significantly shorter than the lifetime of the process under investigation (1 ps - 10 ns); (b) the ion pulses must be well synchronized to a pulsed laser for the pump-probe scheme with a laser-ion time jitter of less than the ion pulse duration; (c) the solid surface must allow many ion impacts before it degrades too much, still showing a large-enough response to the ion impact to be probed by the laser. The method here can produce ion pulses of many different ion species in consistency with (a) and (b). The method allows an ion transport from point of ionization to the pump-probe interaction spot over several cm making it possible to fit standard material samples and optical equipment for flexible measurement strategies addressing (c). The researchers used a miniaturized UEBIS from D.I.S. Germany GmbH where the standard thermal cathode is replaced by a LaB6 photocathode (Kimball Physics, ES-423E-9015, flat tip apex). The cathode emits pulsed photoelectrons by illumination with 280 fs - 259 nm laser pulses. The UV laser pulses are produced by IR fourth harmonic generation utilizing two consecutive β -barium oxide (BBO) nonlinear crystals. The UV laser beam is guided through a half-wave plate (HWP) and polarizer, followed by a second HWP to freely adjust the power and polarization. Finally, the UV laser beam is focused on the cathode (FWHM ∼50 µm) by a lens with 50 cm focal length. For all experiments with pulsed ions, the UV laser repetition rate is set to 100 kHz with pulse energy ~ 31 nJ. In standard operation of an EBIS ~ 1/1000 of photoelectrons impact the drift tubes in front of the ion collector contributing to a blind current. The researchers here make use of these stray electrons to drive UESD at the surface of the last drift tube. Positioning of the UV laser beam at the cathode is done by first measuring the black-body radiation intensity from ohmic heating the LaB₆ crystal after a collimator and guiding the laser beam on the same path with the help of two irises. The surface of the mirror in the vacuum chamber is Au coated to avoid charging effects influencing the passing ion pulse. Desorbed ions from the last drift tube are immediately accelerated by applying a positive potential to the last drif tube, collimated by the electron collector and repeller, and are time-stamped using the MCP assembly in coincidence with a fixed trigger signal from a photo diode (PD, Femto, FWPR-20-IN-FST) picking up part of the initial IR laser intensity. The rate at which particles desorb from the surface scales linearly with the intensity of impinging electrons and the abundance of the specific adsorbed entities present on the surface. In here, thermal desorption can be excluded due to the low (CW-equivalent) pulsed electron current. The setup was designed with a long flight distance (~ 20 cm) to make the alignment easier. This distance can be reduced in future designs to minimize the pulse broadening of ions. A reduction by ~ 10 cm would reduce the dispersion by ~ 90 ps so this needs to be considered for heavier species. For H the estimated dispersion by the initial kinetic energy spread is ~ 19 ps in this setup, thus the system size is not the limiting factor for shorter pulses. The initial position spread of the desorbed H⁺ could be narrowed down by decreasing the diameter of the electron focal spot on the metallic plate and narrowing the collimator formed by the collector and repeller electrodes. UESD is suitable for integration into ultrafast SEM, where a few nm focus of ultrafast pulsed electrons is available and would then allow a nm precise ionization point in 3D space from which ion pulses with ultimate timing performance could be formed. Ions are detected on a 2" MCP without spatial resolution. Different ionization points at the last drift tube lead to different trajectories through the collector and different impact points on the MCP. Using a position-resolved MCP detector would allow significantly shorter pulses by further geometric filtering. RF compression can effectively reduce pulse duration at the cost of introducing a broad spectrum of kinetic energies within the compressed pulse (RF imparts different energy changes to particles at different positions within the pulse).

For more information: Phys.org, September 17 (2024); Phys. Rev. Res., September 16 (2024).

WEEK OF SEPTEMBER 16, 2024 [No. 1593]

Nucleus - electron spin interaction controlled:   researchers at Delft University of Technology in Delft have caused a ⁴⁷Ti nucleus spin interaction with the spin of one of the outermost electrons (which can be manipulated and read out through a STM). They studied the coherent evolution between the nuclear spin and the electron spin in a single atom by using ESR and STM to control nuclear spins of single atoms on a surface even with the nuclear weak coupling to the tunneling electrons. Using pump-probe spectroscopy, they revealed the collective coherent dynamics of the internal spin dynamics inside a single atom. The magnetized STM tip functioned here as a control knob to locally tune the nature of the dynamics with the STM permitting to address individual electron spins in ESR experiments (with sub-nm resolution). By fine-tuning the electronic Zeeman energy using the local field of the probe tip, they identified a parameter space where electronic and nuclear spin states hybridize. They then probe the free coherent evolution of the coupled system by electric DC pump-probe experiments. They reveal an emerging beating pattern, that originates from multiple quantum oscillations with different frequencies at the points of hybridization. The very weak hyperfine interaction here requires a very small, precisely tuned magnetic field. The researchers used a commercial low-temperature STM equipped with high frequency cabling to send both RF signals and ns DC pulses down to the tip. The sample system consists of ⁴⁷Ti atoms deposited on bilayer MgO islands grown on Ag(100), that become hydrogenated by residual H. For all measurements, they use spin-polarized tips that are created by picking up co-deposited Fe atoms onto the tip apex. They studied individual Ti adsorbed onto the O sites of MgO, well-isolated from neighboring spins using atom manipulation. They focused on ⁴⁷Ti isotopes, that carry a nuclear spin with magnitude I = 5/2. Once all experimental conditions were met, the researchers used a voltage pulse to push the electron spin out of equilibrium, after which both spins wobbled together for ~ 1 µs. The experiment resolves the ns coherent dynamics of a hyperfine-driven flip-flop interaction between the spin of an individual nucleus and that of an orbiting electron. They use the unique local controllability of the magnetic field emanating from the STM probe tip to bring the electron and nuclear spins in tune, as evidenced by a set of avoided level crossings in ESR-STM. They polarized both spins through scattering of tunneling electrons and measured the resulting free evolution of the coupled spin system using a DC pump-probe scheme. This revealed a complex pattern of multiple interfering coherent oscillations. Excellent agreement with theoretical calculations was obtained.

For more information: Physics.org, September 12 (2024); Nat. Comm., September 11 (2024).

Entangled photoionization electrons controlled:   researchers at the MPIK in Heidelberg have shown emission control of entangled electrons in photoionization of a H₂ molecule. They demonstrate sub-fs control of the phase between entangled states in molecular H₂ by few-photon dissociation and coincident detection of all participating particles. They have achieved ultrafast steering of entangled electrons from the dissociative photoionization of H₂ using IR and XUV laser pulses with variable delay. They demonstrate optical control of the emission direction of the photoelectron with respect to the outgoing neutral fragment. Depending on the relative delay between the two laser fields, adjustable with sub-fs time resolution, the photoelectron is emitted into the same hemisphere as the H-atom or opposite. This emission asymmetry is a result of entanglement of the two-electron final-state involving the spatially separated bound and emitted electron. Normally, the emission direction of a photoelectron is symmetric relative to the ejected H atom, with no preferred direction, if the parity of the electron wave function is maintained during the process. However, when the H₂ molecule is set in a superposition of states with opposite parities, an asymmetric electron emission pattern emerges. In the experiment, the researchers demonstrate the asymmetric emission in the photoelectron/ion system by detecting them in coincidence while they are spatially separated; and, they steer their relative emission direction using the delay between XUV and weak IR pulses. An analysis of the resulting entanglement of ions and photoelectrons is presented. The researchers found that the emission direction of electrons with respect to the protons can be modified by delaying as pulses with respect to the maxima and minima of a laser light wave on a time-scale < 1 fs. The emission direction of a photoelectron relative to the remaining bound electron is controlled here by the time interval between two laser flashes on ~ 100's as. The adjustable emission asymmetry is based on the entanglement between the bound electron and the spatially separated emitted electron. These states, can be modified by as delays between the XUV and the IR laser pulses. This effect is different from the localization of the bound electron on one of the nuclei in the laboratory frame using a spatially asymmetric strong laser field during dissociation. In those experiments, only the proton (deuteron) is detected and its ejection direction with respect to the laser polarization is controlled by the phase of the light field irrespective of the direction of the ejected photoelectron. The work here measures both photo-electrons and protons in coincidence, assessing the emission direction in the molecular frame. In contrast to experiments where the lab-frame asymmetry was analyzed under the condition that ions appear with large kinetic energies (this is indicative of a fragmentation mechanism that involves the decay of doubly-excited states in the molecule), the study here focused on ground-state fragmentation via coherent superpositions of mixed-parity states. This approach is applicable to experiments using weak IR fields that do not introduce lab-frame asymmetry; they observed no lab-frame asymmetry for either photoelectrons or ions. The work here achieves entanglement control by manipulating the relative phases between the different pathways from outside, achieving control over molecular frame asymmetry at a sub-fs scale. They demonstrate active steering of the asymmetric electron emission by using a few-photon interaction with different colors in combination with the control of their relative delay. They determine the emission asymmetry from the angular distribution of the photoelectron with respect to the ejected H containing the bound electron after the photon dissociation by detecting the electron and proton in coincidence using a reaction microscope, where the H is ejected opposite to the direction of the proton. The spectrometer here includes position-sensitive detectors for electrons and ions. The researchers obtain the momentum components of the ion and the electron using time-of-flights and hit positions on the detectors. They use a fs laser with central wavelength 1030 nm and pulse duration 50 fs. The beam is split into two with the main part used to produce a comb-like XUV spectrum of odd harmonics with energies up to 40 eV (using high-harmonic generation). Both pulses interact with a H jet in the reaction microscope.

For more information: Phys.org, September 12 (2024); Sci. Rep., August 23 (2024).

WEEK OF SEPTEMBER 9, 2024 [No. 1592]

Solid-state optical nuclear-clock operated:   an international group lead by researchers in JILA at the University of Colorado Boulder in Boulder, CO has operated a nuclear clock prototype using crystal-embedded ^229mTh nuclei with a VUV laser exciter / frequency comb. They have measured the frequency ratio of the ^229mTh nuclear isomeric and the ⁸⁷Sr atomic transitions, demonstrating 10⁶ times the precision over the atomic clock. With his set up, they have realized quantum-state-resolved spectroscopy of the ^229mTh isomer and established a direct frequency connection with existing atomic clocks. The researchers used the VUV frequency comb to directly excite the narrow ²²⁹Th nuclear clock transition in a solid-state CaF₂ host material and to determine the absolute transition frequency. They stabilized the fundamental frequency comb to the JILA ⁸⁷Sr clock and coherently upconverted the fundamental to its seventh harmonic in the VUV range by using a fs enhancement cavity. This VUV comb was used to establish a frequency link between nuclear and electronic energy levels and allowed to directly measure the frequency ratio of the ²²⁹Th nuclear clock transition and the ⁸⁷Sr atomic clock transition. The researchers measured the nuclear quadrupole splittings and extracted intrinsic properties of the isomer.

For more information: Nature, September 4 (2024) page 63; Phys.org, September 4 (2024).

Edge state observed in a cloud of ultracold atoms:   researchers at MIT in Cambridge, MA have observed edge state in a cloud of ultracold atoms and captured images of atoms flowing along the boundary without resistance. They report the observation of chiral edge transport in a rapidly rotating quantum gas. They demonstrate the injection of chiral edge modes in a rapidly rotating bosonic superfluid confined by an optical boundary. By tuning the wall sharpness, they reveal the smooth crossover between soft wall behavior in which the propagation speed is proportional to wall steepness and the hard wall regime that exhibits chiral free particles. From the skipping motion of atoms along the boundary they infer the energy gap between the ground and first excited edge bands, and reveal its evolution from the bulk Landau level splitting for a soft boundary to the hard wall limit. They demonstrate the robustness of edge propagation against disorder by projecting an optical obstacle that is static in the rotating frame. They have studied the behavior of ultracold atoms in a setup that mimics the physics of electrons in a solid sample under a magnetic field. In their setup, the same physics occurs in atoms, but over ms / µm space / time scales, rather than fs / nm. They worked with a cloud of ~ 10⁶ Na in a laser-controlled trap, and cooled to nK. They manipulated the trap to spin the atoms around. The trap pulls the atoms inward, balanced by a spinning centrifugal force. Due to the Coriolis effect, atoms are deflected when moving in a line. So the atoms behave as if they were electrons in a magnetic field. The researchers then introduced an edge, in the form of a ring of laser light, which formed a circular wall around the spinning atoms. As they took images of the system, they observed that when the atoms encountered the ring of light, they flowed along its edge, in just one direction with no friction. There was no slowing down, and no atoms leaking or scattering into the rest of the system. There was just coherent flow. Those atoms were flowing free of friction for 100's µm. This frictionless flow held up even when the researchers placed an obstacle in the atoms' path in the form of a point of light, which they shone along the edge of the original laser ring. Even as the atoms came upon the flow obstacle, they did not slow their flow or scatter away, but instead glided right past without feeling friction. The researchers sent in a large, repulsive green blob, and the atoms did not bounce off it; instead they found their way around it, went back to the wall, and continued flowing in their way. The researcher's observations in atoms here document the same behavior that has been predicted to occur in electrons in edge states.

For more information: Phys.org, September 6 (2024); Nat. Phys., June 17 (2024).

WEEK OF SEPTEMBER 2, 2024 [No. 1591]

Atomic Sisyphus-type cooling technique developed:   researchers at NIST in Boulder, CO have developed a sub-recoil clock-line-mediated Sisyphus cooling technique that should improve the precision of atomic clocks. They demonstrate subrecoil Sisyphus cooling using the long-lived ³P₀ clock state in alkaline-earth-like Yb. A 1388-nm optical standing wave nearly resonant with the ³P₀→³D₁ transition creates a spatially periodic light shift of the ³P₀ clock state. Following excitation on the ultranarrow clock transition, they observe Sisyphus cooling in this potential, as the light shift is correlated with excitation to ³D₁ and subsequent spontaneous decay to the ¹S₀ ground state. They observe that cooling enhances the loading efficiency of atoms into a 759-nm magic-wavelength 1D optical lattice, as compared to standard Doppler cooling on the ¹S₀→³P₁ transition. The researchers engineered the energy shift of their excited clock state following a periodically modulated pattern. This allows to precisely control the location where a clock line excitation happens within the Sisyphus cooling process. They configure the excitation condition such that it occurs preferably at the position corresponding to the bottom of the periodic potential landscape. Once excited, atoms lose their kinetic energy by climbing the potential and preferentially exit the potential landscape away from the potential's minimum. The cooling is realized after repetitively climbing the energy potential. The researchers demonstrated their Sisyphus cooling scheme by leveraging the ultra-narrow transition of an Yb-based optical lattice clock. However, the same approach should theoretically be applicable to other systems with narrow linewidth transitions. For the last two decades, the goal of realizing high precision clock spectroscopy of neutral atoms has been best achieved by creating identical trapping conditions for atoms in both the ground state and the excited clock state. This is done by engineering a trap formed by lasers into a standing wave and that operates at a magic-wavelength. In this situation, a difference in the trapping potential felt by the atoms in the two atomic states works against the realization of a high-precision clock spectroscopy. In trying to minimize the trap potential difference between the ground state and excited clock state, the researchers here focused on enhancing the cooling of samples before carrying out the high precision clock spectroscopy. They momentarily introduced an engineered spatially dependent excited state shift, which introduced more trap potential difference for the two clock states. Doing so allowed them to realize the Sisyphus cooling mechanism, which in turn improved the sample condition later for better clock spectroscopy with less trap potential difference. The cooler temperatures helped them to use shallower traps on the atoms (where unwanted trapping effects are even smaller), which also reduced this difference. This additional cooling allows the creation of atomic ensembles with more uniform conditions inside the magic-wavelength standing-wave laser trap; and thus, to more precisely characterize small effects of the trapping laser on the clock frequency. Sisyphus cooling (pulsed or continuous), yields T < 200 nK in the weakly confined, transverse dimensions of the 1D optical lattice. These lower temperatures improve optical lattice clocks by facilitating the use of shallow lattices with reduced light shifts while retaining large atom numbers to reduce the quantum projection noise.

For more information: Phys.org, August 29 (2024); Phys. Rev. Lett., July 31 (2024) page 053401.

Edge supercurrents analyzed in Weyl superconductor:   researchers at Princeton University in Princeton, NJ have shown that edge supercurrents reveal interaction between condensates in a Weyl superconductor. And that the superconducting edge currents in the topological material MoTe₂ can sustain large changes in the potential that keeps the superconducting electrons paired. They show that, when supercurrent is injected into the superconducting Weyl semimetal MoTe₂ from Nb contacts, the invasive s-wave pairing potential from Nb is incompatible with the intrinsic Cooper pair condensate in MoTe₂. This incompatibility leads to strong stochasticity in the switching current and an unusual anti-hysteretic behavior in the I-V loops. There is also an asymmetry in the edge oscillations where, as the magnetic field crosses zero, the phase noise switches from one with a noisy spectrum to one that is noise free. Using the noise spectrum as a guide, they track the anomalous features to field-induced switching of the device gap function between s-wave symmetry and the unconventional symmetry intrinsic to MoTe₂. They conclude that the behavior of the gap function along the edges is different from that in the bulk. When MoTe₂ becomes superconducting, the supercurrent oscillates in a magnetic field. The edge supercurrent oscillates more rapidly than that in the bulk, showing up as a characteristic modulation of the bulk response. To enhance the pairing potential in MoTe₂ and with Nb having a stronger pair potential, the researchers deposited Nb on top of it. Thus, the Nb pair potential affects MoTe₂ and the electrons in MoTe₂ are temporarily subject to the stronger potential. This leakage strengthens the supercurrent oscillations but also reveals incompatibility between the Nb and the MoTe₂ pair potentials. The two cannot seamlessly merge and the wavefunction guiding the edge electrons switches between the Nb and the MoTe₂ pair potential, according to which potential prevails. The choice made by the edge electrons is reflected in the oscillations. These are noisy when the edge pair potential differs from that of bulk MoTe₂, and almost noise free when the two are the same. The work here confirms the existence of edge supercurrents and shows that they can be used to monitor the behavior of superconducting electrons in topological superconductors.

For more information: Phys.org, August 26 (2024); Nat. Phys., January 11 (2024) page 261.

WEEK OF AUGUST 26, 2024 [No. 1590]

⁹Be³⁺ nuclear magnetic moment measured with record precision:   researchers at MPINP in Heidelberg have used atomic spectroscopy in a Penning trap to determine the ⁹Be nuclear magnetic moment with 40 times higher precision than before. They focused on ⁹Be (allows for comparisons between different charge states available for high-precision spectroscopy in Penning traps) to test theoretical calculations obscured by nuclear structure. They performed high-precision spectroscopy of the 1s hyperfine and Zeeman structure in H-like ⁹Be³⁺. They determined the effective Zemach radius with uncertainty ~ 500 ppm, and the bare nuclear magnetic moment with uncertainty ~0.6 ppb. The researchers compared their measurements with the measurements conducted on ⁹Be⁺, thus allowing the testing of calculation of multi-electron diamagnetic shielding effects of the nuclear magnetic moment at ppb level. Then, they tested the QED methods used for the calculation of the hyperfine splitting (HFS). The researchers here trapped the ⁹Be³⁺ ion (considered theoretically here as a two-body system) using the electron used as antenna to measure the nuclear magnetic moment. The trapping magnetic field is obtained from the ion cyclotron frequency measurement. Measurements of the 1s-HFS interval in H-like systems are the most sensitive to the Zemach radius and serve as ideal references to evaluate the nuclear structure effects in other HFS intervals and QED. However, for low nuclear charge Z, these measurements exist only for the H isotopes and ³He; for high Z, tests of the HFS lack accurate experimental values of the nuclear magnetic moments. Their previous high-precision Penning-trap measurement of the Zeeman and HFS of ³He⁺ allowed the researchers to directly measure its Zemach radius and nuclear magnetic moment, simultaneously providing both parameters needed for precise predictions of other HFS intervals. An accurate value of the atom magnetic moment allows absolute magnetometry with hyperpolarized ³He. However, this requires transferring the measured nuclear magnetic moment from the H-like system to the neutral system, which involves the theoretical calculation of diamagnetic shielding parameters. In the past, inadequate calculations of these parameters have led to several discrepancies in precision physics. In those studies, the required nuclear magnetic moments were obtained from measurements using systems with complex electronic structures and calculations of shielding parameters relying on quantum chemistry codes that frequently provided underestimated uncertainties. For systems such as H-like and neutral ³He, these issues are manageable due to their simple electronic structure, that allows for diamagnetic shielding calculations using non-relativistic QED methods. In the work here, the diamagnetic shielding parameters are calculated by perturbation analysis, and the estimated uncertainties are an order of magnitude better than in the experimental value of the ³He nuclear magnetic moment. Improvements to the non-relativistic QED theory value (at the same level as the experimental uncertainty) have been obtained recently, so experimental verification and benchmark for diamagnetic shielding calculations are now required. ⁹Be serves well to test the diamagnetic shielding calculations and to introduce an accurate reference for nuclear structure contributions in the hyperfine interaction. The low nuclear charge permits high accuracy calculations in the H-like system and probing the Zeeman and HFS using spectroscopy in a Penning trap for ⁹Be⁺ and ⁹Be³⁺ charge states. The high accuracy of theoretical calculations possible in H-like ⁹Be³⁺ allows for good determinations of the Zemach radius and the magnetic moment of the bare nucleus. The researchers here do a comparison between the experimental hyperfine and Zeeman splitting of ⁹Be⁺ and ⁹Be³⁺, and use it to eliminate the nuclear structure-dependent terms. This allows for testing of QED (using HFS specific differences) and for testing calculations of multi-electron diamagnetic shielding parameters at the ppb level.

For more information: Phys.org, August 19 (2024); Nature, August 14 (2024) page 757.

Kagome metals host wave-like Cooper pair distributions:   an international group lead by researchers at the Southern University of Science and Technology in Shenzhen and the Julius-Maximilians-Universität Würzburg in Würzburg has experimentally proven that Cooper pairs display wave-like distribution within the sublattices in kagome metals (sublattice modulated superconductivity with each star point containing a different number of Cooper pairs) as previously proposed (by some group members). This overturns the conventional assumption that Kagome metals could only host uniformly distributed Cooper pairs. They detected chiral kagome superconductivity modulations with residual Fermi arcs in KV₃Sb₅ and CsV₃Sb₅ using normal and Josephson STM at 30 mK with resolved electronic energy difference at the µeV level. They identify a superconducting order featuring spatial pair modulations on the kagome lattice subject to on-site Hubbard U and nearest-neighbor V interactions. Within their functional renormalization group analysis, this state appears with a concomitant d-wave superconducting (SC) instability at zero lattice momentum, where it distinguishes itself through intra-unit-cell modulations of the pairing function thus breaking the discrete space group symmetry. The relative weight of the sublattice modulated superconductor and d-wave SC is influenced by the absolute interaction strength and coupling ratio V/U. Parametrically adjacent to this domain at weak coupling, they find an intra-unit-cell modulated vestigial charge density wave and an s-wave SC instability. They observed a U-shaped superconducting gap with flat residual in-gap states. This gap shows chiral 2a × 2a spatial modulations with magnetic-field-tunable chirality, which align with the chiral 2a × 2a pair-density modulations observed by Josephson tunneling. These findings demonstrate a chiral pair density wave (PDW) that breaks time-reversal symmetry. Quasiparticle interference imaging of the in-gap, zero-energy states reveals segmented arcs, with high-temperature data linking them to parts of the reconstructed Va d-orbital states within the charge order. The detected residual Fermi arcs can be explained by the partial suppression of these d-orbital states through an interorbital 2a × 2a PDW and thus serve as candidate Bogoliubov Fermi states. The researchers differentiate the observed PDW order from impurity-induced gap modulations. Initially, their theoretical research on Kagome metals focused on the quantum effects of individual electrons, which, although not superconducting, can exhibit wave-like behavior in the material. Their initial theory (a kagome Hubbard model) on electron behavior with charge density waves (CDW) was experimentally confirmed two years ago . The results indicated that at ~ 80 K, the electrons in the material reorganize and distribute in waves. At ~1 K, Cooper pairs condense into a quantum fluid that spreads in waves through the material, enabling resistance-free superconductivity with this wave-like distribution transmitted from electrons to Cooper pairs. The finding here is that these pairs can be distributed not just evenly, but also in a wave-like pattern within the Kagome metal (sublattice-modulated superconductivity). The presence of PDWs in KV3Sb5 is due to the wave-like electron distribution at ~ 80 K. The direct measurement of the Cooper pairs' distribution here utilized a STM with a single-atom superconducting Josephson tip.

For more information: Nature, August 21 (2024) page 775; Phys.org, August 23 (2024); Nat. Phys. Rev. B, July 1 (2024) page 024501.

WEEK OF AUGUST 19, 2024 [No. 1589]

Fermi level meV-tuning by fast ion implantation:   researchers at MIT in Cambridge, MA have demonstrated ultra-fine carrier doping and Fermi level (meV) tuning on 3D TaP , a topological Weyl semimetal (WSM), using accelerator-based high-energy H^1- implantation. The doping experiment uses a two-stage Tandem ion accelerator (1.7 MV) located at the CSTAR center that consists of two successive linear accelerators with the same power supply. A Cs sputtering source is used on a solid target, which produces negative ions that are then subjected to electron exchange with neutral Cs. Negative ions are initially introduced into the first stage, where they are accelerated toward the positive HV terminal. H^1- was chosen as the dopant here so as to not significantly alter the structure of the sample. With this system, high-precision doping level control is possible as the accelerator allows high precision control of the beam energy and the ion flux. The researchers could set a doping target value as defined through a combination of DFT and Monte Carlo calculations, and showing quantitative agreement with experimentally obtained results. By calculating the desired carrier density and controlling the accelerator profiles, the Fermi level was experimentally fine-tuned from 5 meV below, to 3.8 meV below, and to 3.2 meV above the Weyl nodes. High-resolution TEM with high-angle annular dark-field (STEM-HAADF) and XRD imaging, show high-quality crystal preservation after ion implantation with stacking faults within depth ~ 200 nm. Electrical transport measurements indicate that Weyl nodes are preserved and carrier mobility is largely retained.. Due to the H^1- low stopping power, its implantation, preserves the structural integrity of TaP without generating visible amorphous tracks, voids, or defects. Despite the observation of stacking faults within a 200-nm depth from the sample surface, likely induced by the kinetic energy of H^− ion bombardment causing rigid lattice shifts, electric transport and carrier mobility remains intact. Transport measurements show increment in the charge neutral point temperature through precise tuning of the Fermi level in proximity to the Weyl nodes. The temperature needed to reach the charge neutral point is nearly doubled. Inversion symmetry is not restored as required for Weyl node formation, possibly explaining the preservation of Weyl nodes under irradiation. The researchers doped the sample with H^1- by substituting a H^1- for a Ta. When optimal doping is achieved, the Fermi level coincides with the energy level of the Weyl nodes. For WSMs, the Fermi level (and the Weyl nodes) is especially sensitive to doping. If the Fermi level is not set close to the Weyl nodes, the material's properties are significantly worse than ideally possible. The reason for this extreme sensitivity is its peculiar geometry of the Weyl node. Since the researchers cannot measure the Fermi level while the sample is in the accelerator chamber, they used a theoretical model to predict how many electrons were needed to increase the Fermi level to the preferred level and to translate that to the number of negative H^1- that must be added to the sample (depending on the energy of the ion beam, the exposure time, and the size and thickness of the sample) and to the time the sample ought to be kept in the accelerator chamber. This model results roughly agree with those of proven computationally intensive models. The technique here can be used for any inorganic material (bulk and thin film).

For more information: Phys.org, August 13 (2024); App. Phys. Rev., June 25 (2024).

Leggett-Garg inequality measured with neutron interferometry:   researchers at TU Wien in Vienna have utilized the S18 instrument at the Institut Laue-Langevin in Grenoble to demonstrate the violation of the Leggett-Garg inequality (LGI) using ideal negative measurements in neutron interferometry to conclude the LGI violation and, thus, that no classical macroscopic theory can truly describe reality. The final measured value of the Leggett-Garg correlator K =1.120±0.007(stat)±0.019(sys), obtained in a neutron interferometric experiment, is clearly above the limit K =1 predicted by macrorealistic theories. This reaffirms that a particle can be in a superposition of two states associated with different locations even when these locations are apart. If classical realism holds, Leggett and Garg showed that the degree of these correlations cannot exceed a certain level. Specifically, for a set of three measurements, the quantity K ≡ C₂₁ + C₃₂ – C₃₁ (C_ij is a correlation function in different measurements) must be < 1 (K > 1 if quantum mechanics holds). Previous experiments have already demonstrated LGI violations in several quantum systems, including photonic qubits, nuclear spins in diamond defect centers, superconducting qubits and impurities in Si. The neutron interferometer is a Si-based crystal interferometer in which an incident neutron beam is split into two partial beams (cms apart) at a crystal plate and then recombined by another piece of Si. This configuration means there are three distinct regions in which the neutrons’ locations can be measured: in front, inside and behind the interferometer. The actual measurement of the two-level system’s state probes the presence of the neutron in two particular regions of the interferometer. Their measurement probes the absence rather than the presence of the neutron in the interferometer (ideal negative  measurement, performed that way to prevent disturbance of the time evolution of the system). After combining several neutron measurements, the researchers showed that the LGI is indeed violated. The obtained result cannot be explained within the framework of macro-realistic theories, only by quantum theory. One consequence is that the idea that maybe the neutron is only traveling on one of the two paths and we just don’t know which one, cannot be true. There is no time inside the interferometer when the neutron is in a given state ( either in path 1 or in path 2). Instead, the neutron must be in a coherent superposition of system states. The experimental results were analyzed within the framework of dynamical theory of neutron diffraction reproducing the obtained values as well.

For more information: Physicsworld, August 12 (2024); Phys. Rev. Lett., June 24 (2024) page 260201.

WEEK OF AUGUST 12, 2024 [No. 1588]

Magnon-phonon Fermi resonance observed in antiferromagnet:   an international group lead by researchers at Radboud University in Nijmegen and the University of Cologne in Cologne has found evidence of strong coupling between the spin and the crystal lattice in antiferromagnetic CoF₂. The results revealed an energy transfer channel between magnons and phonons in an antiferromagnet under the condition of Fermi resonance. They have uncovered a regime of magnon-phonon dynamics that in the vicinity of the Fermi resonance condition, facilitates a mutual, anharmonic energy exchange between magnons and phonons. The non-linearly coupled magnon-phonon dynamics is accompanied by a nontrivial energy exchange between the subsystems. They find that by tuning the eigenmode frequencies, one can control this process, enhancing magnon-phonon coupling. They demonstrate a broadening of phonon spectra and an asymmetric redistribution of the phonon weight upon tuning the magnon frequency with an external magnetic field. This suggests the formation of a strongly coupled two magnon-phonon hybridization state. They study the strongly coupled regime of Fermi resonance and propose that a strongly coupled two-magnon-one phonon state in an antiferromagnet allows to coherently control magnon-phonon dynamics. Utilizing intense narrow-band THz pulses and tunable magnetic fields up to μ₀H_ext = 7 T, they experimentally realize the conditions of magnon-phonon Fermi resonance in the antiferromagnetic CoF₂. These conditions imply that both the spin and the lattice anharmonicities harvest energy from the transfer between the subsystems if the magnon eigenfrequency f_m is half the frequency of the phonon (2f_m = f_ph). Performing THz pump-IR probe spectroscopy in conjunction with simulations, they explore the coupled magnon-phonon dynamics in the vicinity of the Fermi-resonance to reveal the nonlinear interaction facilitating energy exchange between these subsystems. Using the intense and spectrally bright accelerator-based superradiant THz source at HZDR's ELBE Center for High-Power Radiation Sources, the researchers selectively excited the antiferromagnetic spin resonance and tuned its center frequency by high external magnetic field (up to several T). This configuration allowed them to tune the spin resonance frequencies to half the lattice vibration frequency, thus satisfying the Fermi resonance condition. This regime of coupled magnon–phonon dynamics allows energy exchange between these two subsystems at the Fermi resonance. By tuning the frequencies of the magnons, the researchers can control this process and enhance the magnon–phonon coupling. This regime was observed as a broadening of the phonon spectra and an asymmetric redistribution of the phonon spectral weight. The results here suggest a hybridized two-magnon-one-phonon state, and a way to manipulate spin-lattice coupling on demand. This allows for an increase in operational frequency from the conventional GHz using ferromagnetic materials up to the THz scale in antiferromagnetic materials. The recently demonstrated nonlinear excitation of a phonon mode mediated by a magnon state suggested the strongly coupled regime of Fermi resonance in antiferromagnetic CoF₂. The researchers here anticipated that in the vicinity of the resonance, two-magnon-one-phonon interaction would determine the coupled dynamics. In zero applied magnetic field the double magnon frequency 2f₀ at T = 6 K is higher than the frequency f_ph of the phonon (2f₀ > f_ph) and thus the system is not in resonance. Thus, they applied an external magnetic field along the magnetic easy-axis, which splits the magnon branches, while the phonon frequency remains unchanged. A field of μ₀H_ext = 4T was expected to be sufficient to reach the frequency matching condition with a lower energy magnon branch 2f_m = f_ph, where the conditions of Fermi resonance might be satisfied. Performing THz pump- IR probe spectroscopy in combination with simulations, they revealed the corresponding fingerprints of coherent energy exchange driven by a THz stimulus in the vicinity of the Fermi-resonance. In contrast to previous theoretical studies focused on incoherent lattice dynamics and thus revealing stochastic acts of energy exchange between the modes, the experiments here revealed the manifestation of the Fermi resonances for the case of coherent dynamics. The nature of mode interaction opens the possibility to control the modes’ scattering rate, which, in conjunction with the pulsed excitation regime, provides a model system to study strong coupling interactions in the time domain. Thus, using the methods of coherent control one should be able to steer the energy flow between spins and lattices.

For more information: Phys.org, August 7 (2024); Nat. Comm., June 28 (2024).

Hard XR-based nuclear quantum memory demonstrated:   an international group lead by researchers at DESY in Hamburg has used the synchrotrons PETRA III in DESY and ESRF in Grenoble to experimentally demonstrate a way of storing and releasing hard XR pulses at the single photon level as proposed by researchers at TA&M University in College Station, TX. The XR quantum memory protocol utilizes mechanically driven nuclear resonant ⁵⁷Fe absorbers to form a comb structure in the nuclear absorption spectrum induced by motion's Doppler frequency shift. A short pulse with the spectrum matching a comb absorbed by such a set of nuclear targets would be re-emitted with the delay determined by the inverse Doppler shift as a result of the constructive interference between different spectral components. This RT nuclear frequency comb (NFC) allows control of the waveform of XR photon wave packets to a high accuracy and fidelity using solely mechanical motions. Trying to extend the quantum memory concept to the hard XR regime, the researchers experimentally have implemented a robust NFC with up to seven teeth. After excitation with synchrotron XR pulses, the prepared NFC reshapes the temporal response of the coherent collective nuclear excitation of ⁵⁷Fe nuclei in the forward scattering direction. This results in the formation of XR wave packets on the single-photon level with durations inverse to the frequency comb’s bandwidth. When adding a thin-film XR cavity, spectrally designed to match the frequency comb’s bandwidth, the wave packet of the emitted weak coherent field by the cavity is stored and retrieved by the NFC with efficiency and fidelity. The set of Doppler-shifted ⁵⁷Fe absorbers allows for reshaping the emission characteristics of the nuclear polariton to produce strong and short echo signals well above the noise level. The achieved storage time (~ 30 ns) exceeds the wave packets duration by an order of magnitude. The quantum memory idea for information storage and retrieval here is to imprint the photon information into a quasi-stationary medium in the form of polarization or spin wave with a long coherence time and release it back via re-emission of the original photons. By using nuclear rather than atomic ensembles this system delivers much longer memory times at high solid-state densities and RT. The longer memory times are a direct result of the lower sensitivity of nuclear transitions to perturbations by external fields (due to small nuclei sizes). This together with a tight focusing of the hard XR photons, could lead to long-lived broad-band compact solid-state quantum memories. The idea is realized here with one stationary and six synchronously moving absorbers forming a seven-teeth frequency comb. Nuclear coherence lifetime is the limiting factor that determines the maximum storage time for this type of quantum memory. Thus, using longer-lived isomers than the ⁵⁷Fe isotope used here should result in a longer memory time. ⁵⁷Fe nuclear resonances (including its prominent 14.4 keV nuclear resonance) exhibit ultrahigh quality factors and long coherence lifetimes (Q ~10¹³ and Ï„₀ = 141 ns) at RT. These are important prerequisites for quantum memories where phase and amplitude of the stored qubit must be preserved. Coherent temporal control of the excited nuclear quantum state was demonstrated here by induced phase shifts via motion-induced Doppler shifts. This allows for the formation of a quantum memory for hard XR photon wave packets using the set of moving resonant nuclear ⁵⁷Fe absorbers. With the small transition linewidths and the typical high transition energies E₀, a sizeable Doppler shift ∆E = E₀v/c, is achieved at comparatively small velocities v (0.1 mm/s shifts the ⁵⁷Fe transition by its natural linewidth). This proposed quantum memory concept is based on moving multiple resonant absorbers with different but equidistantly spaced velocities to form a Doppler NFC in the resonant absorption structure. An incident resonant photon wave packet entering such a nuclear ensemble gets stored in the coherent collective nuclear excitation and is reemitted with a high probability at certain moments in time T_k (k = 1,2,3, …), which are determined by the velocity spacing ∆v between neighboring absorbers as T_k = k hc / E₀ ∆v (periodic reemissions or echoes). With the frequency comb parameters (bandwidth, teeth width, and number of teeth) tuned correctly, an efficient and reliable quantum memory can be constructed. Working at a single-photon level without losing information is expected to qualify this nuclear frequency comb protocol as a quantum memory. The tunable, robust, and flexible system demonstrated here provides a platform for a compact RT solid-state quantum memory for hard XR.

For more information: Phys.org, August 9 (2024); ScienceAdvances, June 23 (2024).

WEEK OF AUGUST 5, 2024 [No. 1587]

Hyper-Raman emission observed:   an international group lead by researchers at the University of Bath in Bath have experimentally demonstrated the molecular hyper-Raman effect, a chiral optical effect (less intense than the Raman effect) where two photons impact the molecule simultaneously and then combine to create a single scattered photon that exhibits the Raman wavelength shift. They use a doubly resonant system of plasmonic Au nanohelices and crystal violet (CV) molecules, where the chirality of the electromagnetic field of Au nanohelices is conferred to the achiral molecules (1/100 in size). It has been theorized (1979) that chiral light used for the hyper-Raman effect could deliver 3D molecular information including its chirality. The experimental difficulty has lied on the balance between molecular absorbing light chiral purity and molecular non-damaging laser illumination. The researchers here rather than measuring the effect directly from chiral molecules, employed achiral molecules that are made chiral by assembly on a chiral scaffold. They augmented the hyper-Raman signal by depositing achiral molecules on Au nanohelices that made the molecules chiral and served as nanoantennas to focus light onto the molecules. This chirality conferral system was designed as doubly resonant, with the nanohelices and molecules resonating at the fundamental frequency and at the second-harmonic, respectively (thus, helping with Fermi's golden rule compliance here). They took care to eliminate possible experimental artefacts from extrinsic chirality in the nanohelices, from the degree of light circularity, from photodecomposition and from differences in the number of illuminated molecules. The researchers demonstrate chirality conferral from the electromagnetic field of chiral plasmonic nanoparticles (Au nanohelices) to CV molecules that are achiral (on average). With illumination at 1,064 nm, they recorded the difference of hyper-Raman spectra (~ 532 nm) for right- and left-handed circularly polarized light (RCP and LCP), that is, the circular intensity difference (CID). The CID spectrum from the achiral CV changes sign, depending on the handedness of the nanohelices. They demonstrate that their results correspond to intrinsic chirality. The arrays of Au nanohelices exhibit extrinsic chirality in linear reflection, in second-harmonic generation circular dichroism (CD) and in second-harmonic generation optical rotation. The extrinsic chirality results in a sign change of the chiroptical response upon rotating the sample (sample rotation effects also affect Raman scattering). They demonstrate that the sign of the Raman and hyper-Raman peaks in their CID spectra does not depend on sample rotation as expected in chirality. They show that no hyper-Raman emission is observed from achiral Au nanostructures coated with CV, thereby confirming the role of the nanohelices’ geometry. The original paper on hyper-Raman emission focused on chiral molecules with no use of chirality conferral by plasmonic nanohelices to achiral molecules. To take this difference into account, they expand the original theory to surface-enhanced hyper-Raman scattering including the chirality conferral mechanism. The origin of the effect is shown by identifying the lead light–matter interaction processes and by calculating their quantum amplitudes, whose modulus square produces the hyper-Raman signal intensity observed. They show that the chiroptical response originates from the sum of cross-terms between the leading, fully electric dipole quantum amplitude contribution and terms that include contributions from magnetic dipoles or electric quadrupoles. Further optimization of surface enhanced Raman scattering (SERS) materials and substrates should lead to greater enhancements in Raman signals, allowing for the detection and analysis of lower concentrations of analytes.

For more information: Phys.org, July 31 (2024); Nat. Phot., July 31 (2024).

S superconducting gap measured at Mbar pressures:   researchers at MPI in Mainz and LANL in Los Alamos, NM have developed a planar tunnel junction technique for diamond anvil cells to make measurements of superconducting S at pressures over 100 GPa using tunneling spectroscopy. At pressures greater than 90 GPa, S (part of the high-temperature superconductor H₃S) changes from being a nonmetal to behaving like a metal. Then, at low temperatures, it becomes superconducting. Here, a µm-sized S sample was subjected to 160 GPa and 17 K in a diamond anvil. Extreme pressure affects the properties and integrity of the tunneling barrier in the planar tunneling junction (where an applied voltage allows electrons to tunnel across an insulating barrier between two conductors) loaded into the pressure cell transducer, and the device suffers leaked currents and a reduction in the size of the potential energy barrier. Thus, the researchers fabricated an in-situ planar tunnel junction within the diamond anvil cell. The planar tunnel junction's tunneling gap (~ 50 µm x 50 mm) was placed between S and Ta and made of TaO₅ (with its high density and stability ensured the width of the tunneling gap would not change under high pressures). This set up allowed tunneling spectroscopy to measure the S energy levels, the local density of electronic states and the band gap using an STM over 100 GPa. To measure the superconducting gap they used its relation to the current that tunnels from a normal conductor to a superconductor when the two materials sit on either side of an insulating barrier. The researchers confirmed S's T_c ~ 17 K and determined the β-Po phase S to be a type II superconductor with a single s-wave gap with gap value 5.6 meV. They measured the S’s superconducting gap, finding a value that matched theoretical predictions.

For more information: Phys.org, July 31 (2024); Physics, July 17 (2024); Phys. Rev. Lett., July 17 (2024) page 036002.

WEEK OF JULY 29, 2024 [No. 1586]

Atomic clock with record systematic uncertainty demonstrated:   researchers in JILA at the University of Colorado Boulder in Boulder, CO have created a highly precise optical lattice clock (OLC) using 10⁵ trapped Sr atoms as reference. They measured the frequency of a Sr atomic transition with a systematic uncertainty of 8.1 × 10^−19 in fractional frequency units, more than a factor of 2 over the previous record holder (another Sr OLC). The clock relies on interrogating the ultranarrow ¹S₀ → ³P₀ transition (the least magnetically sensitive clock transition in Sr atoms) in a dilute ensemble of fermionic Sr atoms trapped in a vertically-oriented, shallow, 1D optical lattice. Using imaging spectroscopy, they had previously demonstrated record high atomic coherence time and measurement precision enabled by precise control of collisional shifts and the lattice light shift. Here they revised the black body radiation shift correction by evaluating the 5s4d ³D₁ lifetime, necessitating precise characterization and control of many body effects in the 5s4d ³D₁ decay. In previous Sr OLCs, accuracy was limited mainly by the uncertainty in the shift of the ³P₀ level. They performed a precise calibration of the second-order Zeeman coefficient. All other systematic effects have uncertainties below 1×10^-19. The Sr atoms probed by the clock laser to determine the transition frequency, are trapped within a standing wave of light produced by two mirrors (lattice parameter 0.5 µm) at UHV. When a light signal is resonant with the transition, its frequency is defined precisely. After the frequency comb converts optical frequencies into microwave frequencies, the periods of these frequencies are analyzed by electronics to output 1 s (tied to the frequency of the Sr transition). Every few seconds they shined the laser on the atoms for 2.4s. If the laser has drifted from the atomic resonance, they correct this drift. Every time the researchers make a measurement of the Sr transition frequency they can simultaneously measure many atoms, giving a precise measurement. This is in contrast with ion optical clocks that use an electronic transition within a single trapped ion, and thus, every measurement is noisier. Since the trapping light could shift the transition frequency, the researchers utilize lower intensity trapping light than in previous systems to boost clock accuracy. The calibration here reduces the uncertainty due to the second-order Zeeman shift to 1 × 10^−19, twice better than in previous calibrations. The OLC here operates at RT so the researchers considered the dynamic black-body-radiation correction (Sr atom’s energy levels shift via the radiation’s electric field). The size of the shift of the ³P₀ level is tied to the ³D₁ transition that lies within the energy spectrum of the black-body radiation and can be determined by measuring the ³D₁ state’s lifetime. In this way, the researchers reduced the uncertainty in the black-body-radiation shift to 7.3 × 10^−19 (down from the 1.5 ×10^−18 value that they had achieved before). Combining the reduction of black-body-radiation shifts with other environmental control measures including temperature stabilization, they determined the sum of all systematic effects on the clock transition’s energy levels < one part in 1 × 10¹⁸. In an optical lattice trap, the atoms’ energy levels can be shifted by the electric fields of the laser beams. To control and measure the atoms, the researchers used a magic wavelength optical lattice (trapping potential is the same for all atoms regardless of their electronic state), and thus, the relative energy shift that the laser beams induce between the clock transition states is minimized and the transition’s linewidth is made narrower. The size of the energy shift induced by the laser beams is greater when the atoms are more tightly confined, so they used a cooling process that allowed confinement in a shallow lattice potential. These techniques made their system to surpass the precision of all previous OLCs, with a timekeeping error < 1 s / 39.6 By.

For more information: Physics, July 29 (2024); Physorg, July 24 (2024); Phys. Rev. Lett., July 10 (2024) page 023401.

Spontaneous strain-induced nanopattern observed in metal-insulator transition:   researchers at Cornell University in Ithaca, NY have used XR nanodiffraction assisted by phase-retrieval algorithms and machine learning, cryogenic electron microscopy, and local resistivity measurements to observe a supercrystal state forming spontaneously in a Ca₂RuO₄ Mott insulator thin film, during a strain-engineered metal-insulator transition. They report a hierarchically ordered supercrystal state with its intrinsic formation characterized in-situ during a Mott transition. They show that under strain the atomic structure in the sample forms an anisotropic, organized nanopattern with multi-scale spatial periodic domain formation during cryogenic cooling at and below the film transition temperature (T_Film ≈ 200–250 K) and a separate anisotropic spatial structure at and above that temperature. The 10-nm structure was observed to be embedded in a larger supercrystal with the orientation of this larger structure dictating the electronic properties and creating a current switch in the current's preferred direction. Local resistivity measurements imply an intrinsic coupling of the supercrystal orientation to the material's anisotropic conductivity.

For more information: Phys.org, July 23 (2024); Adv. Mat., June 17 (2024).

WEEK OF JULY 22, 2024 [No. 1585]

Isolated γ-γ nuclear decay measured:   an international group lead by researchers at Ruprecht-Karls-Universität Heidelberg in Heidelberg and Universite Paris-Saclay in Paris has observed a 2γ decay (discovered in the 1980s at the MPIK in Heidelberg) on a bare atomic nucleus and directly measured the partial half-life for the 2γ emission. The measurements on ⁷²Ge nuclei were carried out at the experimental storage ring (ESR) at GSI in Darmstadt. To be able to directly measure the 2γ decay rate in the low-energy regime below the electron-positron pair-creation threshold, they combined the isochronous mode of a storage ring with Schottky resonant cavities. They report the direct measurement of the 2γ decay of the first excited 0+ state in stored, fully ionized ⁷²Ge³²⁺ nuclei. This isomer, with an excitation energy of 691.43(4) keV, possesses a half-life of 444.2(8) ns in neutral atoms. However, when it is fully ionized, the partial half-life for this isolated decay can be estimated to extend to several hundred ms, using the average value of the previously determined Mγγ matrix elements. By combining the isochronous mode of the storage ring with nondestructive single-ion-sensitive Schottky detectors, the experimental technique here (Schottky plus isochronous mass spectrometry, S+IMS), the researchers resolved the isomeric state and measure the time evolution of the number of observed isomers with resolution ~ ms. The nuclear 2γ decay involves the decay of an excited nucleus through the simultaneous emission of 2γ via the virtual excitation of intermediate states. The partial half-life of this decay gives access to observables such as the (transitional) electromagnetic polarizability and susceptibility, which constrain the nuclear equation of state and the nuclear matrix elements of the neutrinoless double-β decay, and determine the neutron skin thickness. The experiment was conducted at the GSI accelerator facility, using a primary ⁷⁸Kr beam that was accelerated to an energy of 441  MeV/nucleon using the heavy-ion synchrotron SIS-18. After fast extraction the beam was impinged on a 10-mm thick ⁹Be production target placed in the transfer beamline towards the ESR. Few-nucleon removal reactions at relativistic energies produce low-lying isomeric states with relatively high probability (up to 10%). The ⁷²Ge fragments emerged from the target with a mean energy of 367.9  MeV/nucleon. At this relativistic energy the ⁷²Ge ions were fully ionized and were transported and injected into the ESR. No additional beam purification was necessary, allowing the storage of a large number of fragments within the acceptance of the ESR. The stored ions revolved in the ESR with frequencies ~ 2 MHz. A beam of Kr ions was accelerated to ~ 0.7c with the GSI accelerator facility and subsequently passed through a Be plate with thickness ~ 1 cm. In the collision, the required ⁷²Ge ions are produced in a specific excited state, which has the same spin-parity quantum number 0+ as the ground state. In this situation, the usually dominating decay by the emission of a single γ is forbidden due to angular momentum conservation, since the γ must take away an intrinsic spin of one unit. Other competing decay modes, such as the transfer of the energy to an electron of the atomic shell, are also not possible here. Therefore, the 2γ decay becomes the dominant decay mode. In a nuclear reaction at relativistic energies the produced ions have a large velocity spread. To force the same ion species to have identical revolution frequencies, they tuned the ESR into a isochronous mode, such that the differences in velocities are exactly compensated by the lengths of ion trajectories. Thus, they reliably separated the ground and the isomeric states of ⁷²Ge ions despite their relative ppm mass difference. They tracked each ion in the isomeric state non-destructively and precisely determined the time of its decay. The measured half-life for the 2γ decay of the first excited 0+ state in bare ⁷²Ge ions was 23.9(6) ms, that is 5 x 104 longer than in the atomic state and strongly deviates from theoretical expectations. The measured half-life is by at least two orders of magnitude shorter than the shortest lifetime directly measured previously for stored highly charged ions. The technique here can be applied to isomers with excitation energies down to ~ 100  keV and half-lives down to ~10  ms. Additional experiments using direct γ spectroscopy of the 2γ decay branch in ⁷²Ge are planned to clarify the role of the magnetic and electric-dipole terms.

For more information: Phys.org, July 17 (2024); Phys. Rev. Lett., July 11 (2024) page 022502.

Solid state electron motion measured with record resolution:   researchers at the University of Stuttgart have used time-resolving STM (including pump-probe THz spectroscopy) to record the collective movement of electrons at the atomic level with ultra-high spatio-temporal resolution. By utilizing THz pump–probe spectroscopy in a STM, the researchers measure the ultrafast collective dynamics of the charge density wave (CDW) in the transition metal dichalcogenide 2H-NbSe₂ with atomic spatial resolution. The tip-enhanced electric field of the THz pulses excites oscillations of the CDW that vary in magnitude and frequency on the scale of individual atomic impurities. Overlapping phase excitations originating from the randomly distributed atomic defects in the surface create this spatially structured response of the CDW. After excitation by a THz pulse, the CDW state relaxes through low-energy phase excitations in the sub-THz range that are enabled by atomic-sized defects that pin the CDW and are heterogeneous on the nm scale. The atomic spatial resolution of the STM-based THz spectroscopy locally detects charge density dynamics. The researchers used THz spectroscopy of collective CDW dynamics at the atomic scale to study the collective motion of electrons in a CDW in 2H-NbSe₂ and specifically how a single impurity can stop this collective movement. The technique here allows direct real-space observation of phase excitation dynamics of defect-induced charge modulation. When applying a 1-ps electrical pulse to the sample, the CDW is pressed against the impurity and sends nm-sized distortions into the electron collective, which cause transient complex electron motion distortions in the material. For high signal quality the developed TR-STM system is operated at 41 THz and utilizes high isolation against vibrations, noise, air motion, and temperature and humidity fluctuations. The researchers here report direct real-space observation of low-energy CDW dynamics in 2H-NbSe₂ at frequencies ranging from 0.15 THz to 0.9 THz. They use STM to spatially resolve the CDW and excite the tunnel junction with single-cycle THz pulses to measure the CDW’s ultrafast dynamics. They find that the oscillating electric field of the THz pulses drives a strong screening current in the sample surface that excites the phase of the CDW locally and creates a spatially heterogeneous pattern of modes by the interplay between THz excitation and pinning at atomic defects. NbSe₂ hosts a quasi-2D incommensurate CDW below a critical temperature ~ 33 K that appears in STM images as a supermodulation of the charge density with average periodicity of 3.06 unit cells of the host lattice. Thus, the CDW’s complex order parameter (given by the amplitude of the charge modulation and its phase relative to the lattice) can be resolved in real space. Static STM shows that atomic impurities are strong pinning sites that locally lock the CDW, making NbSe₂ a good candidate for observing phase excitations that emerge from the interaction of the CDW state with the disorder potential of the randomly distributed defects. This highly localized dynamics can be detected by exciting the STM’s tunnel junction with pairs of THz pulses: the first THz pulse excites the CDW, and the time-delayed second THz pulse probes the resulting dynamics locally by THz-induced electron tunneling at a defined time after excitation. Sweeping the time delay between the pulses while recording the tunnel current constructs a time trace of the junction conductivity that depends on the ultrafast modulation of the surface charge density. The THz pulses used here consist of only one optical cycle, which provides a time resolution of better than 0.4 ps. The pulses have a peak electric field strength of 130 V cm^−1 in the optical far field, which is too weak to alter the static properties of NbSe₂, but strong field enhancement under the STM tip enables local excitation. Time traces recorded on NbSe₂ in the CDW phase at 20 K show an ultrafast relaxation with a time constant ~ 0.6 ps followed by oscillatory dynamics that persist for > 20 ps The TR-STM capability to observe collective charge order dynamics with local probes allows the study of the dynamics of correlated materials at the intrinsic length scale of their underlying interactions.

For more information: Phys.org, July 16 (2024); Nat. Phys., July 15 (2024).

WEEK OF JULY 15, 2024 [No. 1584]

Nuclear decay recoil measured in microsphere:   researchers at Yale University in New Heaven, CT have detected nuclear decays in the recoil of a levitating SiO₂ bead in a HV optical trap. They report the detection of individual nuclear α decays through the mechanical recoil of the entire µm-sized particle in which the decaying nuclei are embedded. Momentum conservation ensures that such measurements are sensitive to any particles emitted in the decay, including neutral particles that may otherwise evade detection with existing techniques. Detection of the recoil of an object more than 10¹² times more massive than the emitted particles is made possible by recently developed techniques in levitated optomechanics, which enable high-precision optical control and measurement of the mechanical motion of optically trapped particles. Observation of a change in the net charge of the particle coincident with the recoil allows decays to be identified with background levels at the µBq level. The detections relied on two measurement schemes: a position detector that recorded recoils to the microsphere from exiting α particles, and a set of electrodes that recorded changes in the microsphere’s electric charge. They used momentum conservation to determine when a radioactive atom emitted a single He nucleus. Levitated nanospheres in optical traps have been slowed via feedback to their quantum-mechanical ground state of motion and it is possible to measure forces ~ 10^−20 N and accelerations ~ 10^−7g with observation time ~ 1 s. They implanted SiO₂ microspheres with radioactive ²¹²Pb atoms with half-life =10.6 h (a few dozen atoms within 60 nm from the surface). Following implantation, one microsphere at a time was levitated using a focused circularly polarized laser beam (optical tweezer) with laser induced rotation frequency > 100 kHz (providing gyroscopic stability that fixed the orientation of the particle’s rotational axis). Then, the chamber enclosing the microsphere was pumped down to ~ 10^−10 atm and recoil data were continuously recorded for each of 6 microspheres over 2-3 d. The researchers were looking for evidence of nuclear decay to the stable isotope ²⁰⁸Pb through α and β emissions The microsphere’s response to an oscillating electric field revealed how much excess charge it carried, which could be determined at the level of a single electron or proton. Any change in this value signaled that a nuclear decay had caused the ejection of charged particles. A microsphere without implanted ²¹²Pb showed no change in the excess charge over three days. Light scattered by the microsphere provided precise information about the microsphere’s motion in the trap. The researchers used the charge data to identify 83 events in which charge was carried away, then reconstructed the impulse received by the microsphere for each event from the scattered light. A histogram of the reconstructed impulse amplitudes were found to be consistent with the predicted response from α and β decays. It is the α decays that contribute to the recoil signal; the β decays contribute to the background but do not carry away sufficient momentum for the recoil to be resolved. By measuring both recoil and charge in parallel, the researchers boosted the sensitivity of their measurement so that it can detect events that occur up to once per day. The recoil-based detection addresses a shortcoming of conventional nuclear-decay detectors, which rely on the decay products to interact with the detection medium. The researchers propose studying neutrino properties with a sphere mass 100 times smaller than the one used here.

For more information: Physics, July 8 (2024); Phys.org, July 26 (2024); Phys. Rev. Lett., July 8 (2024) page 023602.

Chiral Majorana edge state induced:   a group lead by researchers at the University of Cologne have shown induction of superconducting (SC) correlations in a magnetic 2D thin film that is a proximitized quantum anomalous Hall insulator (QAHI) in contact with a SC Nb electrode. Theoretically this could give rise to topologically protected Majorana fermions. The idea here is to induce Cooper pairing in a thin film of ferromagnetic topological insulator material (that can host the quantum anomalous Hall effect without the need for an external magnetic field) to realize topological superconductivity with the associated chiral Majorana edge states. They inject an electron into one terminal of the insulator material that reflects at another terminal as a hole.(crossed Andreev reflection), and that allows detection of the induced superconductivity in the topological edge state in cryogenic conditions. They demonstrate crossed Andreev reflection across a narrow SC Nb electrode that is in contact with the chiral 1D edge state of the QAHI. In the crossed Andreev reflection process, an electron injected from one terminal is reflected out as a hole at the other terminal to form a Cooper pair in the superconductor. This is a signature of induced SC pair correlation in the chiral edge state. The characteristic length of the crossed Andreev reflection process is found to be much longer than the SC coherence length in Nb, which suggests that the crossed Andreev reflection is, indeed, mediated by superconductivity induced on the QAHI surface. For the 1D helical edge state of a 2D topological insulator (TI), the induced SC correlations have been detected in Josephson junctions. The SC correlations in the quantum Hall edge states are less trivial due to the chiral nature of the edge and large magnetic fields required, but strong evidence has been obtained in terms of the crossed Andreev reflection (CAR) or the formation of Andreev edge states, which cause a negative nonlocal potential in the downstream edge. In the CAR process, an electron in the chiral edge entering a grounded SC electrode creates a Cooper pair by taking another electron from the other side of the electrode, causing a hole to exit into the downstream edge. This hole is responsible for the negative nonlocal voltage observed experimentally. SC correlations are induced in the chiral edge state through the CAR process. The CAR process has been observed even in the fractional quantum Hall edge states. If the 1D edge state of a QAHI can be proximitized, one could create a non-abelian Majorana zero mode by coupling two counter-propagating edges by the CAR process through a superconductor. If the 2D surface of the QAHI is proximitized, a chiral Majorana edge state may occur. The QAHI can be realized by doping Cr or V into a very thin film (≲10 nm thickness) of the 3D TI (Bi_xSb_1−x)₂Te₃ in which the chemical potential is fine-tuned into a magnetic gap that opens at the Dirac point of the surface states as a result of a ferromagnetic order. A QAHI is insulating in the 3D bulk and in the 2D surface. Inducing SC correlations in bulk-insulating TIs is more difficult than in bulk-conducting TIs (one of the reasons for the lack of evidence for the SC proximity effect in a QAHI). The researchers here observed the signature of CAR with a narrow Nb finger electrode (160 nm width) in contact with the QAHI edge. The finger-width dependence of the CAR signal gives the characteristic length of the CAR process that is much longer than the SC coherence length of Nb, which suggests that it is not the superconductivity in the Nb electrode but the proximity-induced pairing in the QAHI beneath the Nb that is mediating the CAR process. The researchers plan experiments to directly confirm the emergence of chiral Majorana fermions.

For more information: Phys.org, July 10 (2024); Nat. Phys., July 10 (2024).

WEEK OF JULY 8, 2024 [No. 1583]

Laser excitation of ^239mTh nucleus:   a group lead by researchers at UCLA in Los Angeles, CA has made the most precise measurement to date of the excited nuclear state of ^239mTh. They have induced the nuclear excitation by using a laser on a highly transparent LiSrAlF₆ crystal doped with ²³⁹Th . In analyzing the nuclear isomeric transition, two spectroscopic features near the nuclear transition energy were observed: a broad excitation feature that produces red-shifted fluorescence that decays with a timescale of a few seconds. And a narrow, laser-line width-limited spectral feature at 148.382 19(4)stat(20)sys nm [2020 407.3(5)stat(30)sys  GHz] that decays with a lifetime of 568(13)stat(20)sys  s. This feature is assigned to the excitation of the ^239mTh nuclear isomeric state, whose energy is found to be 8.355 733(2)stat(10)sys eV. The group has realized a series of experiments to stimulate ^239mTh nuclei in ²³⁹Th:LiSrAlF₆ crystals with a laser over time. Getting nucleons in the atomic nucleus to react to laser light is difficult because they are surrounded by electrons, which react readily to light and can reduce the number of photons actually able to reach the nucleus. Since F can form especially strong bonds with other atoms, the electrons were so tightly bound with the F that the amount of energy it would take to excite them was very high, allowing lower energy light to reach the nucleus ²³⁹Th known to have a low energy excited state accessible to laser illumination. The ²³⁹Th nuclei could then absorb these photons and re-emit them, allowing the excitation of the nuclei to be detected and measured. By changing the energy of the photons and monitoring the rate at which the nuclei are excited, the researchers were able to measure the energy of the nuclear excited state. As part of the demonstration of the laser excitation of the nuclear isomeric transition, researchers also determined the energy of the excited nuclear state more precisely than done two months ago at PTB setting the value at 8.35574 eV. That previous result and the new one were both achieved using different crystals doped with ²³⁹Th (CaF₂ crystals in PTB vs. LiSrAlF₆ crystals in UCLA), an idea proposed by some members of the group here over a decade ago. The experiments here indicate that in some cases the host crystal can dampen the nuclear transition of ^239mTh.

For more information: Physics, July 2 (2024); Phys.org, July 2 (2024); Phys. Rev. Lett., July 2 (2024) page 013201.

Superconductivity quantum bound states controlled:   an international group lead by researchers at Pohang University of Science and Technology in Pohang has controlled the quantum mechanical properties of Andreev bound states in bilayer graphene-based Josephson junctions from short to long junction limits using gate voltage. They have demonstrated that the mode number of Andreev bound states in bilayer graphene Josephson junctions can be modulated by controlling the superconducting coherence length. By exploiting the quadratic band dispersion of bilayer graphene, they control the Fermi velocity, and thus, the coherence length via the application of electrostatic gating. The researchers here used gate voltage to control the quadratic energy dispersion of bilayer graphene as well as the superconducting coherence length in real time. The number of energy levels in the Andreev bound states depends on the ratio of the conduction channel length to the superconducting coherence length (the length along which the superconducting state can be maintained in the normal conductor). In the short junction limit, the conduction channel is short and the number of Andreev bound state levels is limited to a pair; in the long junction limit there are more than two pairs. Tunneling spectroscopy of the Andreev bound states reveals a crossover from short to long Josephson junction regimes as they approach the charge neutral point of the bilayer graphene. They observed the Andreev bound states in the short and long Josephson junction limits. The researchers observed the change of the Andreev bound states at different gate voltages in real time and confirmed that the experimental results matched theoretical predictions. The phase-dependent Josephson current was estimated from the analysis of different mode numbers of the Andreev energy spectrum. The researchers anticipate that the number of energy levels can be adjusted by gate voltage.

For more information: Phys.org, July 1 (2024); Phys. Rev. Lett., May 30 (2024) page 226301.

WEEK OF JULY 1, 2024 [No. 1582]

Repulsive bound magnons observed:   a group lead by researchers at the University of Cologne in Cologne has found evidence in BaCo₂V₂O₈ synthetic crystals (containing screw chains of magnetic Co atoms) that magnetic excitations can be held together not only by attraction but by repulsive interactions (that even in the lower stability state of the repulsively bound states). The researchers studied the collective magnetic excitations by THz radiation in the crystal structure in high magnetic fields. In addition to the usual elementary magnetic low-energy magnon excitations, they discovered two- and three-magnon bound states held together by repusive interactions. They present spectroscopic signatures of repulsively bound three-magnon states and bound magnon pairs in the Ising-like chain antiferromagnet BaCo₂V₂O₈. In large transverse fields, below the quantum critical point, they identify repulsively bound magnon states by comparing THz spectroscopy measurements to theoretical results for the Heisenberg–Ising chain antiferromagnet. Their experimental results show that these high-energy, repulsively bound magnon states are well separated from continua, exhibit notable dynamical responses and, despite dissipation, are sufficiently long-lived to be identified.

For more information: Nature, June 26 (2024); Phys.org, June 27 (2024).

Chiral excitations observed in kagome topologicsl magnet:   a group lead by researchers at ANL in Ames, IA have observed chiral quasiparticle excitations in the kagome-lattice layered topological magnet TbMn₆Sn₆. They confirmed the existence of localized flat band magnons associated with a frustrated kagome lattice geometry. They observe elementary magnetic excitations within the ferromagnetic Mn kagome layers in TbMn₆Sn₆ using inelastic neutron scattering (INS). Specifically, they observe sharp, collective acoustic magnons and identify flat-band magnons that are localized to a hexagonal plaquette due to the special geometry of the kagome layer. They also observe a chiral magnetic quasiparticle that is localized on a hexagonal plaquette. The short lifetime of localized flat-band and chiral quasiparticles suggest that they are hybrid excitations that decay into electronic states. The researchers had previously mapped out the material's magnetic excitations at low energies and studied how these excitations evolve through a temperature-dependent magnetic spin reorientation transition. The findings here come from studying the material's high energy excitations. Based on theoretical models, they expected to find localized flat-band magnons due the special geometry of the kagome lattice. Aside from observing these expected localized, flat-band magnons, they discovered a different hexagon-localized chiral excitation. They concluded that while TbMn₆Sn₆ has ferromagnetic kagome layers, they also harbor a competing chiral antiferromagnetic instability. The ferromagnetic state is more stable than the chiral antiferromagnetism but they believe that perhaps tweaking the chemical makeup of the material they could stabilize the chiral order. The researchers describe the discovery of magnetic chiral quasiparticles in ferromagnetic kagome layers by measuring their elementary excitations using INS. Recent INS studies of a variety of ferromagnetic kagome metals, reveal well-defined and collective acoustic magnon modes at low energies. However, the higher-energy optical and flat band magnon modes and their associated topological features were obscured by heavy damping. The researchers here perform experiments on much larger sample volumes and with increased incident neutron energies that reveal two broad, high-energy excitations in TbMn₆Sn₆ that are better described as localized magnetic quasiparticles, rather than collective magnon modes. The first excitation consists of dynamical spin correlations around a hexagonal plaquette, corresponding to the expected kagome Wannier states associated with localized flat-band magnon quasiparticles, thereby providing the clearest experimental evidence for a magnonic flat band in a kagome metal. The second excitation exhibits unexpected chiral spin correlations around a hexagonal plaquette which are truly anomalous and cannot be captured from simple magnetic models. The observed short-lifetime of chiral and flat-band magnetic quasiparticles is caused by decay into other quasiparticles, likely of electronic origin. The uniaxial ferromagnetic kagome layers in TbMn₆Sn₆ host conventional, collective acoustic magnon quasiparticles representing the in-phase precession of Mn moments within a kagome layer. The thermal population of acoustic spin-1 magnons controls the magnetization of the kagome layer. However, chiral (optical) and flat-band modes whose precessions are out-of-phase have an incoherent character and remain localized to a single hexagonal plaquette with heavy damping. The confinement of the flat magnon band to a hexagonal plaquette is known to arise from the special kagome lattice geometry which leads to phase cancellation of spin precession on the triangular vertices surrounding a hexagon. The flat bands observed here would be consistent with linear spin wave theory were it not for the heavy damping. The observation of chiral magnetic quasiparticles is not expected from linear spin wave theory. Unlike the flat-band magnons, the kagome lattice geometry does not guarantee the localization of chiral excitations to a hexagon. Chiral magnetic quasiparticles are often associated with topological, vortex-like spin textures (ex: skyrmions), that are localized by a balance of exchange interactions and spin-orbit coupling. Skyrmions have a non-zero scalar spin chirality where broken inversion symmetry of the crystal lattice imparts a handedness to the quasiparticle’s spin texture. For TbMn₆Sn₆, spatial inversion is preserved, yet the observed chiral magnetic quasiparticles also consist of vortex-like excitations that possess a vector spin chirality with left and right-handed versions.

For more information: Phys.org, June 24 (2024); Nat. Comm., February 21 (2024).

WEEK OF JUNE 24, 2024 [No. 1581]

Electron spin control by spin polarized current:   researchers at ETH in Zurich have shown spin torque–driven electron paramagnetic resonance (EPR) of a single spin in a pentacene molecule, and thus, that the quantum state of a single electron spin can be controlled by a spin-polarized electron current. They demonstrate coherent driving of a single spin by a RF spin-polarized current injected from the tip of a STM into an organic molecule. The researchers use EPR excitation to establish dynamic control of single spins by spin torque using a local electric current. They carried out the measurements using a spin-polarized current to perturb the resonant magnetization dynamics of a ferromagnet (or antiferromagnet) and then determined the response. This approach has been used successfully over the past decade, but only in macroscopic systems. The researchers here utilize a spin-polarized STM to measure the spin-orbit torque effect of a spin current on EPR at single-molecule scale. They characterized the electron clouds in the molecule using the STM. Electrons with spins aligned in parallel tunnel from the W tip to the sample's pentacene molecules on a thin insulating MgO layer over an Ag substrate. The tunnel current is spin-polarized by using the W tip to pick up a few Fe atoms from the insulating layer. The researchers applied a constant voltage plus a fast-oscillating voltage to the magnetized W tip, and they measured the resulting tunnel current. By varying the strength of both voltages and the frequency of the oscillating voltage, they observed characteristic resonances in the tunnel current and thus studied the processes that occurred between the tunneling electrons and those of the molecule. The electron spins in the pentacene molecule reacted to the electromagnetic field created by the alternating voltage in the same way as in ordinary EPR. The shape of the resonances indicated that spin transfer torque influenced the spins of the electrons in the molecule too. Spin control by spin-polarized currents, in contrast to the use of electromagnetic fields, acts with local precision (< 1 nm). The work here shows how the dissipative action of the spin-transfer torque, in contrast to the nondissipative action of the magnetic field, allows for the manipulation of individual spins based on controlled decoherence. The researchers demonstrated that it is possible to create quantum superposition states of the molecular spin in this way.

For more information: Science, June 20 (2024); Phys.org, June 20 (2024).

Edge states from topological helix chains:   researchers at Tohoku University in Sendai have discovered a 1D Te-based topological insulator (TI). Recent theoretical predictions have suggested that single helix chains could be !D TIs. The Su–Schrieffer–Heeger model (1979, well before TIs) describing conducting polymers predicts a 1D analogue of 3D and 2D topological insulators, hosting topological bound states at the endpoints of a chain. To establish this state, one needs to identify the low-energy excitations stemming from bound states, but this has remained elusive because of the absence of suitable platforms. The researchers here found unusual electronic states that support bound states in elemental Te, with single helices as in an extended version of the SSH chain. Using ESR and ARPES with a micro-focused beam, they have shown spin-polarized in-gap states confined to the edges of the (0001) surface. Density functional theory calculations indicate that these states come from interacting bound states in a 1D array of SSH Te hexagonally patterned chains. To verify this, the researchers needed to observe the electrical charges confined to the endpoints of these chains. This required preparing clean edges of the Te chains without structural damage which they did by using a home-made gas-cluster ion-beam system capable to modify surfaces to within 1 nm. They visualized the spatial distribution of electric charges using ARPES with a micro-focused beam. The results confirmed the electric charges at the endpoints of the chains, thus supporting the 1D TI nature of the Te material.

For more information: Phys.org, June 19 (2024); Nature, June 5 (2024).

WEEK OF JUNE 17, 2024 [No. 1580]

Earth spin effect on entangled photons measured:   researchers at the University of Vienna in Vienna have observed the effect of Earth's rotation on a maximally entangled two-photon state. They set up a table-top experiment using maximally path-entangled quantum states of light in a large-scale Sagnac interferometer with a sensitivity of 5 μrad s^−1 enough to measure the rotation rate of Earth and a record in rotation resolution reached with optical quantum interferometers. This result confirms the interaction between rotating reference systems and quantum entanglement with a thousand-fold precision improvement compared to previous experiments. More generally this methodology should enable measurements of general-relativistic effects on entangled photons. The researchers here built a large-scale, quantum , optical-fiber Sagnac interferometer and kept the noise low and stable for several hours. This enabled the detection of enough high-quality entangled photon pairs to outperform the rotation precision of previous quantum optical Sagnac interferometers. In the set up two indistinguishable photons are incident on a beam splitter cube, entanglement between them is created, and they are coupled in the fiber Sagnac interferometer. With two entangled particles, they behave like a single particle testing both directions simultaneously while accumulating twice the time delay compared to the scenario where no entanglement is present. The Sagnac interferometer was built with 2-km of optical fibers wrapped around a 1.4 m-sided square Al frame. In the experiment, two entangled photons propagate inside the 2-km-long optical fiber wounded onto a large coil, realizing an interferometer with an effective area of 715 m². The researchers injected two-photon states into the interferometer, using quantum interference to demonstrate super-resolution while extracting Earth’s rotation rate. This goes beyond previous laboratory demonstrations of measurements probing Sagnac interferometers with quantum states of light, which involved fiber interferometers with at most hundred-meter-length fibers and were only used to measure synthetic and controllable signals. They chose the detection of Earth’s rotation as a benchmark for their large-scale fiber interferometer, as its minute rate, fixed direction, and the absence of ways to manipulate its behavior, make it particularly difficult to observe. The ubiquitous presence of acoustic- and seismic vibrations and thermal fluctuations transduce directly into phase noise in optical fiber and drive the motion of the large apparatus. To solve these problems, they built their rotatable fiber interferometer with an optical switch to turn Earth’s rotation signal on and off, allowing them to fully characterize the angle-dependent Sagnac phase. This technique allows the rotation signal to be referenced to an effectively nonrotating frame. For operation the researchers needed to isolate and extract Earth's steady rotation signal. They needed to establish a reference point for the measurement, where light remains unaffected by Earth's rotational effect. Thus, they split the optical fiber into two equal-length coils and connected them via an optical switch from parallel to anti-parallel propagation. By toggling the switch positions (switching from straight vertical to X connection configurations), the researchers effectively canceled the Earth's rotation signal at will, which also allowed them to extend the stability of their large apparatus. They confirmed an acquired Sagnac phase from Earth’s rotation with an enhancement factor of two because of the two-photon entangled state.

For more information: Science Advances, June 14 (2024); Physorg, June 14 (2024).

Kitaev chain generated in a 2D electron gas:   researchers at Delft University of Technology in Delft have extended their previous work to a 2D domain by demonstrating creation of a Kitaev chain with semiconductor quantum dots connected via superconductors as a way to produce topologically Majorana qubits. They realized a two-site Kitaev chain in a 2D electron gas by coupling two quantum dots through a region proximitized by a superconductor. They demonstrate systematic control over inter-dot couplings through in-plane rotations of the magnetic field and via electrostatic gating of the proximitized region. This allows them to tune the system to sweet spots in parameter space, where robust correlated zero-bias conductance peaks are observed in tunneling spectroscopy. By implementing the Kitaev-chain in 2D, they show that the underlying physics is universal and platform independent. To study the extent of hybridization between localized Majorana bound states, the researchers probe the evolution of the energy spectrum with magnetic field and estimate the Majorana polarization. They have shown reproducibility and protection to local perturbations. The researchers plan to increase the number of sites in the Kitaev chain and systematically study the protection of Majorana bound states.

For more information: Nature, June 12 (2024) page 329; Phys.org, June 12 (2024).

WEEK OF JUNE 10, 2024 [No. 1579]

Condensation of dipolar molecules observed:   researchers at Columbia University in New York, NY have realized a Bose–Einstein condensate (BEC) of dipolar molecules. By strongly suppressing two- and three-body losses via enhanced microwave collisional shielding, they evaporatively cool NaCs molecules to quantum degeneracy and cross the phase transition to a BEC. The BEC reveals itself by a bimodal distribution when the phase-space density exceeds 1. BECs with over 10³ molecules, a temperature of 6(2) nK , and a condensate fraction of 60(10)% , were created and found to be stable with a lifetime of about 2 s. The researchers used a microwaves field to create shields around each molecule and prevent them from colliding. With the molecules shielded against lossy collisions, the hottest ones were preferentially removed from the sample. They increased cooling by adding a second microwave field. In addition to reducing collisions, the second microwave field can also manipulate the molecules' orientation. In stable molecules, the final stage of cooling, to turn molecule clouds into a condensate, has been prevented by chemical reactions between colliding molecules. These interactions heat the molecules and cause them to escape the cloud, leaving too few to work with. The researchers here applied two different kinds of microwave fields to the cloud, one to make the molecules rotate and another to make them oscillate. Together these fields oriented the molecules such that they always repelled each other. This repulsion prevented collisions, allowing them to cool the molecules by forcing out the hottest ones without losing too many. That in turn is a means to control how they interact. The researchers have a validated-experimentally theoretical description of interactions between the molecules.

For more information: Nature, June 3 (2024); Phys.org, June 3 (2024).

²²Al mass measured:   researchers at MSU in East Lansing, MI have used isotope beams from FRIB to measure the ²²Al mass with high precision. They report the mass measurement of the proton-halo candidate ²²Al performed with the low energy beam ion trap facility’s 9.4 T Penning trap mass spectrometer at FRIB. This measurement completes the mass information for the lightest remaining proton-dripline nucleus achievable with Penning traps. ²²Al has been the subject of recent interest regarding a possible halo structure from the observation of an exceptionally large isospin asymmetry. The measured mass excess value of ME = 18 092.5(3) keV, corresponding to an exceptionally small proton separation energy of S_p = 100.4(8) keV, is compatible with the suggested halo structure. The result here agrees well with predictions from sd-shell USD Hamiltonians. While USD Hamiltonians predict deformation in the ²²Al ground state with minimal 1s_1/2 occupation in the proton shell, a particle-plus-rotor model in the continuum suggests that a proton halo could form at large quadrupole deformation. This isotope probably has its nucleus surrounded by a halo of protons that loosely orbit the nucleus beyond the pull of the residual strong force. While all halo structures are rare fleeting phenomena, neutrons are usually observed as halo particles but halos made of protons are rarer. Measurements on nearby isotopes have suggested that ²²Al might be an isotope that could form a proton halo. The researchers here created a high-energy isotope beam of ²²Al using projectile fragmentation at FRIB. They created a beam from a stable nucleus of an Ar isotope and then accelerated the beam to about c/2 against a target. The collisions create rare, short-lived isotopes from which a ²²Al beam was filtered out. Then they slowed it down into a uniform beam and measured the nucleus mass. The researchers now plan to take the next step in verifying the proton halo structure around ²²Al by measuring the charge radius (the distribution of protons around the nucleus) and how the nucleus us deformed from the spherical shape.

For more information: Phys.org, June 3 (2024); Phys. Rev. Lett., April 9 (2024) page 152501.

WEEK OF JUNE 3, 2024 [No. 1578]

CMOS integration of spin-photon quantum elements:   researchers at MIT in Cambridge, MA have demonstrated a scalable, modular hardware architecture that integrates thousands of interconnected qubits onto a customized integrated circuit. They introduce a modular quantum system-on-chip (QSoC) architecture that integrates thousands of individually addressable Sn-vacancy spin qubits in 2D arrays of quantum microchiplets onto an application-specific integrated circuit designed for cryogenic control. They demonstrate fabrication steps and architectural subcomponents, including QSoC transfer by means of a lock-and-release method for large-scale heterogeneous integration, high-throughput spin-qubit calibration and spectral tuning, and efficient spin state preparation and measurement. This QSoC architecture supports full connectivity for quantum memory arrays by spectral tuning across spin–photon frequency channels. The researchers have developed a modular fabrication process to produce a QSoC which integrates an array of artificial atom qubits onto a semiconductor chip. This QSoC architecture enables them to precisely tune and control a dense array of qubits. They have shown the heterogeneous integration of spin–photon interfaces with a CMOS platform. Multiple chips could be connected using optical networking to create a large-scale quantum communication network. By tuning qubits across 11 frequency channels, this QSoC architecture allows for a proposed protocol of entanglement multiplexing for large-scale quantum computing. The researchers develop a process for manufacturing 2D arrays of atom-sized, RT long coherence time qubit diamond color center (DCC) microchiplets and for transferring thousands of them onto a CMOS chip in a single step. The DCC provide photonic interfaces that allow for entanglement with non-adjacent qubits. Each has its own spectral frequency which allows the researchers to communicate with individual atoms by voltage, tuning them into resonance with a laser. They integrated a large array of DCC qubits into a CMOS chip that includes digital logic that reconfigures the voltages, for full inhomogeneous qubit connectivity. To build this QSoC, they developed a fabrication process to transfer DCC microchiplets onto a CMOS backplane at large scale. They started by nanofabricating an array of DCC microchiplets from a solid block of diamond (19 steps) and associated nanoscale optical antennas. Then, they designed and mapped out the chip from the semiconductor foundry. They post-processed a CMOS chip to add microscale sockets that match up with the DCC microchiplet array. They built an in-house transfer setup in the lab and applied a lock-and-release process to integrate the two layers by locking the DCC microchiplets onto the sockets on the CMOS chip. Since the DCC microchiplets are weakly bonded to the diamond surface, when they release the bulk diamond horizontally, the microchiplets stay in the sockets. Because they can control the fabrication of the diamond and the CMOS chip, they can make a complementary pattern. In this way, they can transfer thousands of DCC microchiplets onto their corresponding sockets all at the same time. The researchers demonstrated a 500-µm x 500-µm area transfer for an array with 1,024 DCC nanoantennas, although the process allows for larger DCC arrays and larger CMOS chips. They found that for this architecture tuning the frequencies with more qubits requires less voltage. They demonstrated a chip with over 4,000 qubits that could be tuned to the same frequency while maintaining their spin and optical properties. They built a custom cryo-optical metrology setup to characterize the system and measure its performance on a large scale. They also built a digital twin simulation that connects the experiment with digitized modeling. Design studies building on their measurements indicate scaling potential by means of increased qubit density, larger QSoC active regions, and optical networking across QSoC modules.

For more information: Nature, May 29 (2024) page 70; Phys.org, May 29 (2024).

Nucleation in supercooled liquids measured:   researchers at the European XFEL in Schenefeld have studied the random process of vacuum nuclei crystallization in microcopic supercooled rare-gas-based (Ar and Kr) liquid jets (only systems for which theoretical predictions are considered) by fs XR diffraction and found that it starts later than previously assumed. Their results provide limits to the validity of classical nucleation theory in atomic liquids. The atoms of the liquids used here interact in a particularly simple way, which they hoped would give nucleation theory the best chance of success. They investigated the crystal nucleation rate J(T), a measure of the probability that a crystal will form in a certain volume within a certain time. They bombarded the 3.5-µm liquid jets with 9.7 keV XR pulses lasting < 25 fs, focusing them on a spot < 1 µm. When such a jet travels through vacuum, evaporation at the surface drives rapid cooling (M  °C /s). They XR probed the jets at many locations within the range of 300–800 µm from the jet source. Spectra from these pulses revealed the atomic configuration of each jet as it evolved, starting as a liquid at the source and then showing signs of crystal formation within a few 100's µm. Given the flow speed, the crystallization time was < 10 µs (longer than for mm-sized droplets). They recorded several million diffraction images. According to their results, the crystal nucleation rates are much smaller than those predicted on the basis of simulations and the classical theory. Classical nucleation theory rests on many potentially incorrect assumptions, such as the idea that newly nucleated crystal seeds will be roughly spherical. It assumes that the first portion of a solid that forms from a cooling liquid will adopt the most stable possible crystal arrangement, which would be a face-centered cubic pattern for Ar or Kr. But some computer simulations suggest that, in some cases, small portions of the initial solid adopt a less stable arrangement (body-centered cubic). On the experimental side, accurate measurements are difficult because nucleation happens over ultrafast timescales (ps-ns), at least for the typical mm-scale droplets used in experiments. Any microscopic impurities in a liquid sample can also strongly affect the results. For both liquids, the theory combined with simulations predicted a nucleation rate 100–1000 times higher than the experimental value, but this result was still about 100 times closer to agreement than previous experiments.

For more information: Phys.org, May 27 (2024); Physics, May 17 (2024); Phys. Rev. Lett., May 17 (2022) page 206102.

Excitonic monolayer Fresnel lens fabricated:   researchers at the University of Amsterdam in Amsterdam and Stanford University in Stanford, CA have made a 0.6-nm thick, 0.5-mm wide, metasurface flat lens with concentric WS₂ rings and gaps in between. The lens diffraction-focuses red light at 1 mm with the focal length determined by the rings size and the distance between them. They show how the excitonic decay rates dictate the focusing efficiency of the atomically thin lens. By isolating the coherent exciton radiation from the incoherent background in the focus of the lens, they obtain a direct measure of the role of exciton radiation in wavefront shaping. They investigate the influence of exciton–phonon scattering by characterizing the focusing efficiency as a function of temperature, demonstrating an increased optical efficiency at cryogenic temperatures. WS₂ absorbs light by sending an electron to a higher energy level. Due to the ultra-thin structure of the material, the negatively charged electron and the positively charged hole it leaves behind in the atomic lattice stay bound together by the electrostatic attraction between them, forming an exciton. These excitons quickly disappear again by the electron and hole merging together and sending out light. This re-emitted light contributes to the lens's efficiency. Quantum enhancement effects allow the material to efficiently absorb and re-emit light at specific wavelengths, giving the lens the built-in ability to work better for these wavelengths. The researchers detected a peak in lens efficiency for the specific wavelengths of light sent out by the excitons. This effect is observed at RT operation with the lens becoming more efficient when cooled down. Another unique lens feature is that, while some of the light passing through makes a bright focal point, most light passes through unaffected.

For more information: Phys.org, May 30 (2024); Phys. Rev. Lett., May 17 (2024) page 206102.

WEEK OF MAY 27, 2024 [No. 1577]

Surface solar dynamo generation:   a group lead by researchers at MIT in Cambridge, MA has found that the sun's magnetic field could arise from instabilities within the convection zone's outermost layers which match observations better than deep solar dynamo generation proposed mechanisms. Analytic estimates show that the near-surface magneto-rotational instability explains the spatiotemporal scales of the torsional oscillations and inferred subsurface magnetic field amplitudes. Numerical simulations corroborate these estimates and reproduce hemispherical magnetic current helicity laws. The solar magnetic dynamo cycle features a propagating region of sunspot emergence at ~ 30° latitude that vanishes near the equator every 11 yrs. Longitudinal flows (torsional oscillations) closely shadow sunspot migration.. Helioseismology pinpoints low-latitude torsional oscillations to the near-surface shear layer (outer 5–10% of the Sun). Within this zone, inwardly increasing differential rotation coupled with a poloidal magnetic field strongly implicates the magneto-rotational instability. The researchers generated a precise model of the solar surface and found that when they simulated certain perturbations in the flow of plasma within the top 5–10% of the sun, these surface changes were enough to generate observed solar magnetic field patterns. Their simulations in deeper layers produced less realistic solar activity. Their findings suggest that sunspots and flares could be a product of a shallow magnetic field, rather than a field that originates deeper in the sun. They have shown that isolated perturbations near the solar surface can grow over time to potentially produce the magnetic structures observed in solar flares, sun spots, and solar corona. Rather than conventionally simulating the complex flow of plasma throughout the entire body of the sun, the researchers here wondered whether studying the stability of plasma flow near the surface might be enough to explain the origins of the dynamo process. They used helioseismologic solar surface vibrations observations to infer the average sun structure. Then, they figured if there were plasma flow perturbations that they could superimpose on top of the average structure, that might grow to cause the solar magnetic field. They developed algorithms that were incorporated into a fluid dynamics code to find self-reinforcing changes in the solar average surface flows. The algorithms discovered patterns that could grow and result in realistic solar activity. They found patterns that match the observed locations and timescales of sunspots. In their simulations, they found that certain changes in the flow of plasma, within just the top 5–10% of the sun's surface layers, were enough to generate magnetic structures in the sun spot regions. Changes in deeper layers produced less realistic solar fields that were concentrated near the poles, rather than near the equator as observed. The researchers considered flow patterns near the surface inspired by those conditions resembling the unstable plasma flows in the accretion disks around black holes.

For more information: Nature, May 22 (2024) page 769; Phys.org, May 22 (2024).

Proton gluon helicity analyzed:   researchers at the Jefferson Lab in Newport News, VA have analyzed the gluon contribution to the total proton spin to clarify their spin alignments. They studied the gluon helicity from global analysis of experimental data and lattice QCD Ioffe time distributions. They started with the combined data from experiments taken in facilities around the world. They then added the results from LQCD calculation into their 1D analysis. Their next step will be to improve the datasets. As more powerful experiments provide more detailed information on the proton, these data begin making a picture that goes beyond 1D. The goal is to eventually produce a 3D understanding of the proton's structure. The researchers here performed a global analysis of spin-dependent parton distribution functions with the inclusion of Ioffe time pseudodistributions computed in LQCD, which are directly sensitive to the gluon helicity distribution, Δg. These lattice data have an analogous relationship to parton distributions as do experimental cross sections, and can be readily included in global analyses. They focused on the constraining capability of current LQCD data on the sign of Δg at intermediate parton momentum fractions x, which was recently brought into question by analysis of data in the absence of parton positivity constraints. They found that present LQCD data cannot discriminate between positive and negative Δg solutions, although significant changes in the solutions for both the gluon and quark sectors were observed.

For more information: Phys.org, May 24 (2024); Phys. Rev. D, February 27 (2024) page 036031.

Superconducting spin qubits coupled:   researchers at Delft University of Technology in Delft have demonstrated the strong and tunable coupling between two distant Andreev spin qubits in semiconductor–superconductor hybrid nanowires. In these qubits, the spin degree of freedom of a quasiparticle trapped in a Josephson junction is intrinsically spin–orbit coupled to the supercurrent across the junction. This interaction has previously been used to perform spin readout, but it has also been predicted to facilitate inductive multi-qubit coupling. The researchers here demonstrate a strong supercurrent-mediated longitudinal coupling between two distant Andreev spin qubits. They show that it is both gate- and flux-tunable into the strong coupling regime and that magnetic flux can be used to switch off the coupling in situ. Their results demonstrate that integrating microscopic spin states into a superconducting qubit architecture can combine the advantages of both semiconductors and superconducting circuits and possibly allow to fast two-qubit gates between distant spins. In earlier work, they studied an Andreev spin qubit created by embedding a quantum dot into a superconducting qubit. The researchers here use a method theoretically suggested (2010) to couple two such qubits. They fabricated a superconducting circuit, deposited two semiconductor nanowires on top of this circuit using a precisely controlled needle. The combined nanowire and superconducting circuits configured in two superconducting loops. The special part of these loops is that a part of each loop is a semiconductor quantum dot where an electron can be trapped. The current that flows around the loops depends on the spin of the trapped electron. This effect allows the researchers here to control a supercurrent of billions of Cooper pairs with a single spin. The combined current of the two coupled superconducting loops depends on the spin in both the quantum dots. This also means that the two spins are coupled via this supercurrent. This coupling can be controlled, either via the magnetic field running through the loops or by modulating the gate voltage. The researchers here have demonstrated that they can couple spins over long distances using a superconductor. Normally, spin-spin coupling only happens when two electrons are very close. When comparing qubit platforms based on semiconductors to those based on superconducting qubits, this requirement of proximity is one of the architectural downsides of semiconductors. The current coherence times are low probably due to the nuclear spin bath of the InAs semiconductor used here. A Ge platform is expected to boost coherence times.

For more information: Phys.org, May 22 (2022); Nat. Phys., May 6 (2024).

WEEK OF MAY 20, 2024 [No. 1576]

Electron vortices detected in graphene at RT:   researchers in ETH Zurich in Zurich have used a quantum sensing microscope to image stationary electron vortices in a graphene layer. Using a nanoscale scanning magnetometer, they imaged stationary current vortices (hydrodynamic transport pattern) in a monolayer graphene device at room temperature (RT). By measuring devices with increasing characteristic size, they observed the disappearance of the current vortex and thus verified a prediction of the hydrodynamic model. They observed that vortex flow is present for both hole- and electron-dominated transport regimes but disappears in the ambipolar regime due to a reduction of the vorticity diffusion length near charge neutrality. In graphene, electron-impurity collisions are rare and collisions between electrons play the leading role. In this case, the electrons behave more like a viscous liquid so vortices should occur in the graphene layer. The vortices formed in 1-µm - 3.2-µm circular disks that they had attached during the fabrication process to a conducting 1-µm wide graphene strip encapsulated by hexagonal BN. Electron–electron interactions in high-mobility conductors can give rise to transport signatures resembling those described by classical hydrodynamics. Hydrodynamic calculations here suggested that electron vortices should form in the smaller, but not in the larger disks. To make the vortices visible the researchers measured the tiny magnetic fields produced by the electrons flowing inside the graphene. They used a quantum magnetic field sensor consisting of a NV center embedded in the tip of a diamond needle moving at 70 nm from the graphene. The NV center behaves like a quantum object whose energy levels depend on an external magnetic field. Using laser beams and microwave pulses, the quantum states of the NV center can be prepared in such a way as to be maximally sensitive to magnetic fields. By reading out the quantum states with a laser, the researchers determined precisely the strength of those fields at RT. The researchers made the electron currents visible with a resolution < 100 nm (sufficient for observing the vortices). In their measurements, they observed a reversal of the flow direction in the smaller disks which is a characteristic sign of the expected vortices. While in normal (diffusive) electron transport, the electrons in strip and disk flow in the same direction, in the case of a vortex, the flow direction inside the disk is inverted. As predicted by the calculations, no vortices were observed in the larger disks. By applying an electric voltage from below the graphene, they changed the number of free electrons so the current flow was no longer carried by electrons, but rather by holes. They observed vortices formed by hole carriers. Only at the charge neutrality point, where there is a small and balanced concentration of both electrons and holes, the vortices disappeared completely.

For more information: Phys.org, May 13 (2024); Science, April 25 (2024) page 465.

Higher charge quasiparticle pairing discovered in kagome crystal:   a group lead by researchers at Peking University in Beijing have observed charge-4e and charge-6e flux quantization and higher charge superconductivity in kagome superconductor ring devices, They report evidence for multicharge flux quantization in mesoscopic ring devices fabricated using the transition-metal kagome superconductor CsV₃Sb₅. They perform systematic magnetotransport measurements and observe quantization of magnetic flux in units of h/4e and h/6e in magnetoresistance oscillations. At low temperatures, magnetoresistance oscillations with period h/2e are detected, as expected from the flux quantization for charge-2e superconductivity. They find that the h/2e oscillations are suppressed and replaced by resistance oscillations with h/4e periodicity when the temperature is increased. Increasing the temperature further suppresses the h/4e oscillations, and resistance oscillations with h/6e periodicity emerge associated to charge-6e flux quantization. The observations here provide experimental evidence for the existence of multicharge flux quanta and quantum matter exhibiting higher-charge superconductivity in the strongly fluctuating region above the charge-2e Cooper pair condensate in kagome superconductors. They have discovered in the superconducting kagome metal CsV₃Sb₅ (CVS), that quasiparticles that appear to have 4 and 6 times the charge of an isolated electron, suggesting the formation of Cooper-pair molecules. In the CVS metal crystals, the V atoms occupy 2D lattices with a kagome combination of triangles and hexagons. The finding challenges the expectation that electron binding forces grow substantially weaker with the number of involved electrons. The researchers deposited CVS onto a substrate to form a thin layer. Their sample had a hole in the middle, so that Cooper pairs were compelled to go around the hole (either clockwise or counterclockwise) and experience the Aharonov-Bohm effect. Bulk CVS has a T_c of 2.5 K. However, the resistivity of the CVS layer started to drop at about 4 K and vanished at ~ 1 K. Such a gradual drop in resistivity starting well above the bulk T_c is expected for a thin superconducting compound, marking a fluctuating regime of superconductivity where Cooper pairs are present in a disorganized fashion. One might have expected charge-2e Cooper pairs to persist even up to 4 K, giving rise to h/2e resistance oscillation. Instead, the researchers observed evidence of oscillations at the flux period of h/6e between 2 and 3 K and of h/4e between 1 and 2 K, with the h/2e period becoming dominant only in the zero-resistance regime below 1 K. The h/6e and h/4e periodicities might imply the possibility that two or three Cooper pairs somehow coalesced into Cooper molecules with a 4e or 6e total charge. Theories predicting such a possibility have been around for some time, like the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) superconductors. In FFLO superconductors, the Cooper pairing takes place between electrons having momenta Q/2 + k and Q/2 – k, with a net momentum of Q for the pair. By contrast, ordinary Cooper pairing takes place between electrons of opposite momenta k and –k, with zero net momentum. In the presence of hexagonal crystal symmetry, three types of FFLO orders are possible. As the temperature rises over T_c, the individual ordered states may melt away, but a composite order may survive, according to the theoretical predictions. This state has quasiparticles with charge 6e and could explain the h/6e resistance oscillation observed here. Support for that explanation comes from experiments, backed by theory, that pointed to a feature in the band structure of CVS involving six valleys arranged in momentum space with a 60° angle between them. A potential wrinkle in this CVS experiment is that, unlike the original Little-Parks setup, the magnetic field not only penetrates the hole in the middle of the superconductor but also the entire material. Defining paths for the Aharonov-Bohm effect under such situations is difficult, and thus, the precise determination of the flux periodicity. It has been argued that these higher-charge Cooper pairs could follow the most efficient path, which happens to be the one circling around the boundary of the hole in the middle of the sample.

For more information: Physics, May 13 (2024); Phys. Rev. X, May 13 (2024) page 021025.

WEEK OF MAY 13, 2024 [No. 1575]

Anisotropic exchange interaction between two spin holes in Si transistor:   researchers at the University of Basel in Basel have built a two-hole-spin-qubit system in a Si FinFET and controlled the interaction between the two-qubits. They demonstrate electrical tunability of the exchange splitting from above 500 MHz to close-to-off and perform a conditional spin-flip in 24 ns. The exchange coupling is anisotropic because of the strong spin–orbit interaction. Upon tunneling from one quantum dot to the other, the spin is rotated by almost 180°. The exchange Hamiltonian no longer has the Heisenberg form and can be engineered such that it enables two-qubit controlled rotation gates without a trade-off between speed and fidelity. This behavior applies over a wide range of magnetic field orientations, making the system robust with respect to variations from qubit to qubit. In previous work (2022) it was shown that the hole spins in an existing electronic device can be trapped and used as qubits. Now, the researchers here have achieved a controllable interaction between two qubits within this setup. They coupled two qubits and brought about a controlled flip of one of their spins, depending on the state of the other's spin (a controlled spin-flip). They have shown that hole spins allow creation of fast and high-fidelity two-qubit gates. The coupling of two spin qubits is based on their exchange interaction, which occurs between two indistinguishable particles that interact with each other electrostatically. Surprisingly, the exchange energy of holes is not only electrically controllable, but strongly anisotropic. This is a consequence of spin-orbit coupling, which means that the spin state of a hole is influenced by its motion through space. The anisotropy makes two-qubit gates possible without the usual trade-off between speed and fidelity. All measurements were performed using a dry dilution refrigerator with a base temperature of ~ 40 mK and a three-axis magnet that provides arbitrary control of the magnetic field vector. The researchers measured the dependence of the exchange splitting on the magnetic field direction and found large values in some directions but close-to-zero values in other directions. They have developed a general theoretical framework applicable to a wide range of devices and identify the spin–orbit interaction as the main reason for the exchange anisotropy. From their measurements, they extracted the full exchange matrix and hence accurately determine the Hamiltonian of the two coupled spins, predicting the optimum operating points for the gates. For holes, unlike electrons, the strong exchange anisotropy facilitates controlled rotations with both high fidelity and high speed, for an experimental setting that is robust against device variations. The researchers have investigated the exchange coupling between two hole-spins in a Si FinFET and found it to be both highly anisotropic and tunable, allowing for an interaction strength > 0.5 GHz. They identify the strong spin–orbit interaction as the main microscopic origin of this anisotropy and propose a simple procedure for determining the exchange matrix. This measurement and analysis scheme applies to a wide variety of devices. By fully characterizing the Hamiltonian of the two coupled spins, the best possible configuration for implementing two-qubit gates can be identified. A strongly anisotropic exchange results in extended sweet spots in magnetic field orientation, where both fast and high-fidelity controlled rotations can be performed. By selecting a close-to-ideal configuration they realize a controlled spin-flip in ≈ 24 ns.

For more information: Phys.org, May 6 (2024); Nat. Phys., May 6 (2024).

Optical microscopy with atomic resolution:   researchers at the University of Regensburg in Regensburg have developed all-optical subcycle microscopy on atomic length scales. The near-field optical tunneling emission (NOTE) microscopy here directly observes a quantum version of Hertz's spark jumping between just two atoms by measuring the oscillogram of the light it emits with temporal precision faster than a single oscillation cycle of the lightwave. Super-resolution optical microscopy has circumvented the far-field diffraction limit by controlling optical nonlinearities. By exploiting the linear interaction with tip-confined evanescent light fields, near-field microscopy has reached even higher resolution. Yet the finite radius of the nm-sized tip apex has prevented access to atomic resolution. The researchers here leverage atomic nonlinearities within tip-confined evanescent fields to push all-optical microscopy to pm-spatial and fs-temporal resolutions. On these scales, they discover a non-classical near-field response, in phase with the vector potential of light and strictly confined to atomic dimensions. This ultrafast signal is characterized by an optical phase delay of ≈ Ï€/2 and facilitates direct monitoring of tunneling dynamics. They image nm-sized defects hidden to AFM by subcycle sampling of current transients on a semiconducting van der Waals material. This technique allows the study of quantum light–matter interaction and electronic dynamics at short spatio-temporal scales in conductive and insulating quantum materials. They used an atomically sharp tip to focus light into the near-field region (a few atoms wide) with sub-atomic precision. This ultrafast optical microscopy technique combines the resolution of a scanning probe microscope with all-optical (light in, light out) signal measurement. The oscillating electromagnetic fields that carry light can cause electrons to tunnel back and forth through a potential energy barrier. This alternating current can coherently emit measurable light waves, used here to build an atomic resolution optical microscope. An ultrafast tunneling current flows between the frontier atom of a sharp tip and a sample in response to an incident electromagnetic driving field. This atomically confined current leads to the emission of light, carrying information about the inner workings of quantum processes. The oscillating electric field of light flushes the tunneling electrons back and forth between the frontier atom of the tip and the sample, hence driving the quantum version of Hertz's spark. The researchers detected the Hertzian emission from a handful of electrons per oscillation cycle of light receiving a strong signal (this made possible by the ultra-stable tip acting as an antenna transmitting this wave from the atomic scale). To simulate from first principles the quantum response of 1010 atoms, they used a supercomputer to reproduce the signature time shift of the NOTE signal and provide insights into the lightwave-driven quantum flow of electrons and distortion of atomic orbitals. The electrons have to stay underneath the tip until the light field changes its direction to be able to return. Direct observation of ultrafast tunneling currents allow analysis of electronic dynamics in quantum systems.

For more information: Nature, May 8 (2024) page 329; Phys.org, May 8 (2024).

WEEK OF MAY 6, 2024 [No. 1574]

Targeted broadband optical excitation of the ^229mTh nucleus:   researchers at PTB in Braunschweig and TU Wien in Vienna have resonantly excited the nuclear isomer state in ²²⁹Th in grown, heavily-Th⁴⁺-doped, transparent, CaF₂ crystals (²²⁹Th concentration up to 5 × 10¹⁸/cm³) using a tabletop, home-made, 148-nm, broadband (measured spectral linewidth ≤10 GHz ), tunable, UV laser system. While presently no 148-nm CW laser exists, the researchers generated their laser here by mixing existing laser wavelengths. They cooled the CaF₂ crystal to ≤ 180 K by placing it on a cold plate in a vacuum chamber (where it was exposed to the VUV laser light), to avoid optically damaging it. They used a PMT to collect, focus, and detect the crystal’s fluorescence as they gradually stepped the laser’s wavelength from 148.2 to 150.3 nm. From 20 measurement cycles, each with 50 frequency steps, they observed a distinct fluorescence peak around 148.38 nm from two differently doped CaF₂ crystals. A control measurement on a crystal doped with ²³²Th emitted no fluorescence signal. The nuclear resonance for the Th⁴⁺ ions in Th:CaF2 is measured at the central wavelength 148.3821(5) nm with frequency 2020.409(7) THz. The observed central wavelength of the nuclear transition amounted to a transition energy of 8.35574(3) eV, thus consistent with the 1 σ-uncertainty of the value reported in radiative-decay experiments but with 800-fold improved precision here. In addition, measurements of the fluorescence decay time revealed an overall radiative lifetime of ^229mTh embedded in the CaF₂ crystal matrix of 630(15) s. This corresponds to a isomer half-life of 1740(50) s for a ^229mTh nucleus isolated in vacuum, consistent with earlier findings and theoretical expectations that take into account polarization effects in the crystal (these effects reduce the isomer’s half-life with the inverse third power of the refractive index). The researchers here have developed many-nuclei crystal set ups, determined and proven the involved energy transition (with the required µeV precision), and used a laser to transfer a nucleus into a state of higher energy and then precisely tracked its return to its original state. The large number of ²²⁹Th nuclei in the crystals amplifies the laser absorption effect, shortens the required measurement time, and increases the probability of actually locating the energy transition. Because CaF₂ crystals have a band gap larger than the isomer’s excitation energy, they should prevent the isomer’s deexcitation via interaction with its electron shell. The researchers achieved what is the first targeted laser excitation of an atomic nucleus on November 21, 2023. The correct energy of the ²²⁹Th transition was hit exactly, and the ²²⁹Th nuclei delivered a clear signal. The laser beam had actually switched the state. Now, a narrowband CW VUV laser needs to be developed to allow the efficient driving of a nuclear clock.

For more information: Physics, May 6 (2024); Phys.org, April 29 (2024); Physicsworld, April 18 (2024); Phys. Rev. Let., April 29 (2024) page 182501.

Strange metallicity in a kagome metal:   an international group lead by researchers at MIT in Cambridge, MA has discovered hopping-frustration-induced flat band and strange metallicity in a kagome lattice metal. They show transport and thermodynamic hallmarks of heavy fermion and strange metal behavior that arise in the kagome metal Ni₃In, with the source of localized states being destructive interference-induced band flattening of partially filled Ni 3d states. With magnetic field and pressure tuning, they find evidence that the system is proximate to quantum criticality, extending the analogy to f-electron Kondo lattices. The observations here show the role of hopping frustration in metallic systems as a source for strong correlations. In previous work the researchers reported that their new Kagome metal produced Dirac fermions as expected. However, the strange metal behavior discovered now was unexpected. After that they tried to determine if there existed a flat band at the Fermi level with correlated still electrons. They found it, and began exploring the system's electrical properties while subjected to high pressure and to a magnetic field. They discovered that the electrons in the flat band interact strongly with other electrons in the system resulting on a strange metal behavior.

For more information: Phys.org, May 1 (2024); Nat. Phys., January 25 (2024) page 610.

WEEK OF APRIL 29, 2024 [No. 1573]

Landau levels observed in Si photonic crystals:   researchers at Penn State University in State College, PA have detected Landau levels that arise due to a strain-induced pseudomagnetic field in a Si photonic crystal slab. The Landau levels are dispersive (not flat bands) due to the distortion of the unit cell by the strain. They employ an additional strain of a different form that induces a pseudoelectric potential to flatten them. By acting akin to cavities that are delocalized across space, flat bands such as these have the potential to strongly enhance light–matter interaction as a result of the photonic structure. The researchers developed an analytical framework for understanding the effects of inhomogeneous strain in photonic crystals via gauge fields that helps to guide the design of multiscale non-periodic photonic structures. Although photons do not experience the Lorentz force because they do not carry charge, they can be made to experience pseudomagnetic fields as a result of periodicity-breaking strain. Within the crystal, the light spun in circles and they observed that it formed discrete energy bands (Landau levels) as in electrons. They employed a method of emulating a magnetic field (pseudomagnetic field) for light by precisely manipulating the structure of a photonic crystal. They created these crystals in 100-nm Si slabs with a honeycomb-like lattice of holes. They shined laser light into the crystal-containing slab, and the lattice pattern caused some of the light to bounce around within the crystal. Then, they measured the spectrum of the light when it exited the crystal. To mimic the effects of a magnetic field, they added a strain to the pattern of the lattice. For the unstrained lattice, they fabricated a honeycomb structure out of nanoscale triangular holes that repeats throughout space in 2D. To add the strain, they made another slab, but deformed the pattern. The new pattern looks as if they pulled up on the two sides, while pulling down on the bottom side. When the researchers shined the laser into the unstrained lattice, the light spreaded out evenly in the crystal. In the strained lattice, the light instead moved in circles and the energy spectrum of the light changed, forming discrete bands just like Landau levels. The Landau-level energy spacing is a linear function of strain strength. Unlike Landau levels in electrons, the energy bands are not flat. Instead, they are curved (dispersion), which the researchers believe results from the curved pattern in the strained crystal. To try to mitigate the dispersion, they added an additional strain to the pattern. This added strain, which acts as a pseudo-electric potential, counteracts the dispersion, producing flat-band Landau levels just like those from electrons. The flat bands represent a concentration of photons at certain discrete energies, providing an avenue to increase the interaction of light with matter.

For more information: Phys.org, April 24 (2024); Nat. Phot., April 23 (2024).

1D proximity superconductivity achieved in quantum Hall regime:   an international group lead by researchers at the University of Manchester in Manchester has made Josephson junctions incorporating domain walls in minimally twisted bilayers achieving robust superconductivity in high magnetic fields. They have shown that domain walls in minimally twisted bilayer graphene support robust proximity superconductivity in the quantum Hall regime, allowing Josephson junctions to operate in fields close to the upper critical field of superconducting electrodes. The critical current was found to be non-oscillatory and practically unchanging over the entire range of quantizing fields, with its value being limited by the quantum conductance of ballistic, strictly 1D, electronic channels residing within the domain walls. The built system supports Andreev bound states at quantizing fields. When combining superconductivity and the quantum Hall effect, Cooper-pair transport between superconducting electrodes in Josephson junctions is mediated by 1D edge states. So far no supercurrents through quantum Hall conductors have been detected. The initial experiments here were on line with proximity superconductivity induced along quantum Hall edge states. Then they used previous work demonstrating that boundaries between domains in graphene could be highly conductive. By placing such domain walls between two superconductors, they achieved the desired ultimate proximity between counterpropagating edge states while minimizing effects of disorder. They observed large supercurrents at temperatures up to 1 K in every device they fabricated. They figured that the proximity superconductivity originated not from the quantum Hall edge states propagating along domain walls, but rather from strictly 1D electronic states existing within the domain walls themselves. These predicted 1D states exhibited a greater ability to hybridize with superconductivity as compared to quantum Hall edge states. The inherent 1D nature of the interior states is believed to be responsible for the observed robust supercurrents at high magnetic fields. In their devices, electrons propagate in two opposite directions within the same nanoscale space and without scattering. The researchers manipulate these electronic states using gate voltage and observe standing electron waves that modulated the superconducting properties.

For more information: Nature, April 24 (2024) page 741; Phys.org, April 24 (2024).

WEEK OF APRIL 22, 2024 [No. 1572]

Q value for electron capture in the ¹⁶³Ho nucleus determined:   the international ECHo collaboration at the Max-Planck-Institut für Kernphysik in Heidelberg has used a Penning trap system to measure the change in mass of ¹⁶³Ho when its nucleus captures an electron and turns into ¹⁶³Dy. They realized a direct, microcalorimetry-independent determination of the Q value by measuring the free-space cyclotron frequency ratio of highly charged ions of ¹⁶³Ho and ¹⁶³Dy in the Penning-trap experiment PENTATRAP. Combining this ratio with calculations of the electronic binding energies yields a Q value of 2,863.2 ± 0.6 eV c^−2 . They measured accurately the total energy released in the decay: This corresponds to a maximum of the Q value minus the rest mass of the neutrino released. From this, they calculated its Q value 50 times more accurately than before (from a kinematic study of tritium β-decay in KATRIN) and thus, they determined the most precise upper limit to date of the electron neutrino mass (0.8 eV c^−2). Complementary ECHo is also investigating the electron capture in ¹⁶³Ho → ¹⁶³Dy + ν_e + E_cal, where E_cal is the energy detected in a calorimeter. Within the ECHo collaboration, metallic magnetic calorimeters are used for the measurement of the energy of all emitted radiation, except for the energy carried away by the neutrino. This is obtained by implanting ¹⁶³Ho ions directly into the absorber material of the detector (Au). The calorimetrically measured decay spectrum is subsequently analyzed by fitting it to a theoretical spectral shape from which the Q value can be determined. To quantitatively investigate systematic effects in the interpretation of the calorimetrically measured spectra that might arise due to the ¹⁶³Ho ions being implanted into a metallic material, this Q value is best compared to one obtained from an independent direct measurement. The required accuracy of ~ 1 eV c^−2 can only be reached at present using high-precision Penning-trap mass spectrometry. In Penning-trap mass spectrometry, the Q value is addressed directly through a measurement of the mass difference of the mother and daughter nuclides, ¹⁶³Ho and ¹⁶³Dy, respectively, by measuring the free-space cyclotron frequency ratio of the two species in a strong homogeneous magnetic field B. In a Penning trap, a superimposed weak quadrupolar electrostatic potential confines the ion along the magnetic field lines and modifies the ion’s radial motion: the free-space cyclotron motion splits into the magnetron motion with a frequency ν_− and the modified cyclotron motion with a frequency ν₊. The quadrupolar electrostatic potential also induces a harmonic oscillatory motion with frequency ν_z along the magnetic field lines. From a measurement of all three motional eigenfrequencies, the free-space cyclotron frequency can be reconstructed using the invariance theorem. From subsequent measurements of the free-space cyclotron frequency, the ratio is determined, which allows the Q value to be determined by including calculations of the binding energy difference of the removed electrons. The researchers here used the Pentatrap system consisting of five identical Penning traps arranged one above the other within a superconducting magnet. Deceleration electrodes with appropriately timed voltage pulses are used to capture the ions in the Penning traps. In these identically constructed traps, ions in the excited quantum state and in the ground state can be measured in comparison. In order to minimize uncertainties, the ions were also moved back and forth between different traps for comparative measurements. They removed 38, 39 and 40 electrons from the atoms in three different series of measurements to increase the cyclotron frequency of both nuclei ions and obtain better precision in the mass difference measurements, and thus, in the Q value for electron capture. In addition to the frequency difference between the two ions, the energy stored in the remaining electron system of a highly charged ion has a significant influence on the Q value determined. The calculations resulted in almost exactly the same Q values for the three measured charge states with 38, 39 and 40 electrons removed so systematic uncertainties in experiment and theory could be ruled out.

For more information: Phys.org, April 19 (2024); Nat. Phys., April 19 (2024).

Spin texture created and stabilized by superconductor stray fields at RT:   an international group lead by researchers at the HZB in Berlin has uncovered an approach to create and stabilize complex spin textures in a variety of compounds and conditions. The work includes the study of the size-dependence and high temperature stability of radial vortex magnetic textures imprinted by superconductor stray fields. They have considered the imprint of magnetic radial vortices in soft ferromagnetic compounds making use of the stray field of YBa₂Cu₃O_7-δ (YBCO) superconducting microstructures in ferromagnet/superconductor (FM/SC) hybrids at temperatures below the superconducting transition temperature (T_c). They explore the lower size limit for the imprint of magnetic radial vortices in square and disc shaped structures as well as the persistence of these spin textures above T_c, with magnetic domains retaining partial memory. Structures with circular geometry and with FM patterned to smaller radius than the superconductor island facilitate the imprinting of magnetic radial vortices and improve their stability above T_c, in contrast to square structures where the presence of magnetic domains increases the dipolar energy. Micromagnetic modeling coupled with a SC field model reveals that the stabilization mechanism above T_c is mediated by microstructural defects. They show superconducting control of swirling spin textures, and their stabilization above T_c by means of defect engineering. Here, radial vortices are created with the help of superconducting structures, while the presence of surface defects achieves their stabilization. Samples consist of µm-sized islands made of the high-temperature superconductor YBCO (T_c < 92 K) on which a ferromagnetic compound is deposited. An external magnetic field is applied and immediately removed. This process allows the penetration and pinning of magnetic flux quanta, which in turn creates a magnetic stray field. It is this stray field that produces new magnetic microstructures in the overlying ferromagnetic layer: spins emanate radially from the structure center, as in a radial vortex. As the temperature is increased, YBCO transits from the superconducting to a normal state, the stray field created by YBCO islands disappear and the magnetic radial vortex should disappear as well. However, the researchers have observed that the presence of surface defects prevents the latter from happening: the radial vortices partially retain the imprinted state, even when approaching RT. They use the magnetic field generated by the superconducting structures to imprint certain magnetic domains on the ferromagnets placed on them and use the surface defects to stabilize those imprinted states. Smaller imprinted vortices were ~ 2 µm in diameter (size of typical skyrmions ~ 200 nm). They studied samples with circular and square geometries and found that circular geometries increased the stability of imprinted magnetic radial vortices. A wide category of effects in FM/SC hybrids involves the influence of the ferromagnet on the superconducting ground state. This influence is mediated by the stray fields of ferromagnetic domains or magnetic chiral structures (like magnetic vortices, skyrmions, or domain walls) on the dissipation properties and critical current characteristics of the superconductor. It is known that the use of superconducting stray fields in the magnetic ground state of ferromagnets, can craft swirling spin textures in ferromagnetic systems by making use of the magnetic stray fields generated by the trapped flux in structured type II superconductors. Below the superconducting T_c , the application and removal of an out-of-plane magnetic field yields the generation of screening supercurrents due to the penetration, pinning, and expulsion of magnetic flux quanta. Within the mixed state, these supercurrents flow following the geometrical contour of the SC structure, with a geometry and sense of rotation which depend on magnetic history. Supercurrents present after removal of the external magnetic field, give rise to a stray magnetic field whose strength and direction varies locally. Typical values of the vortex density for 100 mT used here, yield 4.8 × 10¹³ vortices/m², which corresponds to an intervortex distance (assuming a square lattice for YBCO) of 140 nm. In ferromagnetic systems with perpendicular magnetic anisotropy, the out-of-plane component of the superconductor stray-field can be employed to imprint unusual magnetic textures. The imprint is stabilized by the perpendicular magnetic anisotropy so that it remains even when supercurrents have vanished for temperatures above T_c. It is known that in ferromagnetic layers with in-plane magnetic anisotropy, the in-plane components of the SC stray-field can be utilized to imprint magnetic domain distributions akin to radial vortices with a lateral size of 20 μm. In these, the in-plane magnetization can point toward or away from the core along radial directions orthogonal to the contour of the superconducting microstructure. This type of magnetization distribution is not energetically favored due to large dipolar energies. While mm-size structures are expected to retain some memory of the imprinted state at T > T_c , for smaller structures, the disappearance of the SC stray-field T > T_c , is expected to lead to its relaxation to an energetically more favorable magnetic state, such as a conventional vortex or a multidomain configuration. The researchers here explore how the reduction of the lateral size of the SC structure affects the SC imprint of radial vortex-like magnetic spin textures on YBa₂Cu₃O_7−δ/Ni₈₀Fe₂₀ hybrids. Finite-difference micromagnetic modeling coupled with the YBCO field modeling indicate that the radially inhomogeneous field distribution of the superconductor enables the imprint of these topologically nontrivial magnetic domain distributions below T_c for lateral sizes down to sub-µm. The researchers here obtain radial vortex-like imprints down to 2 μm, most likely limited by the presence of surface defects. Although increasing the temperature above T_c leads to the disappearance of the stabilizing SC stray field and the relaxation of the imprinted magnetic domain pattern, the remnant spin texture retains a significant memory of the imprinted state. They discuss the robustness of this state and the origin of this memory effect in terms of pinning of domain walls by YBCO surface defects, which contribute to stabilize its topology.

For more information: Phys.org, April 17 (2024); ACS Appl. Mater. Interfaces., April 2 (2024) page 19681.

WEEK OF APRIL 15, 2024 [No. 1571]

Quantum phase change induced by THz laser at RT:   an international group lead by researchers at Stockholm University have demonstrated THz electric-field-driven dynamical multiferroicity at RT in the paraelectric perovskite STO (SrTiO₃). They have shown how laser light can induce quantum behavior at room temperature and how to make non-magnetic materials magnetic. They subjected STO to short intense polarized FIR laser beams to induced magnetism at RT. The approach is based in the theory of dynamic multiferroicity that predicts that when Ti atoms are stirred up with circularly polarized light in a Ti-Sr oxide, a magnetic field is formed. That allows to make many insulators into magnetic materials. The researchers resonantly drive the IR-active soft phonon mode with an intense circularly polarized THtz electric field and detect the time-resolved magneto-optical Kerr effect. A simple model, which includes two coupled nonlinear oscillators whose forces and couplings are derived with ab initio calculations using self-consistent phonon theory at a finite temperature, qualitatively reproduces the experimental observations. A quantitatively correct magnitude was obtained by considering the phonon analogue of the reciprocal Einstein–de Haas effect in which the total angular momentum from the phonons is transferred to the electrons. The findings here show a path for the control of magnetism by coherently controlling the lattice vibrations with light. It has been shown independently by other researchers that circular phonons induced by circularly polarized light in a substrate can switch the magnetization of a ferromagnet on top of it. This supports the observation that the induced magnetic moment must be of the order of the electronic one. At low temperatures, intense coherent THz excitation of the soft mode in STO can lead to nonlinear phonon responses that overcome the quantum fluctuations to create a ferroelectric order absent at equilibrium that is coupled to the induced magnetic order. The induced magnetic moment can be seen as a quasi-static magnetization created on ps timescale (an order of magnitude faster than the fastest spin switching to date).

For more information: Nature, April 10 (2024) page 534; Phys.org, April 10 (2024).

Dimensional crossover observed in ultracold bosons:   an international group lead by researchers at Universität Innnsbruck has observed how continuous changes in dimensionality affect collective properties of a superfluid. Specifically, they have analyzed the 2D–1D crossover in strongly interacting ultracold bosons. Starting from a 3D BEC, they generate an ensemble of 2D layers and 1D tubes. In mixed dimensionality, they found a characteristic two-slope decay for the one-body correlation function, consistent with particles being 1D and 2D simultaneously. They used the correlation measurements to numerically determine the temperature of quantum liquids in 1D, 2D, and in between. They showed that strongly interacting ultracold bosons perceive their dimensionality as either 1D or 2D, depending on whether they are probed on short or long distances, respectively. They probed this dimensional crossover considering the momentum distribution to study the characteristic decay of the one-body correlation function in the two dimensionalities and track how the decay is modified during the crossover. The observations here demonstrate how quantum properties in the strongly correlated regime evolve in the dimensional crossover as a result of the interplay between dimensionality, interactions and temperature.

For more information: Phys.org, April 10 (2024); Nat. Phys., April 9 (2024).

WEEK OF APRIL 8, 2024 [No. 1570]

Ultrafast Kapitza-Dirac effect used to visualize electron phase:   an international group lead by researchers at Goethe University Frankfurt in Frankfurt am Main have observed the ultrafast Kapitza - Dirac effect with complete temporal resolution and used it to visualize and , theoretically describe, the quantum mechanical phase of electrons (temporal evolution of the electron waves). Kapitza and Dirac predicted (1933) that electrons should be diffracted from a strong standing wave of light in a static description. The researchers here have added a time dimension when probing the evolution of a photoelectron wavepacket using an intense pulsed standing wave. By tracking the spatiotemporal evolution of a pulsed electron wave packet diffracted by a 60-fs standing wave pulse in a pump-probe scheme, they observed time-dependent diffraction patterns. The electron wave packet was exposed to two counterpropagating ultrashort laser pulses (time span from back to front 10 ps). The fringe spacing in the observed pattern differs from that generated by the conventional Kapitza-Dirac effect. By exploiting this time-resolved diffraction scheme, they accessed the time evolution of the phase properties of a free electron. The researchers fired two ultrashort laser pulses from opposite directions at a Xe gas. At the crossover point, these fs pulses produced an ultrastrong transient light field that ionized the Xe atoms. Then, they fired a second pair of short laser pulses at the electrons released in this way, forming a standing wave at the center. These pulses were slightly weaker than the ones before and did not cause any more ionization. They interacted with the free electrons so they could be observed using a home-made COLTRIMS reaction microscope. At the point of interaction, either the electron does not interact with the light or it is scattered (to the left or to the right) with the wave function spatially concentrated accordingly. The temporal evolution of the wave function and its phase is dependent on how much time elapses between ionization and the moment of impact of the second pair of laser pulses.

For more information: Science, March 28 (2024) page 1467; Phys.org, April 3 (2024).

Josephson junction built with superconductor and topological insulator:   an international group lead by researchers at Julius-Maximilians-Universität Würzburg in Würzburg has made a magnetically tunable stable superconductor in a dilute magnetic topological insulator-based Josephson junction. The researchers added magnetic atoms into topological insulators so that they can be controlled by a magnet. The combination generates a proximity-induced Fulde-Ferrell-Larkin-Ovchinnikov (p-FFLO) state in which superconductivity and magnetism coexist. A superconductor, when exposed to a spin-exchange field, can exhibit spatial modulation of its order parameter (the FFLO state). Such a state can be induced by controlling the spin-splitting field in Josephson junction devices, allowing access to a large phase diagram zone. The researchers here demonstrate that a FFLO state can be induced in Josephson junctions based on the 2D dilute magnetic topological insulator (Hg,Mn)Te. They do this by observing the dependence of the critical current on the magnetic field and temperature. The substitution of Mn dopants induces an enhanced Zeeman effect, which can be controlled with high precision by using a small external magnetic field. They observe multiple fluctuating behaviors of the critical current as a response to an in-plane magnetic field, which they assign to transitions between ground states with a phase shifted by Ï€. Whereas the FFLO state is determined by the properties of the bulk superconductor, the proximity-induced version of the effect depends on the properties of the weak link, which, as shown here, can be a semiconductor. Given the relative ease with which the transport properties of semiconductors (as opposed to those of metals) are affected by magnetic and electric fields as well as currents, the set up used here provides the flexibility to explore FFLO phenomena in ways that were previously experimentally inaccessible.

For more information: Phys.org, April 4 (2024); Nat. Phys., April 1 (2024).

WEEK OF APRIL 1, 2024 [No. 1569]

Quantum Barkhausen noise detected:   researchers at Caltech in Pasadena, CA have found that quantum Barkhausen noise is dominated by correlated domain electron tunneling. The spin alignment avalanche effect and its associated noise was classically demonstrated in magnets (Barkhausen, 1919). The researchers here examine the dynamics of a uniaxial rare-earth ferromagnet deep within the quantum regime, so that domain wall motion, and the associated hysteresis, is initiated by quantum nucleation, which then grows into large-scale domain wall motion observable as a form of Barkhausen noise. They observe noncritical behavior in the resulting avalanche dynamics that can not be explained using conventional renormalization or classical domains. They find that this quantum Barkhausen noise exhibits two quantum mechanical domain wall movements with different dependences on an external magnetic field applied transverse to the spin (Ising) axis. These observations can be understood in terms of the correlated motion of pairs of domain walls, nucleated by cotunneling of plaquettes (sections of domain wall), with plaquette pairs correlated by dipolar interactions; this correlation is suppressed by the transverse field. Similar macroscopic correlations may be expected to appear in the hysteresis of other systems with long-range interactions. The researchers here have done the experiment in the quantum material LiYHoF₄ at near 0 K wrapping a coil around it and measuring voltage spikes (the Barkhausen noise) indicating when domains of electron spins sequentially flip their magnetic orientations. They demonstrate that spin flipping occurs by correlated domain electrons quantum tunneling in sync (with domains here containing ≈ 10¹⁵ electrons in a crystal with ≈ 10²¹ electrons). By analyzing this noise, the researchers showed that a magnetic avalanche was taking place even without the presence of classical effects. These effects were insensitive to changes in the temperature of the material. Based on this and other analytical work they concluded that quantum effects were responsible for the process.

For more information: Phys.org, March 28 (2024); PNAS, March 19 (2024).

Parity anomaly detected in a topological insulator:   researchers at Julius-Maximilians-Universität Würzburg in Würzburg have observed fluctuating quantum Hall effect (QHE) with increasing magnetic field, , and have identified it as a signature of parity anomaly, in a microscopic sample of MBE-grown, 3D paramagnetic topological insulator (Hg,Mn)Te. The band inversion of topological materials in three spatial dimensions is connected to the parity anomaly of 2D massless Dirac fermions. At finite magnetic fields, the parity anomaly shows as a non-zero spectral asymmetry (an imbalance between the number of conduction and valence band Landau levels, due to the unpaired zero Landau level). QFT predicts that a single massless 2D Dirac fermion exhibits the parity anomaly. In a metal, the anomaly induces an ambiguity in the sign of the transverse conductivity in the limit of vanishing magnetic field. A connection between the parity anomaly and topological surface states is predicted but there has not been yet an observation of the parity anomaly in the limit of zero magnetic field because topological surface states come in pairs and the anomaly can occur only within a single Dirac state. While in the anomalous QHE one of the entangled surface states is removed, the effect occurs only in ferromagnets and then, the hysteresis connected with ferromagnetism makes the observation of the anomaly at zero field very difficult. An unconventional fluctuating sequence of quantized Hall plateaus in the measured Hall resistance is related to the occurrence of spectral asymmetry in a single topological surface state. The effect should be observable in any topological insulator where the transport is dominated by a single Dirac surface state. As theoretically predicted in the 80's, relativistic Dirac particles at the material top/bottom surfaces should be subject to parity anomaly that leads to spectral asymmetry with an unusual change in the electrical resistance. The researchers here have identified and verified this and another related consequence of the parity anomaly that is generic for any topological insulator, not specific to (Hg,Mn)Te. The device here had to be controlled so the effects from the top/bottom surfaces did not cancel each other. And the signature of spectral asymmetry had to be differentiated from the other features in the measured electrical resistance. The researchers here make the experimental observation of a spectral asymmetry in the transport characteristics of the material. The spectral asymmetry induces an anomalous Hall response (an extra contribution of e²/h to the Hall conductance σ_H from the unpaired zero Landau level when the bulk bands are in inverted order). In the magneto-transport experiments it manifests as a sequence of ν = −1, −2, −1 quantum Hall plateaus for increasing magnetic fields. This observation requires the coexistence of topological Dirac surface states and massive (non-topological) surface states, where the ν = −1 to ν = −2 transition marks the crossing of an unpaired topological zero Landau level with the Fermi level, changing the Hall resistance R_xy from −h/e² to −h/2e². The parity anomaly should be accessible in any topological insulator device, provided that a single surface can be accessed in magneto-transport and that the material is of sufficiently high quality. The researchers demonstrate the observation of a single 2D Dirac system, realized by separate control of the carrier densities on both surfaces of a 3D topological insulator. The fluctuating sequence of quantum Hall plateaus allows them to identify a single 2D Dirac fermion. At this transition from ν = −1 (low-field) to ν = −2 the contribution of e²/h from the bottom surface state vanishes, which is evidence for the presence of a spectral asymmetry. The contribution from spectral asymmetry persists in the limit of zero magnetic field.

For more information: Phys.org, March 26 (2024); Advanced Science, March 13 (2024).

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 T_c ≈ 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 Mo_x Ge_1-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 → B_QCP) → 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 Λ_b⁰ baryons in pp collisions at COM energy √s = 13  TeV. The production rate of Λ_b⁰ baryons relative to B⁰ mesons in pp collisions was measured in the LHCb experiment. The ratio of Λ_b⁰ 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 Λ_b⁰ 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 Λ_b⁰ 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 Λ_b⁰ 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 Λ_b⁰ 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 E_F 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 m_p / m_e 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 m_p / m_e 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 R_xx. R_xy 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 Ï€-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 cm²â€‰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.