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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 APRIL 21, 2025 [No. 1624]

Single spin detection using photovoltage:   researchers at the HZB in Berlin have developed a non-optical method using photovoltage (PV) to detect the individual and local spin states of Nitrogen-vacancy centers (NVCs) in ambient conditions. They report the detection of elementary charges bound to single NVCs several nm below the diamond surface using Kelvin Probe Force Microscopy (KPFM) under laser illumination. The measured signal depends on the NVC’s electron spin state, thus allowing to perform a non-optical single spin readout. A green laser is used to excite charge carriers in NVCs, which are then captured by surface states. They modified a KPFM systen to move the tip over the surface and measure the potential difference around a NVC. The PV depends on the electron spin state of the NVC so they can read out the individual spin. The electron spin states of the NVCs can be manipulated using microwaves, and thus, they can capture the spin dynamics by coherently manipulating the spin states using microwave excitation. In frequency-modulated KPFM experiments, a probe oscillates at a frequency f₀ in the vicinity of the electrically grounded sample surface. An alternating current (AC) potential with an amplitude V_AC at a frequency f_AC applied to the probe induces mechanical oscillations at sideband frequencies f₀ ± f_AC. The amplitude of these sideband oscillations is proportional to the contact potential difference (CPD) between the probe (cantilever) and the sample and is detected using a lock-in technique. A voltage feedback loop is used to apply a direct current (DC) potential to the probe in order to nullify the sideband oscillations, thus measuring the CPD that reflects the electrical potential of the surface. The researchers here present a method for electrical readout of electron spins associated with defect centers in solids based on changes of the surface potential using KPFM under laser illumination. As a test system they use NVCs in diamond located a few nm below the surface. They use a simplified model for a qualitative description of the effects observed in the experiments. The process can be described more completely with 2D simulations of a time-dependent model based on solving Poisson’s equation for the electric potential and drift-diffusion equations for the charge carriers. This must include the generation of charge carriers of both types from various defects during the light illumination, as well as the trapping of defects. The simplified model here is based on partially filled surface acceptor states originating either from sp² defects (double carbon C=C bonds) or oxygen-terminated sites (C=O bonds), where both can produce free holes in the diamond valence band under green laser illumination (2.38 eV). After the excitation, these holes do not freely diffuse but rather drift in the applied AC potential V_AC (which is used for KPFM detection) between the grounding contact and the cantilever leaving the negative charge at the excitation spot. Due to high carrier mobility in diamond, the hole’s drift time is negligible compared to the V_AC’s oscillation period . Thus, one can assume that during half of the oscillation period, the holes gather below the cantilever, and for the next half, they are at the metal grounding contact. This increased hole concentration leads to the trapping of charge carriers by the surface states at the cantilever tip position, resulting in a trapped positive charge at that location. This model allows to qualitatively explain the PV imaging mechanism in the following way. When the laser is focused at the position of the cantilever tip, the surface states can only release holes, because the photon energy is not high enough to excite electrons to the conduction band, thus leading to an accumulation of a negative charge at the illumination spot. Therefore, even though the holes repopulate the surface states when they are next to the cantilever, the net charge trapped there is still negative, resulting in negative PV. When the laser is focused on the diamond surface away from the cantilever, the excited holes drift towards the cantilever position, leading to a positive PV. The excitation of a NVC produces both holes from the surface states as well as electrons and holes from the NVC itself. In this case, charge carriers of both types move towards the cantilever to be trapped there. This reduces the absolute value of the observed PV (compared to the surface illumination), but its sign depends on the carrier concentrations and capture cross-section that in turn depends on the local surface environment. Due to the spin-dependent nature of the charge carrier emission from the NVC, the associated PV signal is spin-dependent, which is demonstrated with Surface Voltage Detected Magnetic Resonance and PV-detected Rabi experiments. The researchers assume that the charge carrier motion is governed by the drift in the applied AC potential, neglecting diffusion and trapping, which would result in a decrease of the amount of carriers reaching the cantilever. They also neglect the influence of substitutional N defects, which are sources of free electrons under the laser illumination, and other donor-type defects, since their concentration is very low, and their measurements imply a transfer of a net positive charge when the laser is focused on the diamond surface. From this mechanism it follows that the amplitude of the AC oscillating voltage V_AC used for KPFM detection is crucial, and the results should show a strong dependence on the applied AC voltage amplitude and probably on its frequency when it becomes comparable to the carrier capture rate of the surface defects. The amplitude dependency is observed in a series of experiments where the PV resuls were taken at different AC voltage amplitudes while keeping the same cantilever-contact distance. To confirm that this effect depends on the strength of the applied electric field, they performed measurements at different distances between the tip and the grounding contact, while keeping the V_AC constant . In both cases, the result changes sign due to the decrease in the electric field and consequent change of the carrier motion from drift to diffusion, which leads to a higher noise and to a lower absolute contrast. Though the application of a higher AC voltage amplitude could boost the signal, it also increases the attractive force between the cantilever and the grounding contact, leading to instability of the cantilever position on the sample surface. The NVC PV signal shows a dependence on the laser power where the saturation of the signal at higher powers is caused by the screening of the applied V_AC by the built-up charge and/or low density of surface states, compared to the local carrier concentration. The right choice of surface orientation and termination could raise carrier capture rates, diminishing the response time of the signal, and increase the density of states, enhancing the measured PV.

For more information: Phys.org, April 15 (2025); Nat. Comm., April 14 (2025).

Maximal entanglement in fragmentation quarks and gluons demonstrated:   researchers at Stony Brook University in Stony Brook, NY have analyzed pp collisional data from the LHC in CERN to show that particles produced in collimated jets collisions retain information about their initial conditions, with a direct connection between the entanglement entropy at the earliest stage of jet formation and the particles that emerge as the jet evolves. They wanted to see if the distribution of the hadrons in the jet was influenced by the level of entanglement among the quarks and gluons at the time the jet initially formed. An earlier study revealed a connection between entanglement among quarks and gluons within protons and the overall distribution of particles emerging from p-p and e-p smashups. In that work, the higher the entanglement entropy among the quarks and gluons, the greater the entropy in the distribution of particles produced. It revealed that there is maximal entanglement among the quarks and gluons within the high-energy proton. This leads to a simple relation between the proton’s parton distributions and the entropy of hadrons produced in high-energy inelastic interactions, which has been experimentally confirmed. In the work here, they extend the approach to the production of jets, which form from the fragmentation of those quarks and gluons. Would there also be maximal entanglement inside these fragmenting high-energy quarks and gluons? Such a state of maximal entanglement among the jet-forming quarks and gluons predicts a connection between the jet fragmentation function and the entropy of hadrons emerging from the jet. This entropy would be observed as a large number of different types of hadrons. Conversely, such an observation of a high degree of disorder among jet particles and its correlation with the initial fragmentation predictions would be evidence of this maximal entanglement in the fragmenting quarks and gluons. The researchers have found that the distribution of jet hadrons matched this prediction based on maximal entanglement in the earliest stage of jet formation. The maximal entanglement predicts a relation between the jet fragmentation function and the entropy of hadrons produced in jet fragmentation. They tested this relation using the ATLAS Collaboration data on jet production at the LHC, and found good agreement between the prediction based on maximal entanglement within the jet and the data. The researchers extended the test of maximal entanglement to hadronization of high-momentum jets. At high momentum, the QCD renormalization group flow turns a jet into a complicated multiparton system that can be expected to exhibit the maximal entanglement. The parton distribution functions and fragmentation functions (FFs), which describe how a parton fragments into hadrons, are related by crossing symmetry, which gives rise to reciprocal relationships between these quantities. It is then natural to expect that the jet state is also maximally entangled, giving rise to the relation between the jet fragmentation function and the entropy of produced hadrons. Recent quantum simulations of hadronization in jet production probed the real-time dynamics of the entanglement entropy production process, but the relation between entanglement entropy and fragmentation function had not yet been explored. The researchers here extend the universal entanglement entropy model, an improved version of the original Kharzeev-Levin model , to relate the jet fragmentation function to the entropy of the final hadron state. Within this approach, they compared the entanglement entropy derived from FFs with the hadron entropy calculated from the charged-hadron multiplicity distribution within jets, as measured by the ATLAS experiment. The researchers explored the connection between the FFs and hadron entropy implied by the maximal entanglement. The entanglement entropy in jets was found to be related to the FF in a manner similar to its relationship with PDFs, while the initial entanglement entropy of partons has a lower bound directly tied to the number of color degrees of freedom, as expected at weak coupling dictated by the asymptotic freedom. The charged particle multiplicity distributions in jets from the ATLAS experiment at 7 and 13 TeV provided an estimate of the hadron entropy associated with jet fragmentation. The results showed that the entanglement entropy agrees well with the ATLAS data, suggesting that the fragmentation process exhibits maximal entanglement for high transverse momentum jets, similar to the behavior of the proton wave function at high energies.

For more information: Phys.org, April 13 (2025); Phys. Rev. Lett., March 19 (2025) page 11902.

WEEK OF APRIL 14, 2025 [No. 1623]

Single-molecule tuning of magnetism-superconductivity coupling:   researchers at Technische Universität Ilmenau in Ilmenau have demonstrated control of Andreev reflection (AR) via a single-molecule orbital, and thus, that a single molecule provides a controllable connection between a normal metal and a superconductor. The researchers have shown that the efficiency of AR across a single-molecule contact can be tuned by the energy of an electronic resonance close to the Fermi level. Charge transport across a single-molecule junction fabricated from a normal-metal STM tip and a conventional superconductor is analyzed as a function of the gradually closing vacuum gap. The phthalocyanine (2H-Pc) molecule and its pyrrolic-hydrogen-abstracted derivative (Pc) tips exhibit very different behaviors. AR across the 2H-Pc contact exhibits a temporary enhancement that diminishes again with increasing conductance. In contrast, the single-Pc junction lacks AR in the same conductance range. The researchers perform spectroscopies and nonequilibrium Green’s function calculations showing the importance of a molecular orbital close to the Fermi energy for AR. They used an atomically sharp tip, made of normal-metal atoms to approach a 2H-Pc molecule on a superconducting lead surface. An electron can jump across the gap along various paths, and this jump is accompanied by the production of a hole that moves in the other direction (AR). Some lead Cooper pairs are generated by AR events. The researchers measured the current across this interface as they varied the height of the tip and the tip-to-surface voltage. The measurements showed that the 2H-Pc molecule has an electron orbital that changes as the STM tip gets closer to it. The energy of the orbital shifts toward the Fermi level. The normal-to-super-current conversion process is expected to occur for electrons at the Fermi level. Once the orbital reaches the Fermi level, they found that AR of the junction is enhanced. They think that the orbital shift is induced by a chemical interaction between the tip and the 2H-Pc. Because of the proximity between the tip and the molecule, the orbitals of the two overlap, giving rise to a partial filling of the lowest unoccupied molecular orbital. This filling shifts the orbital energy closer to the Fermi level. In addition to revealing AR, the data showed that the molecule became magnetic when touched by the tip. They also tested another molecule, a variant of the 2H-Pc molecule that did not exhibit a shifting orbital. For this system no AR and no magnetic effect were measured. The difference between the results for the two molecules is striking, since the molecules are very similar, one with H atoms and the other without them. While for 2H-Pc, AR develops progressively when closing the vacuum barrier, AR is absent for Pc. Moreover, AR for 2H-Pc is weakened again with continued decrease of the tip-molecule distance, which is at odds with expectations from the BTK picture of AR. 2H-Pc, strongly contrasting with Pc, shows signatures of a magnetic moment upon hybridizing with the NC tip, which is evidenced by the Kondo effect.

For more information: Physics, April 9 (2025); Phys. Rev. Lett., April 9 (2025) page 146201.

Hubbard failure to explain cuprates electron dynamics confirmed:   researchers at SLAC in Menlo Park, CA have analyzed the doping dependence of 2-spinon excitations in the doped 1D cuprate Ba₂CuO_3+x through photoemission experiments. They have examined the behavior of pairs of spinons providing further evidence of an unusually strong attraction between electrons in neighboring lattice sites within the 1D chain not captured by the Hubbard model. In 2021, SLAC researchers created a 1D chain of doped cuprate and used x-rays to study the behavior of holons. Their analysis revealed that the attraction between neighboring electrons was ten times stronger than the Hubbard model predicted. They have now synthesized a 1D sample of doped cuprate chains at the Stanford Synchrotron Radiation Lightsource at SLAC, and examined it at the Diamond Light Source in the U.K. and the National Synchrotron Light Source II at Brookhaven National Laboratory. Recent photoemission experiments on the quasi-one-dimensional Ba-based cuprates suggest that doped holes experience an attractive potential not captured using the simple Hubbard model. They examined signatures of an additional potential between holes in doped 1D cuprates Ba₂CuO_3+x by measuring the dispersion of the 2-spinon excitations using Cu L3-edge resonant inelastic x-ray scattering (RIXS). Upon doping, the 2-spinon excitations appear to weaken, with a shift of the minimal position corresponding to the nesting vector of the Fermi points, q_F. The researchers found that the energy scale of the 2-spinons near the Brillouin zone boundary is substantially softened compared to that predicted by the Hubbard model in 1D. Their analysis of the behavior of spinon pairs concluded that the Hubbard model does not accurately predict the behavior of electrons. However, when they add the same extra attractive force seen in the earlier experiment to their calculations, the data aligns more closely with their experimental observations.

For more information: Phys.org, April 9 (2025); Phys. Rev. Lett., April 8 (2025) page 146501.

WEEK OF APRIL 7, 2025 [No. 1622]

Picosecond magnetic pulse demonstrated:   researchers at the MPI in Hamburg have generated ultrafast magnetic field steps by quenching supercurrents in a superconductor. They used high-frequency 100-fs-long, 400-nm-wavelength UV pulses to induce ultrafast optical disruption of diamagnetism in superconducting optimally doped YBa₂Cu₃O₇ thin film discs cooled below T_c triggering a ps unipolar magnetic field quench. The magnetic field pulse set off spin dynamics in a near superconducting crystal sample and changed the optical properties. The disruption of superconductivity in the YBa₂Cu₃O₇ discs using laser pulses was used to perturb the magnetic field profile surrounding the discs and enabled the generation of magnetic field steps with ultrafast rise times and super-ns-long decay times. The generated near-field, ultrafast magnetic pulses that were extremely broadband with frequency content spanning several octaves, from sub-GHz to THz frequencies. This process allows tracking of the magnetic field evolution by analyzing the polarization rotation of a fs laser pulse. The magnetic field steps had mT amplitude, ps rise times and slew rate near 1 GT/s. The researchers tested the technique by coherently rotating the magnetization in a ferrimagnet. To track the temperature and time evolution of the magnetic field surrounding the superconductor after optical excitation, they used a time-resolved magneto-optical imaging technique. They placed a diamagnetic GaP(100) crystal above the superconductor. The fast Faraday response to the local magnetic field was probed with a time-delayed optical pulse with 50-μm spatial resolution. Previous realizations of this field detection technique used ferrimagnetic detectors, limiting the time resolution to ~100 ps. Here the researchers used diamagnetic GaP as a magneto-optic detector, allowing for a time resolution better than ~1 ps at the expense of a smaller signal and longer measurement times. Although in the current geometry, the magnetic field step is not sufficient to achieve complete switching, the researchers believe that suitable improvements in the device geometry will generate longer and faster magnetic steps. Faster rise times could be achieved by reducing the size of the superconducting disc down to the µm scale to lower the geometrical inductance of the superconducting disc.

For more information: Phys.org, April 2 (2025); Nat. Phot., April 2 (2025).

Surface conductivity in 2D materials observed:   researchers at the Hebrew University of Jerusalem in Jerusalem have discovered a superconducting transition in extremely thin films of NbSe₂ using high-resolution scanning SQUID-on-tip magnetometry. The researchers' experiments with thin superconducting films reveal a coexistence between two superconducting states, one in the bulk, the other at the surface. A superconducting probe was used to measure the size of vortices in ultrathin films of NbSe₂. When the number of atomic layers is a few, the observed vortices are spatially extended. Thicker samples show vortices that are confined to a small region. The explanation is that surface superconductivity dominates thin films, whereas bulk superconductivity is more prevalent in thick films. In thin superconducting films, the material’s ability to completely expel magnetic fields is weakened. Instead of being fully excluded, the field enters the film through the vortices narrow columns, around which superconducting screening currents flow. Vortices consist of a core where the superconductivity is locally suppressed on the scale of the coherence length where the magnetic field can penetrate. This field is screened by the surrounding superconducting currents and results in magnetic flux quantization. In bulk superconductors, the magnetic field is screened exponentially with the London penetration depth λ_L . For a film of thickness d < λ_L, screening is less effective and is governed by the Pearl length Λ = 2 λ_L²_/ / d. In this limit, the magnetic field decays as 1 / Λ r near the vortex core and as Λ / r³ for distances greater than Λ , where r is the distance from the vortex’s center. Therefore, near the vortex core, there is no characteristic length scale for the magnetic field screening. Inside each vortex, there is exactly one quantum of magnetic flux. The size of a vortex is indicative of the effectiveness of the screening currents at confining the magnetic field within the vortex. If the currents are strong, then the magnetic field around the vortex will drop abruptly to zero. But if the currents are weak, then the magnetic field will penetrate a certain distance into the material (the Pearl length in thin films). It has been assumed that the Pearl length must increase as superconducting samples become thinner, leading to larger and larger vortex sizes. The layered superconductor NbSe₂ sustains superconductivity with T_c > 4.2 K for any thickness above 3 layers. When these films become thinner than six atomic layers, superconductivity no longer spreads evenly throughout the material, but instead becomes confined to its surface. The researchers here measured vortex sizes in superconducting samples of various thicknesses and found larger-than predicted vortices in films made up of six or fewer atomic layers. The researchers proposed that two superconducting states may be coexisting within NbSe₂. For the thicker films, screening currents flow throughout the bulk of the sample with the strength of the currents depending on the thickness of the sample. But when samples become ultrathin, another state begins to dominate, where screening currents are only present in the surface layers and the material’s screening strength becomes extremely weak and remains constant, even as thickness varies. The results suggest that thin superconductors host two superconducting states: one in the bulk of the material, the other confined to the surface layers. The researchers used scanning SQUID-on-tip microscopy (coupled to a tuning fork designed to measure gradients in minute magnetic signals emitted by a vortex, particularly in cases where the Pearl length significantly exceeds the size of µm-scale flakes). The SQUID magnetometer consists of a tiny pipette coated with low-T superconducting Pb. By scanning the pipette over the NbSe₂ surface, screening currents generated in the Pb coating allows to determine the locations and sizes of the NbSe₂ vortices. From the sizes, the researchers estimated the Pearl length. They used NbSe₂ flakes with thicknesses ranging from 3 to 53 layers. The researchers measured how these samples responded to magnetic fields as their thickness decreased. It is expected that the ability of a superconducting material to expel magnetic fields to become stronger as the material becomes thicker. This study confirmed that rule for samples thicker than ten atomic layers. However, when the films became extremely thin the researchers observed the Pearl length sharply increased and became thickness independent. It seems that below a certain thickness, superconductors host current mostly at their top and bottom surfaces, rather than throughout their volume. For more than 10 layers, they found the expected dependence Λ ∝ 1/d . However, six-layer films show a sharp increase of Λ deviating by a factor of three from the expected value. This value remains fixed for to 3 to 6 layers. For the thicker samples, they found that the Pearl length increased as thickness decreased, as expected. But the situation changed for the thinnest samples, with the Pearl length plateauing at a constant value of around 0.1 mm, far higher than expected. The anomaly in the Pearl length dependence in the few-layer limit is interpreted as a phase transition in thin films of NbSe₂, wherein a reduction in film thickness triggers the suppression of bulk superconductivity, giving rise exclusively to surface superconductivity.

For more information: Phys.org, March 31 (2025); Physics, April 15 (2025); Nat. Comm., March 31 (2025).

WEEK OF MARCH 31, 2025 [No. 1621]

Cooper-pair density modulation observed:   an international group lead by researchers at Caltec in Pasadena, CA have used a STM to observe the PDW (pair density wave) spatial modulation of the superconducting gap in FeTe_0.55Se_0.45 thin flakes and discover the Cooper-pair density state (PDM) that is a modulation amplitude > 30% of the superconducting gap average with the smallest wavelength possible, corresponding to the lattice periodicity. STM experiments of Fe-based superconductors on thin flakes had been hampered for nearly two decades by the presence of severe surface contamination. The researchers here developed an experimental approach to produce a sufficiently clean surface for microscopic probes. Their model suggests that the PDM state modulation arises from the breaking of a sublattice symmetry and a rotational symmetry specific to thin flakes. They find that the observed modulation originates from the large difference in superconducting gaps on the two nominally equivalent Fe sublattices. The experimental findings here are backed up by model calculations and suggest that, in contrast to the density-wave orders, the PDM state is driven by the interplay of sublattice symmetry breaking and a nematic distortion specific to the 2D thin flakes.

For more information: Phys.org, March 27 (2025); Nature, March 19 (2025) page 55.

Lower limit on half-life of ¹⁰⁰Mo 0νββ decay set:   researchers at the AMoRE Collaboration in Yangyang, Gangwon-do, South Korea have searched for the neutrinoless double beta decay using 100 kg of enriched ¹⁰⁰Mo in the form of scintillating crystals. The scintillating ⁴⁰Ca¹⁰⁰MoO₄ crystals coupled with a metallic magnetic calorimeter operated at mK temperatures to measure the energy of electrons emitted in the decay. The low-temperature detector system encapsulating the crystals has record sensitivity and is located 700-meter underground. The exposure was 8.02  kg year (or 3.89  kg ¹⁰⁰Mo year), and the total background rate near the Q value was 0.025±0.002 counts/keV/kg/year. They observed no indication of 0νββ decay. The background-only result set the most improved limit on the decay halflife of Mo-100. They report a lower limit of the half-life of 100Mo 0νββ decay as T_1/2⁰ > 2.9×1024  yr at 90% confidence level. The effective Majorana mass limit range is m <(210–610)  meV using nuclear matrix elements estimated in the framework of different models, including the recent shell model calculations.

For more information: Phys.org, March 23 (2025); Phys. Rev. Lett., February 27 (2025) page 082501.

WEEK OF MARCH 24, 2025 [No. 1620]

Heavy charm baryon spin-parity measured:   researchers at LHCb collaboration at CERN in Meyrin have investigated the nature of Ξ_b (3,055)+,0 baryon excitation modes and determined their spin-parity which describes how these particles behave under certain symmetry transformations, They have observed the Ξ_b^0(-) → Ξ_c(3055)⁺⁽⁰⁾ (→ D⁺⁽⁰⁾ Λ) π⁻ decay chains and determined the spin-parity of Ξ_b(3055)⁺⁽⁰⁾ baryons. The measurement is performed using pp collision data at a center-of-mass energy of √s =13  TeV, corresponding to an integrated luminosity of 5.4  fb⁻¹, recorded by the LHCb experiment between 2016 and 2018. They determined the spin-parity of the Ξ_c(3055)⁺⁽⁰⁾ baryons to be 3/2⁺ with a significance > 6.5 σ (3.5 σ ) compared to all other tested hypotheses. The up-down asymmetries of the Ξ_b^0(-) → Ξ_c(3055)⁺⁽⁰⁾ π⁻ transitions were measured to be −0.92±0.10±0.05 (−0.92±0.16±0.22), consistent with maximal parity violation, where the first uncertainty is statistical and the second is systematic. These results support the hypothesis that the Ξ_c(3055)⁺⁽⁰⁾ baryons correspond to the first D-wave λ -mode excitation of the Ξ_c flavor triplet. This excitation mode means that the angular momentum (L) between the charm quark and the diquark system is two and the flavor triplet refers to the antisymmetric configuration of the lighter quarks. The researchers focused on the weak decay reaction of bottom baryons (Ξ_b) to charm baryons (Ξ_c), allowing them to exploit the instability of one (Ξ_b) to study the properties of the more stable one (Ξ_c). They used amplitude analysis techniques to examine the angular distributions and kinematics of the decay products emitted in different directions. Knowing the spin-parity of a baryon provides information about the arrangement and orientation of the constituent quarks. This information is revealed through the orbital angular momentum between the quarks and the specific excitation modes present in the system. In singly heavy baryons the excitation modes characterize the distribution of energy within the system. The primary excitation modes, the λ-mode and the ρ-mode, describe the excitation between the heavy quark and the diquark system and the excitation between the two lighter quarks, respectively These different modes create distinct patterns of orbital angular momentum, representing both specific energy distributions and spatial arrangements of the quark wavefunctions. The researchers measured the up-down asymmetry of transitions finding values consistent with maximal parity violation (it is strongly affected by the spin orientation). This provides evidence in favor of theoretical predictions on bottom baryon decays to charm baryons.

For more information: Phys.org, March 21 (2025); Physics, February 28 (2025); Phys. Rev. Lett.,, February 28 (2025) page 081901.

Blackbody-shift corrections on ²²⁹Th nuclear transitions within solid state matrix:   researchers at JILA in Boulder, CO have characterized the temperature-induced frequency shifts of a ²²⁹Th nuclear transition embedded in a crystal. They measured the four strongest transitions of ²²⁹Th in the same crystal at various temperatures. They found shifts of the unsplit frequency and the electric quadrupole splittings, corresponding to decreases in the electron density, electric field gradient, and field gradient asymmetry at the nucleus as temperature increases. The m = ±5/2 → ±3/2 line shifts only 62(6) kHz over the temperature range (≈ 0.4  kHz/K). They have analyzed the temperature shifts of the transition frequencies, which are caused by the interaction of blackbody radiation from the environment with the nucleus and the electrons around it. The characterization of the four strongest transitions of ²²⁹Th allowed the researchers to identify the transition with the smallest temperature sensitivity. They have used a VUV frequency comb laser to directly resolve individual quantum states of the nuclear transition in variable-temperature, ²²⁹Th -doped CaF2 crystal with continually monitored temperature. To determine how temperature affects the nuclear transition, they placed the ²²⁹Th -doped crystal at 150K with LN, at 229K with a dry ice-methanol mixture, and at 293K. They focused on the energy structure of ²²⁹Th, whose electric quadrupolar moment causes the ground and excited isomer states to split into four magnetic-dipole-allowed transitions. The resulting energy structure is sensitive to the temperature and to the electric-field gradient, which shift the two lowest-energy transitions and the two highest-energy transitions in opposite directions. At the same time, temperature-induced changes of the electron density at the nucleus induce same-direction shifts of all lines. Observing both the frequencies of the unsplit transitions and those of the split transitions, the researchers figured how temperature affects electron density, electric-field gradient, and the field-gradient asymmetry at the nucleus. They measured how the nuclear transition frequency shifted at each temperature, revealing the two competing physical effects within the crystal. As the crystal warmed, it expanded, altering the atomic lattice and shifting the electric field gradients experienced by the ²²⁹Th nuclei. This electric field gradient caused the ²²⁹Th transition to split into multiple spectral lines, which shifted in different directions as the temperature changed. The lattice expansion also changed the charge density of electrons in the crystal, modifying the electrons' interaction strength with the nucleus and causing the spectral lines to move in the same direction. Under the two effects one transition was observed to be less temperature-sensitive than the others, as the two effects mostly canceled each other out in it. Across the full temperature range examined, this transition shifted by 62 kHz, a shift 30 times smaller than in the other transitions. The researchers believe that in the interval 150K - 229K, the transition frequency would be even easier to temperature stabilize. . The transition’s temperature sensitivity of 0.4 kHz/K means that a crystal-temperature stability of 5 µK would be sufficient to reach a fractional frequency precision of 10⁻¹⁸.

>For more information: Phys,org, March 17 (2025); Physics, March 17 (2025); Phys. Rev. Lett., March 17 (2025) page 113801.

WEEK OF MARCH 17, 2025 [No. 1619]

Andreev reflection observed in a topological insulator nanowire:   researchers at the University of Cologne in Cologne and University of Bassel in Bassel have observed long-range crossed Andreev reflection (CAR) in topological insulator (TI) nanowires (NW) proximitized by a superconductor. The researchers perform local and non-local conductance spectroscopy on mesoscopic devices in which superconducting Nb and metallic contacts are connected to a bulk-insulating nanowire. In their local conductance measurements they detect a hard gap and the appearance of Andreev bound states that can reach zero bias. They occasionally observe a negative non-local conductance when sweeping the chemical potential, providing evidence of crossed Andreev reflection. This signal is detected even over length scales much longer than the expected superconducting coherence length of either Nb or the proximitized nanowire. The researchers etched high-quality clean nanowires from exfoliated topological insulator flakes. The ability to reliably induce and control superconducting correlations in TI nanowires is critical toward engineering Majorana-based qubits based on the TI platform. The researchers plan to focus next in observing and controlling Majorana zero-modes in these systems. To understand the superconductivity (SC) proximity effect in TINWs, the researchers used a combination of local and non-local conductance spectroscopies and attempted to detect CAR signatures. In the local conductance, they found that a hard gap can occur, signaling robust proximity-induced superconductivity, along with the appearance of Andreev bound states (ABSs) that arise in the unproximitized normal sections of the NW and can reach zero energy even at zero magnetic field. Their measurements of the non-local conductance showed that their SC-TINW hybrid device worked as a good Cooper pair splitter and that CAR can take place over length scales as long as 1.5 μm. This implies that a SC correlation that is necessary for the CAR process can be established throughout the TINW for a length scale much longer than the expected SC coherence length ξ_SC, which is estimated to be shorter than 110 nm. This long-range CAR effect goes beyond simple descriptions of non-local conductance in mesoscopic superconductors and points to a role of disorder in proximitized NWs, like overlapping ABSs created by disorder can lead to an Andreev band that extends beyond ξ_SC and support long-range CAR.

For more information: Phys.org, March 11 (2025); Nat. Phys., March 11 (2025).

Transverse Noether spin currents in noncollinear antiferromagnets observed:   researchers at Colorado State University in Fort Collins, CO and Johns Hopkins University in Baltimore, MD have observed Hall spin currents governed by the Hall mass in antiferromagnets (AFM). They study the conserved Noether current due to spin-rotation symmetry of the local spins in noncollinear AFMs. Noncollinear AFMs have spins oriented in different directions but still sum to zero net magnetization. They find that a Hall component of the spin current can be generically created by a longitudinal driving force associated with a propagating spin wave, inherently distinguishing noncollinear AFMs from collinear ones. The researchers name the corresponding Hall coefficient, an isotropic rank-four tensor, as the Hall (inverse) mass, which generally exists in noncollinear AFMs and their polycrystals. They have observed spin currents generated by the Hall mass of a polycrystalline noncollinear AFM. One can see circularly polarized spin waves propagating along x and the d.c. dynamical Hall spin current flowing along y. The Hall mass leads to the splitting between the transverse and longitudinal polarization magnon modes. The resulting Hall spin current can be realized by spin pumping in a ferromagnet-noncollinear AFM bilayer structure as they demonstrate numerically. Because of the noncollinear magnetic order, the localized electron spins on different magnetic sublattices are not conserved even when spin-orbit coupling is neglected, making it difficult to understand the transport of spin angular momentum. The reason the discovered effect appears only in noncollinear AFM is that they have three degrees of freedom describing spin orientations. This extra complexity leads to three branches of spin waves (collective vibrations of the spins), two of which naturally flow sideways in response to a driving force. Experimentally, researchers can measure this Hall mass either by injecting spin waves from a conventional ferromagnet into a noncollinear AFM and detecting spin accumulation along the edges, or by using scattering techniques (neutron or X-ray) to track the low-energy spin-wave spectrum.

For more information: Phys.org, March 13 (2025); Phys. Rev. Let., January 3 (2025) page 016706.

WEEK OF MARCH 10, 2025 [No. 1618]

Fluctuation-dissipation relation satisfied above 400 mK in spin ice:   an international group lead by researchers from the Grenoble Alpes University in Grenoble has identified temperature ranges where spin ices are in equilibrium or out of equilibrium. At ultralow temperatures, the competing interactions of spins in spin ices induce quasiparticles that act like magnetic monopoles. The fluctuation–dissipation relation links a system’s thermal fluctuations to its energy dissipation in response to external perturbations. A system in thermodynamic equilibrium satisfies this relation, while one out of equilibrium does not. Through high-precision measurements of magnetic noise and the out-of-phase part of the alternating-current susceptibility, the researchers tested the relation in two spin ices (Dy₂Ti₂O₇ / Dy₂TiO₅ and Ho₂Ti₂O₇) as they lowered the temperature from a few K to 150 mK. They found that the fluctuation–dissipation relation was satisfied above 400 mK. A detailed analysis revealed that the spin ices were in global thermodynamic equilibrium above 650 mK. From 400 to 650 mK, the equilibrium was only local, with regions of the materials trapped in certain magnetization states. Below 400 mK, the relation was violated, and the spin ices were out of equilibrium. In this regime, the researchers observed a previously unreported dissipation process, in addition to aging effects, in which the system’s properties depended on the time elapsed since the system was prepared. The relation is satisfied at temperatures well into the nonergodic region below 600 mK, indicating local equilibrium. In both materials, below 400 mK, low frequency violations develop, showing an excess of noise as in spin glasses, with a frequency threshold of 0.1 Hz. New relaxation pathways and aging properties are unveiled in this frequency range. The fluctuation–dissipation relation remains valid at higher frequencies down to 150 mK.

For more information: Physics, March 4 (2025); Phys.org, March 4 (2025).

Moire proximity creates pure electron crystal:   researchers at the ETH in Zurich have generated a moire pattern of ferroelectric domains on the nm scale using a monoatomic insulator with a large band gap (hBN) bilayer interface (2° twist) to externally generate a superlattice potential on a TMD atomic layer (semiconductor MoSe₂) placed below. Using resonant optical spectroscopy, they examine how the gate-controlled electron density influences the system and find evidence of long-range Coulomb interactions. They observe the formation of ordered states at certain filling factors of the moire potential, signaling strong electron correlations. At higher doping levels, the optical excitation spectrum changes due to on-site interactions between electrons. The moire pattern creates a strong electric field that penetrates the adjacent TMD layer, affecting the electrons but not the charge-neutral excitons. Thus, the periodic structure of ferroelectric domains in h-BN creates a purely electrostatic potential for charge carriers. They find direct evidence for induced moire potential in the emergence of new excitonic resonances at integer fillings and for enhancement of the trion binding energy by ~ 3  meV. A model for exciton-electron interactions is used to directly determine the moire potential modulation of 30 ±5  meV from the measured trion binding energy shift. The electric field acts on the electrons inside the MoSe₂ monolayer creating a crystal lattice. The researchers optically probe the electron states through their interactions with excitons. From the light frequency at which excitons are excited, the researchers draw conclusions about the behavior of the electrons. By applying an electric voltage, they varied the number of electrons in the semiconductor. From the exciton excitation frequency they proved that when one third or two thirds of the lattice sites were filled with electrons, they arranged themselves in a regular pattern. When the number of electrons was increased further, such that more than one electron occupied a lattice site, the interactions between the electrons led to a change in the states of the electrons. The researchers demonstrate a purely electrostatic moire potential for itinerant electrons and holes in a MoSe2 monolayer, which they probe through differential reflection spectroscopy. The simple nature of the moire potential allows them to extract its modulation depth directly from the optical signatures (the trion binding energy) using a theoretical model for exciton-electron interactions. In addition to measuring a redshift of the trion and the associated attractive polaron resonance, they identify the charge ordered Mott-Wigner states at filling factors 1/3 and 2/3 through the appearance of excitonic umklapp resonances.

For more information: Phys.org, March 7 (2025); Phys. Rev. X, March 5 (2025) page 011049.

WEEK OF MARCH 3, 2025 [No. 1617]

TTG superfluid stiffness measured:   researchers at Harvard University in Cambridge, MA have studied strong electron interactions in twisted trilayer graphene (TTG). They have developed electrical circuits to perform experiments to study the inductance of the superconducting graphene material. From there, they directly measured the inertia and density of the Cooper pairs that lead to superconductivity. The robustness of the macroscopic quantum nature of a superconductor can be characterized by the superfluid stiffness, ρ_s, a quantity that describes the energy required to vary the phase of the macroscopic quantum wavefunction. In unconventional superconductors, such as cuprates, the low-temperature behavior of ρ_s markedly differs from that of conventional superconductors owing to quasiparticle excitations from gapless points (nodes) in momentum space. TTG shows superconducting states and strongly correlated electronic states associated with spontaneously broken symmetries The researchers report the measurement of ρ_s in TTG, revealing unconventional nodal-gap superconductivity. The researchers initially wanted to study the structure of the electron pairs. When particles form a pair, their relative motion does not stop, but different kinds of motion can occur. While in the BCS theory, the relative motion of electrons in a pair is uniform (isotropic), there is evidence that the electrons in unconventional superconductors pair in a non-uniform (anisotropic) way. The experiments here demonstrate that pairing in TTG is strongly anisotropic, and it reinforces the connection between TTG and high T_c cuprates. This has been uncovered by studying how temperature and current break the electron pairs apart. For strongly anisotropic pairs, there are always directions along which the pair is easy to break. Thus, even a tiny temperature change can lead to anisotropic pairs breaking apart. The researchers experimented with other parameters to disrupt the Cooper pairs, such as altering the current, and the results further confirmed that the Cooper pairs in TTG are highly anisotropic. The researchers developed a theoretical model for the behavior of the TTG inductance as a function of temperature and current. By measuring the properties of superconducting pairs of electrons, they obtained information on the underlying electronic structure and underlying properties of single electrons in the highly anisotropic state. Although the conventional expectation is that the more electrons are placed in, the more superconducting pairs form, that is not what was observed here. The electrons in TTG behave unlike those described by BCS theory; but they share properties with high-temperature cuprates. Utilizing RF reflectometry techniques to measure the kinetic inductive response of superconducting TTG coupled to a microwave resonator, the researchers find a linear temperature dependence of ρ_s at low temperatures and nonlinear Meissner effects in the current-bias dependence, both indicating nodal structures in the superconducting order parameter. The doping dependence shows a linear correlation between the zero-temperature ρ_s and the superconducting transition temperature T_c, reminiscent of Uemura’s relation in cuprates, suggesting phase-coherence-limited superconductivity.

For more information: Phys.org, February 24 (2025); Nature, February 5 (2025) page 93.

Multizone trapped-ion qubit control in a QCCD:   researchers at ETH in Zurich have presented a quantitative description of the effects of integrated photonic elements on ion shuttling routines, map out these effects, and compensated for them during ion transport in a QCCD architecture. They use a surface-electrode trap with integrated photonic components which are scalable to larger numbers of zones. They demonstrate coherence between multiple zones through a distributed Ramsey experiment, which requires mapping optical to ground state qubits to avoid sensitivity to the optical phase of the driving beams. They implement a Ramsey sequence using the integrated light in two zones, separated by 375 μm, performing transport of the ion from one zone to the other in 200 μs between pulses. The use of integrated components in multiple trap zones introduces several complications, as exposed dielectric can affect trap performance through additional heating or charging of surfaces, which can play a detrimental role in transport routines. To achieve low motional excitation during transport, they develop techniques to measure and mitigate the effect of the exposed dielectric surfaces used to deliver the integrated light to the ion. They demonstrate simultaneous control of two ions in separate zones with low optical crosstalk and use this to perform simultaneous spectroscopy to correlate field noise between the two sites. They demonstrate transport and coherent multizone operations in an integrated photonic ion trap system. Individual ions are transported above surface electrodes, which are patterned on a dielectric material and have integrated photonics underneath. Light enters the system through optical fibers and is sent to two separate zones where it is launched as a pair of crossing beams to manipulate an ion’s state. They use surface electrodes and integrated photonics to go around problems with ion transport originated by trap-integrated components that distort an ion’s trapping potential. Their trap contains two zones, each with three optical waveguides leading to grating couplers that shoot laser light out of the trap and focus it on ions confined just above the trap’s surface. One of the three waveguides carries light for initializing and detecting the state of the ion qubits. The other two launch crossing beams that create a standing wave, driving an atomic transition that flips between the two qubit states. The grating couplers face the ions through windows in the electrodes, leaving the ions exposed to underlying dielectric material, through which laser light also propagates. Voltages applied to the electrodes control an axial electric potential that confines the ions along the trap’s length and allows for ion shuttling. Shuttling is achieved by changing these voltages over time to produce a trapping potential at multiple locations along the trap’s length. Qubit connectivity requires the ions to move during a quantum computation while maintaining controlled quantum superposition of qubit states. However, light-induced charging in the dielectric windows distorts the trapping potential and the ions shuttling. To determine shuttling distortions, the researchers first cooled an ion to near its lowest-energy motional state in the trap. They then shuttled the ion back and forth between zones before measuring its final motional state. Without any compensation, the distortions caused the ion to have 58 quanta of coherent excitation (back-and-forth wiggling of the ion at its natural frequency) and 25 quanta of incoherent excitation (random jiggling). Such effects would be enough to hamper high-fidelity quantum operations. The researchers tried to compensate for these effects. Changes in the frequency at which an ion oscillates in the trapping potential can cause ion heating. Therefore, for zone 1, they developed a protocol for stabilizing this trap frequency along the whole ion trajectory in the presence of the stray charges from the dielectric windows. They modeled these windows as fictitious electrodes, used spectroscopy to measure the changing trap frequency along the trap’s length, and then modeled window voltages that would induce such changes. Accounting for the modeled window voltages, the researchers generated an updated sequence of time-dependent electrode voltages for keeping the trap frequency constant during ion transport. After a few iterations of this protocol, the applied voltages achieved the required stabilization. Although this procedure worked well for compensating zone 1, another method was needed for zone 2, whose dielectric windows underwent more charging. For this zone, they moved the ion along the same direction as a laser beam and then measured the ion’s velocity by looking at the Doppler shift in the ion’s atomic resonance frequency. The researchers still modeled the windows as fictitious electrodes and used the modeled voltages to generate a revised series of applied voltages for ion shuttling. They then picked the modeled voltages that gave the ion the smoothest ride with the smallest changes in velocity. By combining the compensation methods for zones 1 and 2 as the ion was shuttled between the zones, they reduced the ion’s coherent excitation to only 8 quanta and its incoherent excitation to a negligible level. The objective here was to demonstrate coherent qubit operations between the trap’s two zones. Using the trap-integrated beams, the researchers placed an ion in a quantum superposition in zone 1, transported it to zone 2, manipulated the qubit state in zone 2, and then sent the ion back to zone 1 for detection. In this multizone protocol, they achieved a fidelity of more than 99% for single-qubit logic gates, showing that the effects of transporting the ion over the dielectric windows were sufficiently compensated. The researchers also demonstrated parallel, simultaneous qubit operations in the two zones.

For more information: Physics, February 24 (2025); Phys. Rev. X, February 24 (2025) page 011040.

WEEK OF FEBRUARY 24, 2025 [No. 1616]

CNT upconversion photoluminescence analyzed:   researchers at RIKEN in Waco have shined an IR laser on a single-wall carbon nanotube (SWCNT) air-suspended over a trench in a Si substrate and explained the UV emission response through exciton-phonon interaction (K-momentum–phonon coupling). Nearly linear excitation power dependence of upconversion photoluminescence (UCPL) intensity is observed, indicating a one-photon process as the underlying mechanism. The researchers have shown that the UCPL is a reverse process of the sideband emission from the K -momentum dark exciton observed in PL. The comparative analysis of PL and UCPL measurements has confirmed that the emission in the spectra and excitation images originates from the same nanotube. They have attributed the nearly linear excitation power dependence to the one-photon and one-phonon absorption process. The polarization degree of UCPL has been found to be as high as that of PL, evidence for the intrinsic nature of the upconversion process. Through UCPLE spectroscopy, they have detected three phonon-related peaks, which arise from the sidebands of the K-momentum dark singlet excitons. They applied a second-order exciton-phonon scattering model, precisely identifying phonon energies and relative matrix element amplitudes for the scattering events with phonons involved in the upconversion processes. The model predicts the temperature-dependent upconversion photoluminescence excitation (UCPLE) spectra. It is presently understood that UCPL could only happen in single-walled carbon nanotubes if excitons were temporarily trapped by defects in the nanotube's structure. But the researchers here have found that UCPL occurred with high efficiency even in defect-free nanotubes. They discovered that when an electron is excited by light, it gets a simultaneous energy boost from a phonon to form a dark exciton state. After losing some energy, the exciton still emits light with more energy than the incoming laser. Raising the temperature produced a stronger UCPL effect, as expected given that the phonon population increases at higher temperatures, enhancing the likelihood of phonon-mediated transitions. UCPL spectra across various chiralities of SWNTs are also examined, revealing that UCPL is a universal phenomenon across the chiralities. UCPLE spectra are compared with sidebands in PL spectra to elucidate the excitation process in UCPL. They develop a theoretical model for upconversion that involves photon-exciton and exciton-phonon interactions, quantitatively explaining the features in the UCPLE spectra. They conduct temperature-dependent UCPLE spectroscopy, verifying the model based on phonon-assisted upconversion in pristine SWCNTs. The air-suspended SWCNTs are grown over trenches on Si substrates by CVD. The researchers fabricated the trenches with a depth of ~ 1µm and a width from 0.5 to 4.0 µm by electron-beam lithography and dry etching. Another electron-beam lithography step is conducted to define catalyst areas near the trenches, onto which Co- or Fe-silica catalysts dispersed in ethanol are spin-coated and then lifted off. SWCNTs are synthesized over the trenches using alcohol CVD under a flow of ethanol with carrier gas Ar/ H2 at 800 °C for one minute. A home-built confocal microscopy system is used to collect PL spectra in dry N2 gas . Si substrates are mounted on a motorized 3D translation stage, allowing automated measurements over hundreds of samples. They utilize a CW Ti:Sa laser for excitation and a LN2-cooled InGaAs photodiode array attached to a 30-cm spectrometer for detection. Laser polarization is generally kept perpendicular to the trenches. The excitation beam is focused via an objective lens with NA = 0.65 and focal length = 1.8 mm. The focused spot exhibits 1/e² diameters of 1.46, 1.31, and 1.06 µm for energies of 1.36, 1.46, and 1.60 eV, respectively, where these diameters are determined by PL line scans perpendicular to a suspended tube. The excitation polarization is rotated by a half-wave plate mounted on a motorized rotation stage for polarization-dependent PL measurements. All PL spectra are taken at the center of the nanotubes except for hyperspectral PL imaging. UCPL spectra are also taken with the same microscope using a CW laser diode with an energy of 0.800 eV where the 1/e² diameter of the focused beam is 2.61 µm. The laser is coupled through a single-mode fiber, ensuring a high-quality beam. A short-pass dichroic filter with a transmission band between 0.855 and 1.24 eV is utilized to block the excitation light during UCPL collection. They ensure that excitation by laser sidebands does not occur by inserting a band-pass filter with center energy = 0.800 eV, bandwidth = 6.2 meV, and optical density > 5. They examine the possibility of laser emission beyond the laser peak by analyzing a UCPL spectrum on a SWCNT and a reflection spectrum on a Si substrate. The obtained spectra show the absence of any laser emission at energies higher than the short-pass dichroic filter cutoff of 0.855 eV.

For more information: Phys.org, February 20 (2025); Phys. Rev. B, October 10 (2024) page 155418.

Layer interactions on spin waves observed in kagome ferromagnets:   researchers at the Paul Scherrer Institut in Villigen PSI have used resonant inelastic X-ray scattering (RIXS) at the ADRESS beamline of SLS, to probe the magnetic excitations in the kagome ferromagnet Fe₃Sn₂ to verify that the spin waves on it form nearly flat bands as theoretically predicted although the context and origin of these bands was unexpected. The researchers discovered that the flat bands were created by strong interactions not just within the layers of the kagome material, but also between adjacent layers, thus, concluding that the spin waves are influenced by unexpected interactions between the layers in the material. The researchers utilized RIXS because the usual inelastic neutron scattering used to analyze the magnetic excitations in a material requires grams of sample, which in this case would mean precisely aligning hundreds of crystals. By using RIXS, they could study single crystals of the material. This technique is sensitive not only to spin excitations, associated with the magnetic behavior of the material, but also to electronic excitations. By using circularly polarized light, they were able to subtract out other types of excitations and focus exclusively on low-energy spin excitations that reveal the magnetic topology. Fe₃Sn₂ is a ferromagnet with a high Curie temperature (T_C ≈ 640 K) that consists of kagome bilayers stacked along the c-axis with the crystal structure. The kagome layers are composed of two different sets of equilateral triangles with different Fe–Fe distances and are stacked with an offset along the (1, −1) in-plane lattice direction. It is not clear whether there is any physical property of 3D Fe₃Sn₂ approaching that of an ideal kagome ferromagnet with short-range interactions, including Dirac crossings and perfectly flat bands among the spin waves, which, analogous to the electronic bands, can become topologically non-trivial with additional anisotropic or antisymmetric interactions. The researchers used circularly polarized X-rays for the unambiguous isolation of magnetic signals to discover a nearly flat spin-wave band and large (compared to elemental Fe) orbital moment in the metallic ferromagnet Fe₃Sn₂ with compact AB-stacked kagome bilayers. As a function of out-of-plane momentum, the nearly flat optical mode and the global rotation symmetry-restoring acoustic mode are out of phase, consistent with a bilayer exchange coupling that is larger than the already large in-plane couplings. The defining units of this topological metal are therefore triangular lattices of octahedral Fe clusters rather than weakly coupled kagome planes. The spin waves are strongly damped when compared to elemental Fe. The magnetic circular dichroism of the X-ray absorption reveals that the orbital contribution to the magnetic moment is five times larger than in elemental Fe where it is understood to be almost entirely quenched on account of the crystal field energies being larger than the spin–orbit interaction. Using the magnetic circular dichroism of RIXS, they discovered two spin-wave bands that are ascribed to the even and odd modes, derived from strong bilayer coupling, by measurements of the out-of-plane momentum dependence. This means that the underlying magnetic and concomitant electronic Hamiltonians for Fe₃Sn₂ are remote from the limit of weakly coupled single kagome layers, thus accounting for the difficulty of finding in both computation (DFT) and experiment (ARPES) the flat bands and resolved Dirac points associated with single kagome layers. The fundamental low-energy electronic building blocks are triangular lattices of octahedral Fe molecules, without the possibility for perfectly flat modes in the planar reciprocal space but with many new touching points between the greater number of modes introduced by those molecules. There is a strong mixing of the optical modes with the electron-hole pair continuum. The mixing may be due to attempted rearrangements of the Fe₃Sn₂ Weyl nodes caused by transient magnetization rotations associated with the spin waves.

For more information: Phys.org, February 18 (2025); Nat. Comm., October 16 (2024).

WEEK OF FEBRUARY 17, 2025 [No. 1615]

Nuclear-spin dark state observed:   researchers at University of Rochester in Rochester, NY have used dynamic nuclear polarization to align atomic nuclei spins in a gate-defined Si double quantum dot to provide direct evidence of the nuclear-spin dark state where nuclei synchronize reducing their coupling with electron's spins, thus decoupling nuclei and stabilizing electron spins. They show that the transverse electron–nuclear coupling rapidly diminishes in the dark state, and that this state depends on the synchronized precession of the nuclear spins. The dark state significantly reduces the relaxation rate between the singlet and triplet electronic spin states.

For more information: Phys.org, February 12 (2025); Nat. Phys., January 28 (2025).

Topological chiral superconductivity:   researchers at the MIT in Cambridge, MA have proposed conditions under which superconductivity can arise from the electrons’ own mutual electrostatic repulsion in a 2D lattice. They predicted different superconducting states, all chiral. Some of the states have quartet charge condensations. The electron's dispersion ~ k⁴ (rather than having the usual k² energy dependence). As in regular superconductivity their energies must be lower than that of their cooled parent state. That parent state is one of two exotic electronic states characterized by strong repulsive interactions (not clear yet which one). The resulting superconductors break both time-reversal and reflection symmetries in the orbital motion of electrons, and they exhibit nontrivial topological order. Their findings suggest that this topological chiral superconductivity is more likely to emerge near or between a fully spin-valley polarized metallic phase and a Wigner crystal phase. These topological chiral superconductors can be fully or partially spin-valley polarized. For partial spin-valley polarization, the ratios of electron densities associated with different spin-valley quantum numbers are quantized as simple rational numbers. Many of these topological chiral superconductors exhibit charge-4 or higher condensation, neutral quasiparticles with fractional statistics, and/or gapless chiral edge states. Two of the topological chiral superconductors are in the same phases as the “spin”-triplet or spinless p + ip BCS superconductor, while others are in different phases than from any BCS superconductors.

For more information: Physics, February 11 (2025); Phys. Rev. B, January 10 (2025) page 014508.

WEEK OF FEBRUARY 10, 2025 [No. 1614]

1D phase change observed:   researchers at the University of Maryland in College Park, MD and Duke University in Durham, NC have observed a finite-energy phase transition in a 1D ion array quantum simulator. They show that finite-energy states can be generated by time-evolving prepared initial states and letting them thermalize under the dynamics of a many-body Hamiltonian. By preparing initial states with different energies, they study the finite-energy phase diagram of a long-range interacting quantum system. They observe a ferromagnetic equilibrium phase transition as well as a crossover from a low-energy polarized paramagnet to a high-energy unpolarized paramagnet, in agreement with numerical simulations. The researchers use Au electrodes to create electromagnetic fields that trap dozens of ions in a chain geometry just above the surface of the electrodes. They use laser beams to induce interactions between the ions, thus, establishing a long-range interaction 1D magnet. The researchers simulated a system of 23 ⁷⁰Yb ions arranged into a 1D chain. The difficulty on effectively heat the system and observe a phase transition as a function of energy lies in coupling to a heat bath without disrupting the quantum state. To solve the problem of realizing interactions over sufficiently long distances and preparing equilibrium states, the researchers prepared the ions in a heated initial condition and then allowed them to evolve following their own dynamics via long range interactions. This evolution mimicked the effects that would follow an increase in temperature. Using this method, they observed the system transition from a ordered magnetized state to a disordered unmagnetized state, confirming the occurrence of the phase change.

For more information: Phys,org, February 6 (2025); Nat. Phys., January 17 (2025).

Ambient pressure HTS superconductivity in bilayer nickelate demonstrated:   researchers at SLAC in Menlo Park, CA have demonstrated that lateral compression from substrates can stabilize a superconducting state in thin film La₃Ni₂O₇ at room pressure. They present signatures of superconductivity in La₃Ni₂O₇ thin films at ambient pressure, facilitated by the application of epitaxial compressive strain. The onset T_c varies approximately from 26 K to 42 K, with higher T_c values correlating with smaller in-plane lattice constants. They observed the co-existence of other Ruddlesden-Popper phases within the films and dependence of transport behavior with ozone annealing, suggesting that the observed low zero resistance T_c ≈ 2 K can be attributed to stacking defects, grain boundaries, and O stoichiometry. The nickelate studied here has been shown to have Tc ≈ 80 K at high pressure. The researchers observed that the sample's T_c ≈ 26 K to 42 K depending on the level of compressive strain. While the material enters the superconducting phase at these temperatures, defects in the nickelate and the O atom ratio produce T_c ≈ 2 K. They have demonstrated that lateral compression from substrates can stabilize the material, even though it differs from the uniform compression achieved through squeezing it evenly from all directions, similar to that produced by a diamond anvil cell. The researchers plan to refine the material's crystalline quality and explore doping strategies.

For more information: Phys.org, February 5 (2025); Nature, December 19 (2024) page 935.

WEEK OF FEBRUARY 3, 2025 [No. 1613]

Chirality-induced directional spin selection observed:   researchers at the Johannes Gutenberg -Universität Mainz in Maiz have studied the chirality effect on spin in hybrid metal/chiral molecule thin-film heterostructures. Their observation of spin-dependent transmission of electrons through chiral molecules has confirmed the existence of the chiral-induced spin selectivity (CISS) effect with chiral-induced unidirectional spin-to-charge conversion. They inject a pure spin current via spin pumping and investigate the spin-to-charge conversion at the hybrid chiral interface, observing chiral-induced unidirectionality in the conversion. Angle-dependent measurements reveal that the spin selectivity is maximum when the spin angular momentum is aligned with the molecular chiral axis. They did not pass the charge current directly through the chiral molecules themselves. Instead, they created a hybrid system that consisted of a thin film of Au with chiral molecules on it. Although the major part of the current flows through the Au film, the presence of the chiral molecules alters the state of the Au component. They were interested in how the spin current was converted to a charge current. In a film consisting of pure Au, ~ 3% of the spin current is converted to charge, regardless of electron spin orientation. In the hybridized system of the Au layer with chiral molecules if the molecules on the surface of the Au are right-handed, currents with electron spin-up are converted much more efficiently to charge than those with spin-down. The outcome is the opposite if molecules on the Au surface are left-handed. This effect occurs only if the spin is in the same or opposite direction to the helix structure of a chiral molecule. If the direction of spin is not aligned with the direction in which the helix structure is arranged, the effect does not occur. The researchers demonstrate the impact of chiral molecules on the inverse spin Hall effect (ISHE), which originates from a collection of relativistic spin-orbit-coupling (SOC) phenomena. For this, they inject a pure spin current generated by ferromagnetic resonance in a ferromagnetic insulator into a hybrid metal/chiral molecule bilayer. The SOC of the hybrid layer converts the spin current into an electromotive force via the ISHE, measurable as a voltage signal across the metal layer. The results show a chirality and spin polarization–dependent unidirectional ISHE in the hybrid chiral system confirming that SOC plays a key role in this CISS effect.

For more information: Phys.org, January 30 (2025); Science Advances, January 1 (2025).

Weakly-driven spin squeezing entanglement in atomic arrays:   researchers at JILA in Boulder, CO have investigated the driven-dissipative dynamics of multilevel atomic arrays interacting via dipolar interactions at subwavelength spacings. They find that unlike two-level atoms in the weakly excited regime, multilevel atoms can become strongly entangled. The entanglement manifests as the growth of ground state spin waves persisting after turning off the drive. They propose the 2.9  μm transition between ³P₂↔︎³D₃ in ⁸⁸Sr with 389 nm trapping light as a platform to test their predictions. In dipole-dipole atom interactions in a lattice, the state of the system can become correlated. In the absence of an external drive, the generated entanglement typically disappears since all atoms relax to the ground state. If atoms have more than two levels participating in the process, then system interactions and complexity drastically increase. The researchers here have studied atom-light interactions in the case of effective four-level Sr atoms, two metastable ground and two excited levels arranged in specific 1D and 2D crystal lattices, with atoms closer to each other than the wavelength of the laser light used to excite them. The study concentrated on a set of internal levels with a much smaller energy separation than typical optical transitions. Instead of using truly ground-state levels, they proposed using long-lived metastable levels. By creating a long-lived metastable excited state, a 2.9-µm wavelength transition between this state ³P₂ and another excited-state ³D₃ state.(about eight times longer than the usual separation between nearby atoms trapped in an optical lattice) is accessed here. By having a transition wavelength much longer than the trapping light wavelength, they can realize strong and programmable interactions via the photon exchange that happens when the atoms are set close to each other. The atoms need to be very close, as interactions weaken with distance, eventually becoming lost due to other sources of decoherence. Keeping atoms close allows interactions to dominate, preserving the growth of entanglement. They work in the weak and far-from-resonance regime where atoms are allowed to virtually exchange photons, moving them between ground states without permanently occupying an excited state. In the metastable state dynamics, they observe growing correlations, which can be preserved when the laser is turned off. In this regime where the excited levels are only virtually populated, and only atoms can occupy the metastable state levels, the four-level problem can be reduced back to a two-level system although dealing with much more complex interactions including multi-atom interaction. Considering the far-from-resonance regime (in leading order, only two atoms interact at a given time), the Hamiltonian describing the metastable state dynamics maps back to a know spin model. Thus, they studied spin waves across the lattice arrangement. By controlling the polarization and propagation direction of the photons of the laser exciting the atoms, the researchers could determine which spin-wave pattern became dominantly entangled. The entanglement observed was spin-squeezing with increased sensitivity to external noise. The spin squeezing in the system can be experimentally measured and serves as a witness of quantum entanglement. This finding implies that quantum systems could maintain entanglement over long periods, without needing constant intervention to prevent decoherence. One key limitation here is dipole-dipole interactions, which involve long-range forces that couple atoms both near and far in the lattice. These couplings are anisotropic and depend on the relative orientation of the atomic dipoles, making the system more complex. Each atom interacts differently with its neighbors spaced along different directions in the lattice, leading to varying interaction strengths and signs across the array.

For more information: Phys.org, January 27 (2025); Phys. Rev. Lett., December 3 (2024) page 233003.

WEEK OF JANUARY 27, 2025 [No. 1612]

Rydberg state thermometry with mm blackbody emissions demonstrated:   researchers at NIST in Boulder, CO have performed primary quantum thermometry of mm-wave blackbody radiation (BBR) via induced state transfer in Rydberg states of cold atoms. Rydberg states of alkali-metal atoms are highly sensitive to electromagnetic radiation in the GHz-to-THz regime because their transitions have large electric dipole moments. The researchers track the BBR-induced transfer of a prepared Rydberg state to its neighbors and use the evolution of these state populations to characterize the BBR field at the relevant wavelengths, primarily at 130 GHz. They use selective field ionization readout of Rydberg states and substantiate their ionization signal with a theoretical model. Using this detection method, they measure the associated BBR-induced time dynamics of these states, reproduce the results with a simple semiclassical population transfer model, and demonstrate that this measurement is temperature sensitive with a statistical sensitivity to the fractional temperature uncertainty of 0.09 , corresponding to 26 K at room temperature. This represents a calibration-free SI-traceable temperature measurement, for which they calculate a systematic fractional temperature uncertainty of 0.006, corresponding to 2 K at room temperature when used as a primary temperature standard. They used 10^{6 85}Rb laser-excited Rydberg atoms (size ~ 100 nm) and laser-cooled to 0.5 mK in a MOT. The researchers make non-contact, calibration-free, 0-100 C, absolute temperature measurements by tracking atomic energy jumps induced by the emitted BBR over time. Every 300 ms, they load a new packet of ⁸⁵Rb atoms into the trap, cool them and excite them from the 5S energy level to the 32S Rydberg state. They then allow them to absorb black-body radiation from the surroundings for around 100 μs, causing some of the 32S atoms to change state. Then, they apply a strong, ramped electric field, ionizing the atoms. The higher energy states get ripped off easier than the lower energy states, so the electrons that were in each state arrive at the detector at a different time. In that way they get the readout indicating the population in each of the states. The researchers use this ratio to infer the spectrum of the BBR absorbed by the atoms and, then, the black body temperature. After pulsing a two-photon excitation to a Rydberg state, they wait 100 µs for BBR to couple from this Rydberg state to other states. Then, they sweep an electric field to selectively ionize Rydberg state atoms and collect the ions and stripped electrons using electron avalanche detectors. Each measurement takes 354 ms, consisting of 231 ms of experiment and 123 ms of dead time. The 3D MOT consists of three retroreflected laser beams ( 20 MHz detuned from the D2 line with 80 mW of power in a 1-cm beam radius) and two coils in an anti-Helmholtz configuration. The current through the coils is controlled with an insulated-gate bipolar transistor which allows the field to be switched off in 300 µs. They estimate the cloud to contain 2×10⁶ atoms, of which 5400 participate in the measurement. After the trap is released, the atoms are excited to a Rydberg state via laser 1 (resonant to the D2 line with 9 mW in a 5-mm beam radius) and laser 2 (resonant on the 5S3/2→32S1/2 transition with 57 mW in a 5 mm beam radius, locked to a two-photon electromagnetically induced transparency in a reference cell). After the blackbody coupling time, ionization is performed with two electrodes placed 56 mm apart that are swept from 0 to 3 kV in 7µs, and the ions and their electrons are collected using CEM detectors). The current incident on the anode of the CEM is converted into a voltage using a transimpedance amplifier with a gain of 10³ V/A and recorded on a scope.

For more information: Phys.org, January 23 (2025); Physicsworld, February 2 (2025); Phys. Rev. Res., January 23 (2025) page L012020.

Topological electronic crystals in TBLG:   an international group lead by researchers from the University of British Columbia in Vancouver, BC and the University of Washington in Seattle, WA has identified topological electronic crystal states in 2D twisted bilayer–trilayer graphene. They report signatures of a generalized version of the anomalous Hall effect driven by the moiré potential. The crystal forms at a band filling of one electron per four moiré unit cells (ν = 1/4) and quadruples the unit-cell area, coinciding with an integer quantum anomalous Hall effect. The Chern number of the state is tunable, and it can be switched reversibly between +1 and −1 by electric and magnetic fields. Several other topological electronic crystals arise in a low magnetic field, originating from ν = 1/3, 1/2, 2/3 and 3/2. The quantum geometry of the interaction-modified bands is expected to be very different from that of the original parent band.

For more information: Nature, January 22 (2025) page 1084; Phys.org, January 22 (2025).

WEEK OF JANUARY 20, 2025 [No. 1611]

High-fidelity long molecular entanglement demonstrated:   researchers at Durham in Durham have used rotationally magic-wavelength optical tweezers to create a controlled stable environment that supports long-lived (≈ 1 s) coherence between entangled ultracold polar molecules. They prepared two-molecule Bell states, using dipolar spin exchange and directed microwave excitation, with fidelities 0.924 (+0.013/-0.016) and 0.76 (+0.03/-0.04), respectively, limited by detectable leakage errors. When correcting for these errors, the fidelities were 0.976 (+0.014/-0.016) and 0.93 (+0.03/-0.05), respectively. This despite the Hz-scale interactions at their 2.8 μm particle spacing. They have shown that the second-scale entanglement lifetimes are limited solely by these errors. The speed and fidelity of their Bell-state preparation may be improved by changing the confinement of the molecules to access smaller separations. Transferring the molecules into a magic-wavelength optical lattice should give access to sub-µm separations and increased molecular confinement, resulting in increased interaction strengths with reduced noise.

For more information: Nature, January 15 (2025) page 827; Phys.org, January 15 (2025).

²⁵²Rf nucleus produced and decay measured:   researchers at the GSI in Darmstadt have discovered the shortest-lived superheavy nucleus, from the most neutron deficient ₁₀₄Rf isotope, at the boundary of the stability island in the sea of unstable superheavy nuclei. They report the discovery of ²⁵²Rf with ground state fission half-life 60(+90 / −30) ns, shorter than the previous minimum for spontaneously fissioning nuclei, thus, expanding the range of half-lives of the known superheavy nuclei by about 2 orders of magnitude. The researchers utilized an isomeric state with inverted fission stability for the measurement. The results here confirm a smooth onset of decreasing ground-state spontaneous fission half-lives in the neutron-deficient Rf isotopes toward the isotopic border of 10 fs (boundary determined as the time needed to form an atomic shell). The island of stability predicted in the 60's has been confirmed with the observation of increasing half-lives in the heaviest currently known nuclei as the predicted next magic number of 184 neutrons is approached. The short-lived ²⁵²Rf was synthesized in a gas recoil separator and guided to the detection system in its high-K isomeric state ^252mRf (for which they measured a half-life of 13(+4 / −3) μs) taking advantage of inverted fission-stability where excited states are more stable than the ground state. The researchers used an intense pulsed beam of ⁵⁰Ti available at the GSI/FAIR UNILAC accelerator to fuse Ti nuclei with ²⁰⁴Pb nuclei on a target foil. They used four different beam energies that resulted in excited ²⁵⁴Rf, that emits either one neutron to leave ²⁵³Rf or two neutrons to leave ²⁵²Rf. These isotopes were separated in the TransActinide Separator and Chemistry Apparatus TASCA. After a flight of 3.5 m (flight-time ≈ 0.6 µs), those were implanted into a Si detector that registered their implantation as well as their subsequent decay. The large beam energies used here favored the production of ²⁵²Rf over ²⁵³Rf. In total, 27 atoms of ²⁵²Rf decaying by fission with half-life 13 µs were detected. The electrons emitted after the implantation of the isomer ^252mRf in its decay to the ground state, were detected using a home-made fast digital data acquisition system. In all the three registered cases, a subsequent fission followed within 250 ns. From these data, a half-life of 60 ns was deduced for the ground state of ²⁵²Rf, making it the shortest-lived superheavy nucleus currently known. It was determined that almost all the 13-µs span belonged to the decay of the excited isomeric state in the ²⁵²Rf nucleus whose existence allowed the measurement of the 60-ns ground-state fission because the excited state survived the time-of-flight of the separator *so the ground state appeared and decayed in the detector rather than in the separator). In future experimental campaigns, the researchers plan the measurement of isomeric states with inverted fission stability in the next heavier element ₁₀₆Sg to further map the isotopic border of the stability island.

For more information: Phys.org, January 15 (2024); Physics, January 14 (2025); Phys. Rev. Lett., January 14 (2025) page 022501.

WEEK OF JANUARY 13, 2025 [No. 1610]

Fractional excitons observed:   researchers at Brown University in Providence, RI have observed excitons in the fractional quantum Hall regime. They used a bilayer graphene sandwiching an insulating hBN layer to control the movement of electrical charges and generate excitons under a high magnetic field. Some of the excitons arise from the pairing of fractionally charged particles and have non-bosonic properties that are different from fermions and anyons as well. The researchers present transport signatures of excitonic pairing in the fractional quantum Hall effect states. By probing the composition of these excitons and their impact on the underlying wavefunction, they discovered two new types of quantum phases of matter. One of those can be viewed as the fractional counterpart of the exciton condensate at a total filling of 1, whereas the other involves a more unusual type of exciton that obeys non-bosonic quantum statistics. The researchers will next study how these fractional excitons interact and whether their behavior can be controlled.

For more information: Nature, January 8 (2025) page 327; Phys.org, January 8 (2025).

Proximity ferroelectricity detected:   researchers in Penn State University at University Park, PA have detected proximity ferroelectricity in a non-ferroelectric polar material induced by one or more adjacent ferroelectric materials (wurtzite ferroelectric heterostructures). Proximity ferroelectricity enables polarization reversal in wurtzites without the chemical or structural disorder that accompanies elemental substitution. They had previously developed a ferroelectric material, Mg-substituted ZnO thin films. The ZnO has desirable properties, but it is not ferroelectric by itself. Adding Mg makes the material ferroelectric but degrades properties like heat dissipation during device operation and the ability to transmit light over very long distances. Using proximity ferroelectricity, the researchers found they could turn pure ZnO ferroelectric by stacking it with a ferroelectric material like the Mg-substituted ZnO thin films. The ZnO here can exhibit polarization reversal in its pure state. The ferroelectric layer can be just 3% of the total volume of the stack, meaning the vast majority is material with the most-desired properties. The ferroelectric, or switching layer, can be placed on the top or bottom or as an isolated internal layer. The researchers observed proximity ferroelectricity in oxide nitride and combined nitride-oxide systems, suggesting that there is a generic mechanism involved. The non-ferroelectric layers here are AlN and ZnO, whereas the ferroelectric layers are Al_1−xB_xN, Al_1−xSc_xN and Zn_1−xMg_xO. The layered structures include nitride–nitride, oxide–oxide and nitride–oxide stacks that feature two-layer (asymmetric) and three-layer (symmetric) configurations. Ferroelectric switching in both layers is validated by polarization hysteresis, anisotropic chemical etching, second harmonic generation, piezo response force microscopy, electromechanical testing and atomic resolution polarization orientation imaging in real space by STEM. The researchers present a physical switching model in which antipolar nuclei originate in the ferroelectric layer and propagate towards the internal non-ferroelectric interface. The domain wall leading edge produces elastic and electric fields that extend beyond the interface at close proximity, reducing the switching barrier in the non-ferroelectric layer, and allowing complete domain propagation without breakdown. DFT calculations of polymorph energies, reversal barriers and domain wall energies support this model.

For more information: Nature, January 8 (2025) page 574; Phys.org, January 8 (2025).

Time-domain oscillations between distant on-chip spins probed:   researchers at TU Delft in Delft have demonstrated coherent interaction between two semiconductor electron spin qubits 250 μm apart, using a superconducting resonator coupled to two gate-defined double dots. The separation is several orders of magnitude larger than for the commonly used direct interaction mechanisms in this platform. The researchers here demonstrate the time-domain control of a dot–resonator–dot system and realize two-qubit iSWAP oscillations between distant spin qubits. The two qubits are encoded in single-electron spin states and they are coupled via a 250-μm-long superconducting NbTiN on-chip resonator. The resonator is also used for dispersively probing the spin states. They demonstrate operations on individual spin qubits at the flopping-mode operating point and characterize the corresponding coherence times. They realize iSWAP oscillations between the two distant spin qubits in the dispersive regime. They study how the oscillation frequency varies with spin–cavity detuning, spin–photon coupling strength and frequency detuning between the two spin qubits, and compare the results with theoretical simulations. The researchers operate the system in a regime in which the resonator mediates a spin–spin coupling via virtual photons. Their observations are consistent with iSWAP oscillations between the distant spin qubits, and suggest that entangling operations are possible in 10 ns. The fabricated device was characterized by recording the microwave transmission from the input port via the resonator to the output port. The researchers initialized one spin in the ground state and the other in the excited state. When they activated the interaction between these spins, the two qubits transferred their quantum states back and forth. When one spin transitions to the ground state, the other simultaneously transitions to the excited state, and vice versa. After previous spectroscopic measurements relying on coherent spin-photon interactions, the researchers observed time-domain oscillations here. The researchers plan to increase the quality factor of the oscillations and study time-domain oscillations between each of the spins and real photons in the resonator in the form of vacuum Rabi oscillations

For more information: Physics, January 7 (2025); Nat. Phys., December 9 (2024).

WEEK OF JANUARY 6, 2025 [No. 1609]

No electronic correlation at twist angle 4.4° in dichalcogenide bilayers:   an international group lead by researchers at the University of Groningen in Groningen has used nano-ARPES to investigate structural relaxation in small angle twisted bilayer WS₂ and found electronic behavior inconsistent with theory predictions of collective behavior. They present here a systematic nano-ARPES investigation of bulk, single-layer, and twisted bilayer samples with a small twist angle (4.4°). The experimental results are compared with theoretical calculations based on DFT along the high-symmetry directions. The electronic band structure measurements suggest a structural relaxation occurring at twist angle 4.4° and the formation of large, untwisted bilayer regions replacing most of the twisted area with the twisted bilayer partially reverting to a lower-energy, untwisted configuration.

For more information: Phys.org, December 30 (2024); Phys. Rev. Mat., December 26 (2024) page 124004.

Spin-orbit coupled superconducting electrons:   researchers at the University of Minnesota have proposed that in certain materials, pair spin–orbit interaction (PSOI) is strong enough to engender unconventional superconductivity. They analyzed the PSOI arising from Coulomb interaction in a class of materials that exhibit spin–orbit coupling associated with a strong Rashba effect. This effect has been studied for decades, owing to the possibility of creating spin-polarized currents of electrons without the need to apply a magnetic field. The Rashba effect can arise in a crystal lacking inversion symmetry, where spin-up and spin-down electrons split into different conduction bands. PSOI can induce p-wave superconducting order without the need for attraction mediator. Depending on the sign and strength of the PSOI coupling, two distinct superconducting phases emerge in 3D systems, analogous to the A and B phases observed in superfluid ³He. In contrast, 2D systems exhibit order parameter like p^x ± iq^y, leading to time-reversal-invariant topological superconductivity. A sufficiently strong PSOI can induce ferromagnetic ordering. It is associated with a deformation of the Fermi surface, which eventually leads to a Lifshitz transition from a spherical to a toroidal Fermi surface, with a number of experimentally observable signatures. In pure Rashba materials, ferromagnetism and p-wave superconductivity may coexist. This state resembles the A₁ phase of ³He, yet it may avoid nodal points due to the toroidal shape of the Fermi surface. Their calculations show that the PSOI is strong in the considered Rashba materials and can induce electrons to pair up and produce a superconducting state. Although the pairing symmetry differs in 2D and 3D, in both cases it has odd parity, meaning that the system would be an unconventional superconductor. Such a state would be disrupted by modest concentrations of impurities and could be detectable in ultrapure samples at 100's mK.

For more information: Physics, January 2 (2025); Phys. Rev. B, January 2 (2025) page 035104.

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.