Motivated by recent experiments demonstrating intricate quantum Hall physics on the surface of elemental bismuth, we consider proximity coupling an $s$-wave superconductor to a two-dimensional electron gas with strong Rashba spin-orbit interactions in the presence of a strong perpendicular magnetic field. We focus on the high-field limit so that the superconductivity can be treated as a perturbation to the low-lying Landau levels. In the clean case, wherein the superconducting order parameter takes the form of an Abrikosov vortex lattice, we show that a lattice of hybridized Majorana modes emerges near the plateau transition of the lowest Landau level. However, unless magnetic-symmetry-violating perturbations are present, the system always has an even number of chiral Majorana edge modes and thus is strictly speaking Abelian in nature, in agreement with previous work on related setups. Interestingly, however, a weak topological superconducting phase can very naturally be stabilized near the plateau transition for the square vortex lattice. The relevance of our findings to potential near-term experiments on proximitized materials such as bismuth will be discussed.

In our theoretical study in which we combine a nonequilibrium Green's function approach with density functional theory we investigate branched compounds containing Ru or Os metal complexes in two branches, which due to their identical or different metal centers are symmetric or asymmetric. In these compounds the metal atoms are connected to pyridyl anchor groups via acetylenic and phenyl spacer groups in a metaconnection. We find there is no destructive quantum interference (DQI) feature in the transmission function near the Fermi level for the investigated molecules regardless of their symmetry, either in their neutral states or in their charged states. We map the structural characteristics of the range of molecules onto a simplified tight-binding model in order to identify the main differences between the molecules in this study and previously investigated ferrocene compounds in order to clarify the structural sources for DQI, which we found for the latter but not for the former. We also find that local charging on one of the branches changes only the conductance by about one order of magnitude, which we explain in terms of the spatial distributions and charge-induced energy shifts of the relevant molecular orbitals for the branched compounds.

We study the entanglement spectrum of topological systems hosting non-Abelian anyons. Akin to energy levels of a Hamiltonian, the entanglement spectrum is composed of symmetry multiplets. We find that the ratio between different eigenvalues within one multiplet is universal and is determined by the anyonic quantum dimensions. This result is a consequence of the conservation of the total topological charge. For systems with non-Abelian topological order, this generalizes known degeneracies of the entanglement spectrum, which are hallmarks of topological states. Experimental detection of these entanglement spectrum signatures may become possible in Majorana wires using multicopy schemes, allowing the measurement of quantum entanglement and its symmetry resolution.

We study spin transport in a Hubbard chain with a strong, random, on-site potential and with spin-dependent hopping integrals $t_{\sigma }$. For the SU(2) symmetric case $t_{\uparrow }=t_{\downarrow }$, such a model exhibits only partial many-body localization with localized charge and (delocalized) subdiffusive spin excitations. Here, we demonstrate that breaking the SU(2) symmetry by even weak spin asymmetry, $t_{\uparrow }\ne t_{\downarrow }$, localizes spins and restores full many-body localization. To this end we derive an effective spin model, where the spin subdiffusion is shown to be destroyed by arbitrarily weak $t_{\uparrow }\ne t_{\downarrow }$. Instability of the spin subdiffusion originates from an interplay between random effective fields and singularly distributed random exchange interactions.

We have investigated the microscopic charge/lattice/spin states in the neutral-ionic transition material $\mathrm{DMTTF}-{\mathrm{QCl}}_4$ under pressure by ${}^{35}\mathrm{Cl}-\mathrm{NQR}$ and ${}^1\mathrm{H}-\mathrm{NMR}$ measurements to reveal the mechanism of the phase transition and the nature of the pressure-temperature phase diagram. At ambient pressure, the ${}^{35}\mathrm{Cl}-\mathrm{NQR}$ frequency indicates low ionicity and shows neither discontinuous jump nor precursory decrease around the transition, suggesting that the spin degrees of freedom are not involved in a dimerization transition. These experimental features are consistent with the theoretically proposed picture that the electronic energy gain is acquired by modulations in transfer integrals (so-called Peierls mechanism) with the lattice dimerization. As pressure is applied, the ionicity is increased and the system is driven into an ionic state with the degree of charge transfer exceeding 0.5 above $\sim 9\mathrm{kbar}$ at room temperature, where the ${}^1\mathrm{H}-\mathrm{NMR}$ captures paramagnetic spin fluctuations indicative of the emergence of a nondimeric ionic phase. Furthermore, ${}^{35}\mathrm{Cl}-\mathrm{NQR}$ and x-ray-diffraction measurements found a distinct antiferroelectric state at high pressures above 11 kbar. Based on these results, we propose a pressure-temperature phase diagram.

Observation of large nonsaturating magnetoresistance in rare-earth monopnictides has raised enormous interest in understanding the role of its electronic structure. Here, by a combination of molecular-beam epitaxy, low-temperature transport, angle-resolved photoemission spectroscopy, and hybrid density functional theory we have unveiled the band structure of LuSb, where electron-hole compensation is identified as a mechanism responsible for large magnetoresistance in this topologically trivial compound. In contrast to bulk single crystal analogues, quasi-two-dimensional behavior is observed in our thin films for both electron and holelike carriers, indicative of dimensional confinement of the electronic states. Introduction of defects through growth parameter tuning results in the appearance of quantum interference effects at low temperatures, which has allowed us to identify the dominant inelastic scattering processes and elucidate the role of spin-orbit coupling. Our findings open up possibilities of band structure engineering and control of transport properties in rare-earth monopnictides via epitaxial synthesis.

Quantum Hall inter-plateau transitions are physical exemplars of quantum phase transitions. Near each of these transitions, the measured electrical conductivity scales with the same correlation length and dynamical critical exponents, i.e., the critical points are superuniversal. In apparent contradiction to these experiments, prior theoretical studies of quantum Hall phase transitions within the framework of Abelian Chern-Simons theory coupled to matter found correlation length exponents that depend on the value of the quantum critical Hall conductivity. Here, we use non-Abelian bosonization and modular transformations to theoretically study the phenomenon of superuniversality. Specifically, we introduce a new effective theory that has an emergent $U\left(N\right)$ gauge symmetry with any $N>1$ for a quantum phase transition between an integer quantum Hall state and an insulator. We then use modular transformations to generate from this theory effective descriptions for transitions between a large class of fractional quantum Hall states whose quasiparticle excitations have Abelian statistics. We find the correlation length and dynamical critical exponents are independent of the particular transition within a controlled 't Hooft large $N$ expansion, i.e., superuniversal! We argue that this superuniversality could survive away from this controlled large $N$ limit using recent duality conjectures.

Recently, we have calibrated the diagonal components $S_{11}=S_{xxxx}$ of the GaAs gradient elastic tensor $S_{ijkl}$ using nuclear magnetic resonance (NMR) and photoluminescence spectroscopy of GaAs/AlGaAs quantum dot structures in Faraday geometry [E. A. Chekhovich et al., Phys. Rev. B 97, 235311 (2018)]. Here we measure quantum dot NMR spectra in oblique magnetic fields giving access to the off-diagonal components $S_{yzyz}$. We find the ratios $S_{yzyz}/S_{xxxx}\approx +1.{98}_{-0.27}^{+0.21}$ for ${}^{75}\mathrm{As}$ and $\approx -0.{40}_{-0.31}^{+0.23}$ for ${}^{69}\mathrm{Ga}$. Combined with our previous results, we find all independent nonzero components of $S_{ijkl}$: $Q{S}_{xxxx}\approx +0.758\times {10}^{-6}$ V, $Q{S}_{yzyz}\approx +1.51\times {10}^{-6}$ V for ${}^{75}\mathrm{As}$ and $Q{S}_{xxxx}\approx -0.377\times {10}^{-6}$ V, $Q{S}_{yzyz}\approx +0.151\times {10}^{-6}$ V for ${}^{69}\mathrm{Ga}$, where $Q$ is the corresponding nuclear quadrupolar moment. Our results show that earlier nuclear acoustic resonance experiments [R. K. Sundfors, Phys. Rev. B 10, 4244 (1974)] most likely overestimated the GaAs gradient elastic tensors, especially for ${}^{69}\mathrm{Ga}$. We further use NMR spectroscopy in oblique fields for assumption-free measurement of the intrinsic strain in GaAs/AlGaAs quantum dots. We find deviations of the strain principle directions from the sample growth axis, which vary between individual quantum dots.

Reversible martensitic transformations (MTs) are the origin of many fascinating phenomena, including the famous shape memory effect. In this work, we present a fully ab initio procedure to characterize MTs in alloys and to assess their reversibility. Specifically, we employ ab initio molecular dynamics data to parametrize a Landau expansion for the free energy of the MT. This analytical expansion makes it possible to determine the stability of the high- and low-temperature phases, to obtain the Ehrenfest order of the MT, and to quantify its free energy barrier and latent heat. We apply our model to the high-temperature shape memory alloy Ti-Ta, for which we observe remarkably small values for the metastability region (the interval of temperatures in which the high- and low-temperature phases are metastable) and for the barrier: these small values are necessary conditions for the reversibility of MTs and distinguish shape memory alloys from other materials.

The similarities to cuprates make iridates an interesting potential platform for investigating superconductivity. Equally attractive are their puzzling complex intrinsic interactions. Here, we report an ultrafast optical spectroscopy investigation of a coherent phonon mode in ${\mathrm{Sr}}_3{\mathrm{Ir}}_2{\mathrm{O}}_7$, a bilayer Ruddlesden-Popper perovskite iridate. An anomaly in the $A_{1g}$ optical phonon $(\nu =4.4\mathrm{THz})$ is unambiguously observed below the Néel temperature $\left(T_{\mathrm{N}}\right)$, which we attribute to pseudospin-lattice coupling (PLC). Significantly, we find that PLC is the dominant interaction at low temperature, and we directly measure the PLC coefficient to be $\lambda =150\pm 20\mathrm{c}{\mathrm{m}}^{-1}$, which is two orders of magnitude higher than that in manganites ($<2.4\mathrm{c}{\mathrm{m}}^{-1}$) and comparable to that in CuO ($50\mathrm{c}{\mathrm{m}}^{-1}$, the strongest PLC or spin-lattice coupling (SLC) previously known). Moreover, we find that the strong PLC induces an anisotropic negative thermal expansion. Our findings highlight the key role of PLC in iridates and uncovers another intriguing similarity to cuprates.

Artificial spin ices (ASIs) are nanoscale geometrically engineered arrays of magnetic bars or islands that display magnetic frustration and provide a fertile system for modeling and testing many complex statistical mechanics theories and emergent phenomena. Aperiodic ASIs display spatially varying magnetic frustration as a result of the aperiodicity. We have explored the magnetization reversal of quasicrystalline ASIs (QC-ASIs) patterned on P2 and P3 Penrose tilings, using a statistical graph-theory approach that enables quantitative comparison between the two tilings. Our approach offers insight into the QC-ASIs. We show that the P2 tiling leads to enhanced topological frustration compared to the P3 tiling, and we elucidate the emergence of local antiferromagnetic ordering as well as disordered states in these ferromagnetic aperiodic lattices. Our method enables quantitative exploration of frustration behavior on any periodic or aperiodic lattice, thereby opening pathways to study collective behavior in various topologically frustrated systems.

A temperature gradient applied to a ferromagnetic metal induces not only independent flows of electrons and magnons but also drag currents because of their mutual interaction. In this paper, we present a microscopic study of the electron flow induced by the drag due to magnons. The analysis is based on the s-d model, which describes conduction electrons and magnons coupled via the s-d exchange interaction. Magnetic impurities are introduced in the electron subsystem as a source of spin relaxation. The obtained magnon-drag electron current is proportional to the entropy of magnons and to $\alpha -\beta$ (more precisely, to $1-\beta /\alpha$), where $\alpha$ is the Gilbert damping constant and $\beta$ is the dissipative spin-transfer torque parameter. This result almost coincides with the previous phenomenological result based on the magnonic spin-motive forces, and consists of spin-transfer and momentum-transfer contributions, but with a slight disagreement in the former. The result is interpreted in terms of the nonequilibrium spin chemical potential generated by nonequilibrium magnons.

Thermal generation of magnonic spin current by the spin Seebeck effect (SSE) and spin-polarized current by the anomalous Nernst effect (ANE) are fascinating phenomena in the field of spin caloritronics. In this paper, we investigate the thermal spin current driven by light in the framework of the SSE and the ANE. Unlike heating by electrical means, the spin current can be enhanced or reduced, and its sign can even be reversed, thanks to the intrinsic characteristic of the penetrating light. Most importantly, by reversing the direction of the incident light, we are able to separate the contributions from the interfacial and bulk temperature gradients to the transverse spin accumulation. Notably, although the overall spin signals are strongly influenced by the frequency of light, the interfacial and bulk spin Seebeck coefficients are intrinsic and frequency independent. Furthermore, with light excitation, we observe a nearly instantaneous steplike response for both the SSE and the ANE, highlighting the importance of spin-charge conversion by spin-orbit coupling for ultrafast spin caloritronics.

We investigate theoretically the phase transition in a three-dimensional cubic Ising model utilizing state-of-the-art machine learning algorithms. Supervised machine learning models show high accuracies (99%) in phase classification and very small relative errors ($<{10}^{-4}$) of the energies in different spin configurations. Unsupervised machine learning models are introduced to study the spin configuration reconstructions and reductions, and the phases of reconstructed spin configurations can be accurately classified by a linear logistic algorithm. Based on the comparison between various machine learning models, we develop a few-shot strategy to predict phase transitions in larger lattices from a trained sample in smaller lattices. The few-shot machine learning strategy for a three-dimensional (3D) Ising model enables us to study the 3D Ising model efficiently and provides an integrated and highly accurate approach to other spin models.

Instabilities relating to cooperative octahedral tilting is common in materials with perovskite structures, in particular in the subclass of halide perovskites. In this work the energetics of octahedral tilting in the inorganic metal halide perovskites ${\mathrm{CsPbI}}_3$ and ${\mathrm{CsSnI}}_3$ are investigated using first-principles density functional theory calculations. Several low-energy paths between symmetry equivalent variants of the stable orthorhombic $\left(Pnma\right)$ perovskite variant are identified and investigated. The results are in favor of the presence of dynamic disorder in the octahedral tilting phase transitions of inorganic halide perovskites. In particular, one specific type of path, corresponding to an out-of-phase “tilt switch,” is found to have significantly lower energy barrier than the others, which indicates the existence of a temperature range where the dynamic fluctuations of the octahedra follow only this type of path. This could produce a time averaged structure corresponding to the intermediate tetragonal $(P4/mbm)$ structure observed in experiments. Deficiencies of the commonly employed simple one-dimensional “double-well” potentials in describing the dynamics of the octahedra are pointed out and discussed.

Anisotropy of multiphoton carrier generation in monocrystalline IIa diamond and its dependence on the polarization state of excitation light (near-infrared few-cycle laser pulses) are investigated using photoluminescence and nonlinear absorption measurements. We observe anisotropy between multiphoton transition rates for light linearly polarized along the 〈100〉 and 〈110〉 crystallographic directions of diamond and a strong intensity-dependent decrease of the transition rate for circularly polarized light. Measured results are compared with numerical simulations using time-dependent density functional theory and with an analytical model assuming a parabolic two-band system and the Houston function as the time-dependent wave function of the valence and conduction bands.

We study the finite-temperature superfluid transition in a modified two-dimensional (2D) XY model with power-law-distributed “scratch”-like bond disorder. As its exponent decreases, the disorder grows stronger and the mechanism driving the superfluid transition changes from conventional vortex-pair unbinding to a strong randomness criticality (termed scratched XY criticality) characterized by a nonuniversal jump of the superfluid stiffness. The existence of the scratched XY criticality at finite temperature and its description by an asymptotically exact semi-renormalization group theory, previously developed for the superfluid-insulator transition in one-dimensional disordered quantum systems, is numerically proven by designing a model with minimal finite-size effects. Possible experimental implementations are discussed.

In this article, we show that the two-dimensional electron gas formed at the ${\mathrm{LaAlO}}_3/{\mathrm{SrTiO}}_3$ interface behaves as a Josephson junction array. In particular, it exhibits a stochastic switching of the superconducting critical current, which qualitatively follows the dynamics of the resistively and capacitively shunted Josephson junction model. The switching current distribution (SCD) has been measured as a function of temperature and back-gate voltage. At low temperatures a clear saturation of the standard deviation of the SCD is observed, possibly indicating the presence of a macroscopic quantum tunneling regime with phase diffusion. Through the gate voltage we modify the damping of the array and compare it to artificial arrays of junctions.

Understanding the appearance of local electrical connectivity in liquid filled layered graphene oxide subjected to an external electric field is important to design electrically controlled smart permeable devices and also to gain insight into the physics behind electrical effects on confined water permeation. Motivated by recent experiments [K. G. Zhou et al. Nature (London) 559, 236 (2018)], we introduce a new model with random percolating paths for electrical connectivity in micron thick water filled layered graphene oxide, which mimics parallel resistors connected across the top and bottom electrodes. We find that a strong nonuniform radial electric field of the order ∼10–50 mV/nm can be induced between layers depending on the current flow through the formed conducting paths. The maxima of the induced fields are not necessarily close to the electrodes and may be localized in the middle region of the layered material. The emergence of electrical connectivity and the associated electrical effects have a strong influence on the surrounding fluid in terms of ionization and wetting which subsequently determines the permeation properties.