We investigate pressure-induced structural changes to the Peierls-type distorted low-temperature phase of the low-dimensional ${\mathrm{Sc}}_3\mathrm{Co}{\mathrm{C}}_4$ as a possible origin of its pressure-enhanced superconductivity. By means of cryogenic high-pressure x-ray diffraction experiments we could reveal subtle, but significant structural differences between the low-temperature phase at ambient and elevated pressures. We could thus establish the structure of the superconducting phase of the title compound, which interestingly still shows the main features of the Peierls-type distorted low-temperature phase. This indicates that in contrast to other low-dimensional materials a suppression of periodic structural distortions is no prerequisite for superconductivity in the transition-metal carbide.

Topological states in quantum materials are defined by bulk wave functions that possess nontrivial topological invariants. While edge modes are widely presented as signatures of nontrivial topology, how bulk wave functions can manifest explicitly topological properties remains unresolved. Here, using high-resolution inelastic x-ray spectroscopy (IXS) combined with first principles calculations, we report experimental signatures of chiral wave functions in the bulk phonon spectrum of BaPtGe, which we show to host a previously undiscovered twofold-degenerate quadruple Weyl node. The chirality of the degenerate phononic wave function yields a nontrivial phonon dynamical structure factor, $S(\mathbf{Q},\omega )$, along high-symmetry directions, that is in excellent agreement with numerical and model calculations. Our results establish IXS as a powerful tool to uncover topological wave functions, providing a key missing ingredient in the study of topological quantum matter.

Anharmonic phonon-phonon scattering serves a critical role in heat conduction in solids. Previous studies have identified many selection rules for possible phonon-phonon scattering channels imposed by phonon energy and momentum conservation conditions and crystal symmetry. However, the crystal-symmetry-based selection rules have mostly been ad hoc so far in selected materials, and a general formalism that can summarize known selection rules and lead to new ones in any given crystal is still lacking. In this work, we apply a general formalism for symmetry-based scattering selection rules based on the group theory to anharmonic phonon-phonon scatterings, which can reproduce known selection rules and guide the discovery of new selection rules between phonon branches imposed by the crystal symmetry. We apply this formalism to analyze the phonon-phonon scattering selection rules imposed by the in-plane symmetry of graphene, and demonstrate the significant impact of symmetry-breaking strain on the lattice thermal conductivity. Our work quantifies the critical influence of the crystal symmetry on the lattice thermal conductivity in solids and suggests routes to engineer heat conduction by tuning the crystal symmetry.

We present here the details of a method [A. B. Culver and N. Andrei, Many-body wavefunctions for quantum impurities out of equilibrium, Phys. Rev. B 103, L201103 (2021)] for calculating the time-dependent many-body wavefunction that follows a local quench. We apply the method to the voltage-driven nonequilibrium Kondo model to find the exact time-evolving wavefunction following a quench where the dot is suddenly attached to the leads at $t=0$. The method, which does not use Bethe ansatz, also works in other quantum impurity models and may be of wider applicability. We show that the long-time limit (with the system size taken to infinity first) of the time-evolving wavefunction of the Kondo model is a current-carrying nonequilibrium steady state that satisfies the Lippmann-Schwinger equation. We show that the electric current in the time-evolving wavefunction is given by a series expression that can be expanded either in weak coupling or in strong coupling, converging to all orders in the steady-state limit in either case. The series agrees to leading order with known results in the well-studied regime of weak antiferromagnetic coupling and also reveals a universal regime of strong ferromagnetic coupling with Kondo temperature $T_K^{\left(F\right)}=D{e}^{-\frac{3{\pi }^2}{8}\rho \left|J\right|}$ ($J<0, \rho \left|J\right|\to \infty$). In this regime, the differential conductance $dI/dV$ reaches the unitarity limit $2e^2/h$ asymptotically at large voltage or temperature.

We extend the general formalism discussed in the previous paper (A. B. Culver and N. Andrei, [Phys. Rev. B 103, 195106 (2021)]) to two models with charge fluctuations: the interacting resonant level model and the Anderson impurity model. In the interacting resonant level model, we find the exact time-evolving wavefunction and calculate the steady-state impurity occupancy to leading order in the interaction. In the Anderson impurity model, we find the nonequilibrium steady state for small or large Coulomb repulsion $U$, and we find that the steady-state current to leading order in $U$ agrees with a Keldysh perturbation theory calculation.

Using density functional theory within the generalized gradient approximation, we predict that monolayer vanadium oxyhydroxide (VOOH) and chromium oxyhydroxide (CrOOH) are two-dimensional (2D) ferromagnetic semiconductors. These two kinds of oxyhydroxides possess excellent thermal and dynamical stability as the free-standing 2D monolayer films. Our results show that the monolayer VOOH and CrOOH remain ferromagnetic ordering with the critical temperature up to $\sim 100\mathrm{K}$, and the ferromagnetic ordering is due to a superexchange interaction in the near-${90}^{\circ }$ V(Cr)-O-V(Cr) bonds in monolayer VOOH(CrOOH). However, the different occupancy of $d$ electrons between the ${\mathrm{V}}^{3+}$ and ${\mathrm{Cr}}^{3+}$ ions gives rise to the different axes or planes of easy magnetization and the different magnetic anisotropy energies between the monolayer VOOH and CrOOH. Our study demonstrates that monolayer transition metal oxyhydroxides could be one category of testing ground for the investigation of 2D ferromagnets.

Optical rogue waves are demonstrated in the far-field scattered radiation from photonic arrays designed according to the aperiodic distributions of prime elements in complex quadratic fields. Specifically, by studying light diffraction from Eisenstein and Gaussian prime arrays, we establish a connection between the formation of optical rogue waves and multifractality in the visible single-scattering regime. We link strong multifractality with the heavy-tail probability distributions that describe the fluctuations of scattered radiation from the fabricated arrays. Our findings pave the way to control high-intensity rogue waves using deterministic arrays of dielectric nanostructures for enhanced sensing and lithographic applications.

Constrained electronic-structure theories enable the construction of effective low-energy models consisting of partially dressed particles. However, the interpretation and physical content of these theories is not straightforward. Here, we carefully explore the properties of downfolding theories for electron-ion problems, in particular constrained density-functional perturbation theory (cDFPT). We show that the dipole selection rules determine whether the partially dressed phonons satisfy Goldstone's theorem, and we prove that electronic screening always lowers the phonon frequencies. We illustrate the theory with cDFPT calculations for minimal example systems: the nitrogen and benzene molecule as well as graphene.

The topological properties and intrinsic anomalous Hall effect of CsCl-type ferromagnets GdZn and GdCd have been studied based on first-principles electronic structure calculations. According to the calculated band structures, both GdZn and GdCd host nodal lines near the Fermi level. Once the magnetization breaks the mirror symmetries, the nodal lines are gapped. This can create a huge Berry curvature. A large anomalous Hall effect is then generated when the Fermi level resides within the opened gaps of the nodal lines. Our works indicate that GdZn and GdCd can provide a promising platform for exploring the interplay between topological property and magnetism.

The compound-tunable embedding potential (CTEP) method developed for simulating the influence of environment on a fragment in the ionic-covalent crystal is presented in the form of a linear combination of particular short-range semilocal pseudopotentials for the atoms of nearest environment and the long-range Coulomb potentials from optimized fractional point charges centered on both nearest and more distant atoms of the environment. A pilot application of the CTEP method to calcium niobate crystal, ${\mathrm{CaNb}}_2{\mathrm{O}}_6$, is performed. A very good agreement of the electronic density and interatomic distances within the relaxed fragment with those of the original periodic crystal calculation is attained. Calcium niobate crystal can be considered as an idealized fersmite $({\mathrm{CaNb}}_{2-x}{\mathrm{Ta}}_x{\mathrm{O}}_6,x\approx 0.3)$ mineral when neglecting the contributions of the impurities with smaller molar fractions, and substitution $\mathrm{Nb}\to \mathrm{Ta}$ is considered here as a point Ta defect in ${\mathrm{CaNb}}_2{\mathrm{O}}_6$. Besides, uranium-containing point defects are also studied since the euxenite group minerals, to which fersmite belongs, are considered as prospective matrices for long-term immobilization of high-level waste. The chemical shifts of $K_{{\alpha }_{1,2}}$ and $K_{{\beta }_{1,2}}$ lines of x-ray emission (fluorescence) spectra in niobium are evaluated to analyze its chemical state in the crystal. Potential of CTEP for studying properties of point defects containing $f$ and heavy $d$ elements with relativistic effects, extended basis set, and broken crystal symmetry taken into account is discussed.

We study the spin-boson model (SBM) with two spins in staggered biases by a numerically exact method based on variational matrix product states. Several observables such as the magnetization, the entanglement entropy between the two spins and the bosonic environment, the ground-state energy, as well as the correlation function for two spins are calculated exactly. The characteristics of these observables suggest that the staggered biases can drive the second-order quantum phase transition (QPT) to the first-order QPT in the sub-Ohmic SBM, while the Kosterlitz-Thouless QPT in the Ohmic SBM goes directly to the first-order one. A quantum tricritical point, where the continuous QPT meets the first-order one, can then be detected. It is found that the staggered biases would not change the universality of the phase transition in this model below the quantum tricritical point.

The eigenvalue problem of quantum many-body systems is a fundamental and challenging subject in condensed-matter physics because the dimension of the Hilbert space (and hence the required computational memory and time) grows exponentially as the system size increases. A few numerical methods have been developed for some specific systems but may not be applicable in others. Here we propose a general numerical method, random sampling neural networks (RSNNs), to utilize the pattern recognition technique for the random sampling matrix elements of an interacting many-body system via a self-supervised learning approach. Several exactly solvable one-dimensional models, including the Ising model with a transverse field, the Fermi-Hubbard model, and the spin-$\frac{1}{2}XXZ$ model, are used to test the applicability of RSNN. Pretty high accuracy ($>96\%$) of energy spectra, magnetization, critical exponents, etc. can be obtained within the strongly correlated regime or near the quantum phase-transition point, even the corresponding RSNN models are trained in the weakly interacting regime. After including data augmentation and transfer learning methods, RSNN can be further applied to systems of larger sizes with much less training data if only pretrained in a smaller system. Our results demonstrate the possibility to combine the existing numerical methods and the RSNN to explore quantum many-body problems in a much wider parameter space.

Several monolayer transition metal dichalcogenides (TMDs) are direct band gap semiconductors and potentially efficient emitters in light emitting devices. Photons are emitted when strongly bound excitons decay radiatively, and accurate models of such excitons are important for a full understanding of the emission. Importantly, photons are emitted in directions uniquely determined by the exciton center of mass momentum and with lifetimes determined by the exciton transition matrix element. The exciton band structures of two-dimensional hexagonal materials, including TMDs, are highly unusual with coexisting particle- and light-like bands. The latter is nonanalytic with emission selection rules essentially opposite to the particle-like states, but has been ignored in analyses of TMD light emission so far. In the present work, we analyze the temperature and angular dependence of light emission from both exciton species and point out several important consequences of the unique exciton band structure. Within a first-principles density-functional-theory$+$Bethe-Salpeter-equation framework, we compute exciton band structures and optical matrix elements for the important TMDs ${\mathrm{MoS}}_2$, $\mathrm{Mo}{\mathrm{Se}}_2$, ${\mathrm{WS}}_2$, and $\mathrm{W}{\mathrm{Se}}_2$. At low temperature, only the particle-like band is populated and our results agree with previous work. However, at slightly elevated temperatures, a significant population of the light-like band leads to modified angular emission patterns and lifetimes. Clear experimental fingerprints are predicted and explained by a simple four-state model incorporating spin-orbit as well as intervalley exchange coupling.

We successfully fabricated a ${\mathrm{MnBi}}_2{\mathrm{Te}}_4/{\mathrm{Bi}}_2{\mathrm{Te}}_3$ heterostructure by incorporating Mn and Te inside the topmost quintuple layer of ${\mathrm{Bi}}_2{\mathrm{Te}}_3$, as unambiguously confirmed by low-energy electron diffraction $I\text{−}V$ and scanning transmission electron microscopy measurements. The surface-state Dirac cone of the heterostructure showed little change compared to that of pristine ${\mathrm{Bi}}_2{\mathrm{Te}}_3$, and x-ray magnetic circular dichroism measurements showed that the system was paramagnetic down to 5.6 K. These results are in contrast to previous works on related materials that showed magnetic order around 10 K as well as theoretical predictions and suggest the intricacy of the magnetic properties of two-dimensional van der Waals magnets.

We present a method for calculating the time-dependent many-body wavefunction that follows a local quench. We apply the method to the voltage-driven nonequilibrium Kondo model to find the exact time-evolving wavefunction following a quench where the dot is suddenly attached to the leads at $t=0$. The method, which does not use Bethe ansatz, also works in other quantum impurity models (we include results for the interacting resonant level and the Anderson impurity model) and may be of wider applicability. In the particular case of the Kondo model, we show that the long-time limit (with the system size taken to infinity first) of the time-evolving wavefunction is a current-carrying nonequilibrium steady state that satisfies the Lippmann-Schwinger equation. We show that the electric current in the time-evolving wavefunction is given by a series expression that can be expanded either in weak coupling or in strong coupling, converging to all orders in the steady-state limit in either case. The series agrees to leading order with known results in the well-studied regime of weak antiferromagnetic coupling and also reveals another universal regime of strong ferromagnetic coupling, with Kondo temperature $T_K^{\left(F\right)}=D{e}^{-\frac{3{\pi }^2}{8}\rho \left|J\right|}$ ($J<0, \rho \left|J\right|\to \infty$). In this regime, the differential conductance $dI/dV$ reaches the unitarity limit $2e^2/h$ asymptotically at large voltage or temperature.

We construct an effective low-energy Hamiltonian, which describes fermions dwelling on a deformed honeycomb lattice with dislocations and disclinations, and with arbitrary hopping parameters of the corresponding tight binding model. Despite the presence of dislocations and disclinations, the tight binding Hamiltonian preserves the connectivity number 3 at each lattice site. This construction is related to fermions with a two-dimensional gravity. The effective theory has a local SU(2) gauge invariance of the group of rotations. We reformulate the model by fermions interacting with the deformation as a fermion lattice model with a phonon field and calculate the response of the fermion currents to the external deformation or phonon field. This indicates a $Z_2$ anomaly. This can be detected experimentally.

To develop a dislocation-based statistical continuum theory of crystal plasticity is a major challenge of materials science. During the last two decades, such a theory has been developed for the time evolution of a system of parallel edge dislocations. The evolution equations were derived by a systematic coarse graining of the equations of motion of the individual dislocations and later retrieved from a functional of the dislocation densities and the stress potential by applying the standard formalism of phase field theories. It is, however, a long-standing issue if a similar procedure can be established for curved dislocation systems. An important prerequisite for such a theory has recently been established through a density-based kinematic theory of moving curves. In this paper, an approach is presented for a systematic derivation of the dynamics of systems of curved dislocations in a single-slip situation. In order to reduce the complexity of the problem, a dipolelike approximation for the orientation-dependent density variables is applied. This leads to a closed set of kinematic evolution equations of total dislocation density, the geometrically necessary dislocation densities, and the so-called curvature density. The analogy of the resulting equations with the edge dislocation model allows one to generalize the phase field formalism and to obtain a closed set of dynamic evolution equations.

A comprehensive study of magnetocrystalline anisotropy of a layered van der Waals ferromagnet $\mathrm{V}{\mathrm{I}}_3$ was performed. We measured angular dependences of the torque and magnetization with respect to the direction of the applied magnetic field within the basal $ab$ plane and a general orthogonal plane to $ab$, respectively. A twofold butterflylike signal was detected by magnetization in the orthogonal plane. This signal symmetry remains conserved throughout all magnetic regimes as well as through the known structural transition down to the lowest temperatures. The maximum of the magnetization signal and the resulting magnetization easy axis is significantly tilted from the normal to the basal $ab$ plane by ∼40°. The close relation of the magnetocrystalline anisotropy to the crystal structure was documented. In contrast, a two-fold-like angular signal was detected in the paramagnetic region within the $ab$ plane in the monoclinic phase, which transforms into a six-fold-like signal below the Curie temperature $T_{\mathrm{C}}$. With further cooling, another six-fold-like signal with an angular shift of ∼30° grows approaching $T_{\mathrm{FM}}$. Below $T_{\mathrm{FM}}$, in the triclinic phase, the original six-fold-like signal vanishes, being replaced by a secondary six-fold-like signal with an angular shift of ∼30°.

Thermal transport in topologically-ordered phases of matter provides valuable insights as it can detect the charge-neutral quasiparticles that would not directly couple to electromagnetic probes. An important example is the edge heat transport of the Majorana fermions in a chiral spin liquid, which leads to a half-quantized thermal Hall conductivity. This signature is precisely what has recently been measured in $\alpha \text{−}{\mathrm{RuCl}}_3$ under external magnetic fields. The plateau-like behavior of the half-quantized thermal Hall conductivity as a function of external magnetic field, and the peculiar sign change depending on the magnetic field orientations, have been proposed to be strong evidence for the non-Abelian Kitaev spin liquid. On the other hand, for in-plane magnetic fields, it has been theoretically shown that such a sign structure can also arise from topological magnons in the field-polarized state. In this paper, we investigate the full implications of topological magnons as heat carriers on thermal transport measurements. We first prove analytically that for any commensurate order with a finite magnetic unit cell, reversing the field direction leads to a sign change in the magnon thermal Hall conductivity in two-dimensional systems. We corroborate this proof numerically with nontrivial magnetic orders as well as the field-polarized state in Kitaev magnets subjected to an in-plane field. In the case of the tilted magnetic field, in which there exists both finite in-plane and out-of-plane field components, we find that the plateau-like behavior of the thermal Hall conductivity and the sign change upon the reversal of the in-plane component of the magnetic field arises in the partially polarized state, as long as the in-plane field contribution to the Zeeman energy is significant. While these results are consistent with the experimental observations, we comment on other aspects that require further investigation in future studies.