Using the truncated Wigner approximation (TWA) we study quench dynamics of two-dimensional lattice systems consisting of interacting spinless fermions with potential disorder. First, we demonstrate that the semiclassical dynamics generally relaxes faster than the full quantum dynamics. We obtain this result by comparing the semiclassical dynamics with exact diagonalization and Lanczos propagation of one-dimensional chains. Next, exploiting the TWA capabilities of simulating large lattices, we investigate how the relaxation rates depend on the dimensionality of the studied system. We show that strongly disordered one-dimensional and two-dimensional systems exhibit a transient, logarithmic-in-time relaxation, which was recently established for one-dimensional chains. Such relaxation corresponds to the infamous $1/f$ noise at strong disorder.

We perform a symmetry analysis of the ABC-stacked kagome planes of Mn atoms common to the $L{1}_2 {\mathrm{Mn}}_{3}X$ and antiperovskite ${\mathrm{Mn}}_{3}AB$ alloys. In addition to the single-ion-anisotropy and Kitaev-type anisotropic exchange known to stabilize ${120}^{\circ }$ spin structures in these materials, our analysis results in a staggered Dzyaloshinskii-Moriya interaction and further gamma-type anisotropic exchange between nearest-neighbor spins. The presence of these new terms is shown not to affect the energetics of the ${120}^{\circ }$ ground states, which explains their absence in prior minimal magnetic models. We go on to show, however, that their influence becomes apparent when spin-wave excitations are considered. We highlight these effects by calculating inelastic neutron-scattering cross sections to illustrate experimental means of probing the existence and relative strengths of these cloaked interactions.

The magnetic-field-dependent critical current $I_{\text{c}}\left(B\right)$ of a Josephson junction is determined by the screening currents in its electrodes. In macroscopic junctions, a local vector potential drives the currents; however, in thin-film planar junctions, with electrodes of finite size and various shapes, they are governed by nonlocal electrodynamics. This complicates the extraction of parameters such as the geometry of the effective junction area, the effective junction length, and the critical current density distribution from the $I_{\text{c}}\left(B\right)$ interference patterns. Here, we provide a method to tackle this problem by simulating the phase differences that drive the shielding currents and use those to find $I_{\text{c}}\left(B\right)$. To this end, we extend the technique proposed by Clem [Phys. Rev. B 81, 144515 (2010)] to find $I_{\text{c}}\left(B\right)$ for Josephson junctions separating a superconducting strip of length $L$ and width $W$ with rectangular, ellipsoid, and rhomboid geometries. We find the periodicity of the interference pattern ($\mathrm{\Delta }B$) to have geometry-independent limits for $L\gg W$ and $L\ll W$. By fabricating elliptically shaped superconductor–normal-metal–superconductor junctions with various aspect ratios, we experimentally verify the $L/W$ dependence of $\mathrm{\Delta }B$. Finally, we incorporate these results to correctly extract the distribution of critical currents in the junction by the Fourier analysis of $I_{\text{c}}\left(B\right)$, which makes these results essential for the correct analysis of topological channels in thin-film planar Josephson junctions.

We report the first comprehensive microscopic description of the effect of strong correlations and thermal fluctuations on the properties of noncentrosymmetric superconductors in presence of an in-plane Zeeman field. Away from the weak coupling regime the Bardeen-Cooper-Schrieffer theory breaks down and the superconducting transitions are dictated by the pairing field phase fluctuations. Using a nonperturbative numerical technique viz. static path approximation, we demonstrate that short-range fluctuating superconducting pair correlations give rise to Fermi-surface segmentation with direction-dependent pair breaking and hot spots for quasiparticle scattering. A fluctuation-driven finite-temperature topological transition of the Fermi surface is realized, characterized by a shift of the corresponding Dirac point from $\mathbf{k}=0$ to $\mathbf{k}\ne 0$. Our results provide key benchmarks for the thermal scales and regimes of thermal stability of the properties of these systems, which are important for device applications. Our numerical estimates are in fairly good qualitative agreement with the recent differential conductance and quasiparticle interference measurements on ${\mathrm{Bi}}_2{\mathrm{Te}}_3/{\mathrm{NbSe}}_2$ hybrid. A generic theoretical framework for the finite momentum scattering of quasiparticles and the associated spectroscopic features is proposed, which is expected to be applicable to a wide class of superconducting materials.

We theoretically study the Kondo effect in type-II Ising superconductors with a single magnetic impurity. Type-II Ising superconductivity was found in two-dimensional centrosymmetric materials with multiple degenerate orbitals, such as in few-layer stanene [Falson et al., Science 367, 1454 (2020)] and ultrathin ${\mathrm{PdTe}}_2$ films [Liu et al., Nano Lett. 20, 5728 (2020)]. The type-II Ising spin-orbit coupling (SOC) in these materials generates out-of-plane effective Zeeman fields orienting opposite directions for opposing orbitals, which strongly protects the interorbital superconducting pairing states against in-plane magnetic fields. We show that the SOC-induced band splitting and the chemical potential significantly influence the formation of a localized magnetic moment, with particle-hole asymmetry in the phase boundary between magnetic and nonmagnetic states. The behaviors of spin-induced Yu-Shiba-Rusinov bound states and low-temperature magnetic susceptibility demonstrate that the SOC suppresses the Kondo screening of the magnetic moment, while the interorbital mixing weakens the SOC at finite momentum, then enhances the Kondo effect. The quantum phase transition between magnetic doublet and Kondo singlet ground states can be tuned through chemical potential in experiments.

A subsystem of an entangled ground state (GS) is in a mixed state. Thus, if we isolate this subsystem from its surroundings, we may be able to extract work applying unitary transformations, up to a maximal amount which is called ergotropy. Once this work has been extracted, the subsystem will still contain some bound energy above its local GS, which can provide valuable information about the entanglement structure. We show that the bound energy for half a free fermionic chain decays as the square of the entanglement entropy divided by the chain length, thus approaching zero for large system sizes, and we conjecture that this relation holds for all one-dimensional critical states.

Despite recent extensive studies of the non-Hermitian topology, understanding interaction effects is left as a crucial question. In this paper, we address interaction effects on exceptional points which are protected by the nontrivial point-gap topology unique to non-Hermitian systems. Our analysis in a two-dimensional parameter space elucidates the existence of exceptional points and symmetry-protected exceptional rings fragile against interactions; they are topologically protected only in noninteracting cases. This fragility of exceptional points and symmetry-protected exceptional rings arises from the reduction of non-Hermitian topological classifications, which is elucidated by introducing topological invariants of the second-quantized Hamiltonian for both noninteracting and interacting cases. These topological invariants are also available to analyze the reduction phenomena of gapped systems. The above results strongly suggest similar reduction phenomena of exceptional points in generic cases and open up a new direction of research in the non-Hermitian topology.

We present a solver for correlated impurity problems out of equilibrium based on a combination of the so-called auxiliary master equation approach (AMEA) and the configuration interaction expansion. Within AMEA one maps the original impurity model onto an auxiliary open quantum system with a restricted number of bath sites which can be addressed by numerical many-body approaches such as Lanczos/Arnoldi exact diagonalization (ED) or matrix product states (MPS). While the mapping becomes exponentially more accurate with increasing number of bath sites, ED implementations are severely limited due to the fast increase of the Hilbert space dimension for open systems, and the MPS solver typically requires rather long runtimes. Here, we propose to adopt a configuration interaction approach augmented by active space extension to solve numerically the correlated auxiliary open quantum system. This allows access to a larger number of bath sites at lower computational costs than for plain ED. We benchmark the approach with numerical renormalization group results in equilibrium and with MPS out of equilibrium. In particular, we evaluate the current, the conductance, as well as the Kondo peak and its splitting as a function of increasing bias voltage below the Kondo temperature $T_{\text{K}}$. We obtain a rather accurate scaling of the conductance as a function of the bias voltage and temperature rescaled by $T_{\text{K}}$ for moderate to strong interactions in a wide range of parameters. The approach combines the fast runtime of ED with an accuracy close to the one achieved by MPS making it an attractive solver for nonequilibrium dynamical mean field theory.

We present the results from torque magnetometry studies of the kagome superconductor ${\mathrm{RbV}}_3{\mathrm{Sb}}_5$ under applied fields up to 45 T and temperatures down to liquid ${}^3\mathrm{He}$ temperature (0.32 K). The torque signal shows clear de Haas–van Alphen (dHvA) oscillations with eight distinct frequencies ranging from $\approx 150$ to 3000 T. Among these, five are above 500 T. Angle-dependent measurement of dHvA oscillations shows that all frequencies follow 1/$\cos \theta$ dependence, where $\theta$ is the tilt angle with respect to the applied field direction, and the oscillations disappear above $\theta$ = ${60}^{\circ }$, which confirms that the Fermi surfaces corresponding to these frequencies are two dimensional. The Berry phase ($\varphi$), calculated by constructing a Landau level fan diagram, is found to be $\approx \pi$, which strongly supports the nontrivial topology of ${\mathrm{RbV}}_3{\mathrm{Sb}}_5$. Using the Lifshitz-Kosevich formula, we estimate the effective mass ($m^{*}$) of charge carriers in ${\mathrm{RbV}}_3{\mathrm{Sb}}_5$, and it is found to be heavier ($\approx 0.7m_o$, where $m_o$ is the free electron mass) than that for other topological insulators. The findings of high frequencies up to 3000 T in ${\mathrm{RbV}}_3{\mathrm{Sb}}_5$ have not been reported previously, and the results regarding the Fermi surface of ${\mathrm{RbV}}_3{\mathrm{Sb}}_5$ are crucial for understanding the charge density wave order, superconductivity, and nontrivial topology in $A{\mathrm{V}}_3{\mathrm{Sb}}_5$ ($A$ = K, Rb, and Cs), as well as the interplay among them.

A theoretical method is presented for angle-resolved photoemission at the transition metal $L$-edge resonance. It combines atomic multiplet calculations for the second-order resonant photoemission amplitude on the core-level site and a single scattering calculation of the photoelectron final state. The theory is applied to a magnetized Ni(111) surface excited with circularly polarized x rays at the Ni $L_{2,3}$-edge resonance with a focus on the circular dichroism (CD) signal. Good agreement with available experimental data is achieved. It is shown that the CD pattern is composed of a slowly varying magnetic signal induced by the atomic resonant process and a signal of fast angular modulations that are due to the interference of primary and scattered waves, known as the Daimon effect. The two types of CD signals are found to be nearly additive. At the Ni $L_2$-edge resonance, the angular dependence of the magnetic CD is well described by a simple expression known from x-ray magnetic CD. At the $L_3$ edge, however, the angular dependence is more complex and shows a pronounced final state multiplet dependence. With the present theory, it becomes possible to extract element- and site-selective magnetic information of surfaces from the CD in angle-resolved resonant photoemission data.

We report on a direct connection between band theory, quasiperiodic topology, and the almost-Mathieu (Aubry-André) metal insulator transition (MIT). By constructing the transfer matrix equations of one-dimensional (1D) quasiperiodic operators from rational approximate projected Green's functions, we relate the quasiperiodic Lyapunov exponents to the chiral edge modes of rational-flux Hofstadter Hamiltonians. We thereby show that the insulating phase is rooted in a topological “bulk-bulk” correspondence, a bulk-boundary correspondence between the 1D Aubry-André system (boundary) and its two-dimensional (2D) parent Hamiltonian (bulk). We extend this connection to random disorder via a Fourier expansion in quasiperiodic modes, demonstrating our results are widely applicable to systems beyond this paradigmatic model. The uncorrelated disorder limit is characterized by the breakdown of bulk-boundary driven quasiperiodic localization.

We examine a wavefunction ansatz in which a doped hole can experience a quantum transition from a charge $+e$ Landau quasiparticle to a neutral spinon as a function of the underlying spin-spin correlation. As shown variationally, such a wavefunction accurately captures all the essential features revealed by exact diagonalization and density matrix renormalization group simulations in a two-leg $t\text{−}J$ ladder. Hence its analytic form can provide an explicit understanding of the mechanism for the unconventional ground state. The transition in the phase diagram is accompanied by a change of the hole composite from a tight charge-spin binding to a loosely-bound hole-spin pair. In the latter, the hole carries a finite spin current but with vanishing charge current in the degenerate ground states. We show that the charge of the hole composite here is dynamically diminished due to an internal relative hole-spin motion, which is fundamentally distinct from a simple charge-spin separation in a one-dimensional case. We further show that the same effect is also responsible for a strong pairing between two doped holes in such a non-Landau quasiparticle regime.

We investigate the stability of the Néel quantum critical point of two-dimensional quantum antiferromagnets, described by a nonlinear $\sigma$ model, in the presence of a Kondo coupling to $N_f$ flavors of two-component Dirac fermion fields. The long-wavelength order parameter fluctuations are subject to Landau damping by electronic particle-hole fluctuations. Using the momentum-shell renormalization group (RG), we demonstrate that the Landau damping is weakly irrelevant at the Néel quantum critical point, despite the fact that the corresponding self-energy correction dominates over the quadratic gradient terms in the IR limit. In the ordered phase, the Landau damping increases under the RG, indicative of damped spin-wave excitations. Although the Kondo coupling is weakly relevant, sufficiently strong Landau damping renders the Néel quantum critical point quasistable for $N_f\ge 4$ and thermodynamically stable for $N_f<4$. In the latter case, we identify a multicritical point which describes the transition between the Néel critical and Kondo runaway regimes. The symmetry breaking at this fixed point results in the opening of a gap in the Dirac fermion spectrum. Approaching the multicritical point from the disordered phase, the fermionic quasiparticle residue vanishes, giving rise to non-Fermi-liquid behavior.

Two-dimensional (2D) Dirac bands and flat bands are characteristics of a kagome lattice. However, experimental studies on their electrical transport are few, because three-dimensional (3D) bulk bands of kagome materials, consisting of stacked 2D kagome layers, dominate the transport. We report a magnetoresistance (MR) study of a kagome material, ${\mathrm{Co}}_3{\mathrm{Sn}}_2{\mathrm{S}}_2$. Based on analysis of the temperature, magnetic field, and field angle dependence of the resistivity, we obtain a complete anatomy of MR. Besides a magnon MR, a chirality-dependent MR, and a chiral-anomaly-induced MR, the most intriguing feature is an orbital MR that scales only with the out-of-plane field, which strongly indicates its 2D nature. We attribute it to the Dirac band of the kagome lattice.

The quantum kinetic approach based on the density matrix offers a complete quantum mechanical description of dynamical optical and transport currents in solid-state systems. Starting from the quantum Liouville equation for the density matrix in an external electric field, we identify a strong photovoltaic response due to the nonlinear optical transition between heavy and light hole sub-bands enabled by $T_d$ symmetry in a quantum well, which we term the quadrupolar photovoltaic effect (QPE). The photovoltaic current exhibits a strong resonance in the vicinity of the heavy hole-light hole splitting, with a magnitude governed by the momentum relaxation time, which can reach nanoseconds in GaAs holes. Since the heavy hole-light hole splitting can be tuned from a few meV to nearly 100 meV, the QPE could serve as the basis for a terahertz photodetector, addressing the famous terahertz gap. We discuss experimental observation and device applications.

Finding reliably and efficiently the spectrum of the resonant states of an optical system under varying parameters of the medium surrounding it is a technologically important task, primarily due to various sensing applications. Computationally, it presents, however, a fundamental challenge owing to the nature of the eigenstates of an open system lacking completeness outside it. We solve this challenge by making a linear transformation of Maxwell's equations which maps perturbations of the surrounding medium onto effective perturbations within the system where the resonant states are complete. By treating such perturbations with a rigorous resonant-state expansion, we find the modified modes of the system for arbitrary homogeneous perturbations of the medium with any required accuracy. A numerically efficient single-mode approximation is shown to be highly accurate, as illustrated for various plasmonic nanoparticles, including gold nanospheres and silver bowtie antennas.

Recently, twisted moiré systems have attracted intense attention, where the emergence of the moiré potential is experimentally observed as an effective way to regulate the electron and exploit novel properties. Various theoretical models have been proposed and attributed the moiré potential as a result from interlayer electrical polarization or atomic relaxation effects. To date, the moiré potential is not yet directly calculated via an exact first-principles method and the universal mechanism in different systems is still unclear. Here, we obtain and analyze the surface superlattice potential by directly using first-principles calculations. We demonstrate that the moiré potential is a widely existing phenomenon in twisted systems with or without interlayer electrical polarization and is generally determined by the local work function. Specifically, the surface moiré potential is positively proportional to the local work function in single-atom-thick materials and negatively in multi-atom-thick ones due to a compensation effect. Interestingly, the surface potential amplitude of an unpolarized phosphorene system ($\sim 430$ meV) is much larger than that of the polarized $h$-BN system ($\sim 170$ meV). Our work directly reveals the universal moiré potential via fully first-principles calculations and provides a different angle in the study of moiré systems and twistronics.

Lead-based relaxor ferroelectric perovskites with the formula $\mathrm{Pb}\left({\mathrm{B}}_{1/3}{\mathrm{B}}_{2/3}^{\prime }\right){\mathrm{O}}_3$, such as $\mathrm{Pb}\left({\mathrm{Mg}}_{1/3}{\mathrm{Nb}}_{2/3}\right){\mathrm{O}}_3$, can have different degrees of compositional ordering even for the same stoichiometry. Despite intensive investigation, determining the nature of the B and ${\mathrm{B}}^{\prime }$ ordering remains a challenging problem. Here, we reveal a hierarchical ordering in $\mathrm{Pb}\left({\mathrm{B}}_{1/3}{\mathrm{B}}_{2/3}^{\prime }\right){\mathrm{O}}_3$ on top of the well-known ${\beta }^{\text{I}}$ and ${\beta }^{\text{II}}$ sublattices, which is the first level of the proposed hierarchy. Such additional ordering, which is found by considering additional nearest neighboring interactions in the model enriches our understanding of the complex nature of the chemical ordering in lead-based ferroelectric relaxors. In addition, we show that $\mathrm{Pb}\left({\mathrm{Cd}}_{1/3}{\mathrm{Nb}}_{2/3}\right){\mathrm{O}}_3$ has a higher degree of ordering than that of $\mathrm{Pb}\left({\mathrm{Mg}}_{1/3}{\mathrm{Nb}}_{2/3}\right){\mathrm{O}}_3$ or $\mathrm{Pb}\left({\mathrm{Zn}}_{1/3}{\mathrm{Nb}}_{2/3}\right){\mathrm{O}}_3$ in which antiphase domains exist, revealing the possible different degree of compositional ordering arising from intralattice ordering.

Antisymmetric exchange interactions lead to nonreciprocal spin-wave propagation. As a result, spin waves confined in a nanostructure are not standing waves; they have a time-dependent phase, because counterpropagating waves of the same frequency have different wavelengths. We report on a Brillouin light scattering (BLS) study of confined spin waves in Co/Pt nanowires with strong Dzyaloshinskii-Moriya interactions. Spin-wave quantization in narrow ($\lesssim 200$-nm-wide) wires dramatically reduces the frequency shift between BLS Stokes and anti-Stokes lines associated with the scattering of light incident transverse to the nanowires. In contrast, the BLS frequency shift associated with the scattering of spin waves propagating along the nanowire length is independent of nanowire width. A model that considers the chiral nature of modes captures this physics and predicts a dramatic reduction in frequency shift of light scattered from higher-energy spin waves in narrow wires, which is confirmed by our experiments.