We study the matrix elements of local and nonlocal operators in the single-particle eigenstates of two paradigmatic quantum-chaotic quadratic Hamiltonians; the quadratic Sachdev-Ye-Kitaev (SYK2) model and the three-dimensional Anderson model below the localization transition. We show that they display eigenstate thermalization for normalized observables. Specifically, we show that the diagonal matrix elements exhibit vanishing eigenstate-to-eigenstate fluctuations and that their variance is proportional to the inverse Hilbert space dimension. We also demonstrate that the ratio between the variance of the diagonal and the off-diagonal matrix elements is 2, as predicted by the random matrix theory. We study distributions of matrix elements of observables and establish that they need not be Gaussian. We identify the class of observables for which the distributions are Gaussian.

We develop a scaling theory of interaction-induced delocalization of few-particle states in disordered quantum systems. In the absence of interactions, all single-particle states are localized in $d<3$, while in $d\ge 3$ there is a critical disorder below which states are delocalized. We hypothesize that such a delocalization transition occurs for $n$-particle bound states in $d$ dimensions when $d+n\ge 4$. Exact calculations of disorder-averaged $n$-particle Green's functions support our hypothesis. In particular, we show that three-particle states in $d=1$ with nearest-neighbor repulsion will delocalize with $W_c\approx 1.4t$ and with localization length critical exponent $\nu =1.5\pm 0.3$. The delocalization transition can be understood by means of a mapping onto a noninteracting problem with symplectic symmetry. We discuss the importance of this result for many-body delocalization, and how few-body delocalization can be probed in cold atom experiments.

The magnetic structure of Fe-doped MnNiGe alloys of nominal compositions ${\mathrm{MnNi}}_{0.75}{\mathrm{Fe}}_{0.25}\mathrm{Ge}$ and ${\mathrm{Mn}}_{0.85}{\mathrm{Fe}}_{0.15}\mathrm{NiGe}$ has been explored through detailed neutron powder diffraction (NPD) study in ambient and high-pressure (6 kbar) conditions. Both these alloys crystallize with ${\mathrm{Ni}}_2\mathrm{In}$-type hexagonal structure (with $P{6}_3/mmc$ space group) at room temperature and undergo a martensitic transition to low-temperature TiNiSi-type orthorhombic structure (with space group $Pnma$). Both the alloys show incommensurate antiferromagnetic ordering at the low-temperature martensitic phase. However, the modulation of the magnetic structures depends strongly on the doping site of Fe. The incommensurate propagation vector $\mathbf{k}$ for ${\mathrm{MnNi}}_{0.75}{\mathrm{Fe}}_{0.25}\mathrm{Ge}$ and ${\mathrm{Mn}}_{0.85}{\mathrm{Fe}}_{0.15}\mathrm{NiGe}$ alloys are found to be (0.1790(1),0,0) and (0.1543(3),0,0), respectively at 1.5 K, and these remain almost unchanged with increasing sample temperature under ambient conditions. The application of the external pressure results in a significant effect on both martensitic transition temperature and low-temperature magnetic structure. The propagation vectors $\mathbf{k}$ for both the alloys show a monotonic decrease with increasing sample temperature in the presence of external pressure. Interestingly, no sign of the magnetically ordered hexagonal austenite phase was observed in any alloys down to the lowest measurement temperature.

A prototypical polycrystalline in-plane exchange bias system exhibited a viscous decrease of the ferromagnetic magnetization upon increasing external magnetic field when measuring first-order reversal curves. The phenomenon is investigated by means of angular-resolved vectorial magneto-optical Kerr magnetometry complemented by fits of model calculations and by Kerr microscopy. It is found that the viscous magnetization decrease is mediated by a rotatable magnetic anisotropy arising from thermally unstable antiferromagnetic grains coupled to the probed ferromagnet. This additionally manifests itself by a creeping domain wall motion in the ferromagnet due to thermally activated processes in the antiferromagnet. The investigations are in agreement with a generalized description of polycrystalline exchange bias systems and emphasize the relevance of understanding minor loop behavior addressing nonsaturated magnetic states for systems susceptible to dynamic changes on the hysteresis loop timescale.

An interstitial skyrmion is the skyrmion state confined in the helical background. While the spin helix naturally provides the one-dimensional channels in helical magnets, the dynamics of an interstitial skyrmion are different from that for free skyrmions in a ferromagnetic or conical background. In this paper, we studied the dynamics of interstitial skyrmions under a temperature gradient numerically and analytically. We show that the interstitial skyrmion moves from the hot area to the cold area in the presence of temperature gradients, in contrast to skyrmions in the ferromagnetic background, where they move towards the hot region. Because the interstitial skyrmion is confined in the one-dimensional channel, it moves in the same direction as the magnon flow induced by the temperature gradient, where the $\beta$-type thermomagnonic torque plays an essential role.

Electron states localized at a magnetic domain wall in a graphene with Rashba spin-orbit interaction and coupled to a magnetic layer are studied theoretically. It is shown that two one-dimensional bands of edge modes propagating along the domain wall emerge in the energy gap for each Dirac point, and the modes associated with different Dirac points $K$ and $K^{\prime }$ are the same. The coefficients describing decay of the corresponding wave functions with distance from the domain wall contain generally real and imaginary terms. Numerical results on the local spin density and on the total spin expected in the edge states characterized by the wave number $k_y$ are presented and discussed. The Chern number for a single magnetic domain on graphene indicates that the system is in the quantum anomalous Hall phase, with two chiral modes at the edges. In turn, the number of modes localized at the domain wall is determined by the difference in Chern numbers on both sides of the wall. These numbers are equal to 2 and $-2$, respectively, so there are four modes localized at the domain wall.

We study the magnetization process of the $S=1$ Heisenberg model on a two-leg ladder with farther neighbor spin-exchange interaction. We consider the interaction that couples up to the next-nearest neighbor rungs and find an exactly solvable regime where the ground states become product states. The next-nearest neighbor interaction tends to stabilize magnetization plateaus at multiples of 1/6. In most of the exactly solvable regime, a single magnetization curve shows two series of plateaus with different periodicities.

Two-dimensional ferroelectrics (FEs) are promising in the miniaturization of memory devices with ultra-high-density data storage and low power consumption. However, many thiophosphate monolayers, i.e., analogs of ${\mathrm{CuInP}}_2{\mathrm{S}}_6$ and referred to as ${\mathrm{ABP}}_2{\mathrm{X}}_6$, lose ferroelectricity and instead exhibit an antiferroelectric (AFE) or paraelectric ordering. We propose to tune the AFE ${\mathrm{ABP}}_2{\mathrm{X}}_6$ monolayers into the FE ordering through interface engineering. The mechanism is that there are couplings between the charge polarizations of the ${\mathrm{ABP}}_2{\mathrm{X}}_6$ monolayers and the local dipoles as well as the induced electronic polarizations in the substrate which have a tendency to stabilize the FE ordering. We further perform first-principles calculations for ${\mathrm{CuInP}}_2{\mathrm{Se}}_6$ and ${\mathrm{CuCrP}}_2{\mathrm{S}}_6$ monolayers and their van der Waals heterostructures. We find that an AFE ${\mathrm{CuInP}}_2{\mathrm{Se}}_6$ monolayer becomes FE as interfaced with graphene, ${\mathrm{MoS}}_2$, and h-BN monolayers. In contrast, the ${\mathrm{CuCrP}}_2{\mathrm{S}}_6$ monolayer remains AFE since there is a large energy difference between the AFE and FE phases. Interfacing it with a ${\mathrm{MoTe}}_2$ monolayer induces a metal-insulator transition for the heterostructure, whereas interfacing with a polar surface MgO(111) can drive it into FE. The interfacing effect can also be used to manipulate the FE properties of ${\mathrm{ABP}}_2{\mathrm{X}}_6$ multilayers. We further find that the AFE-to-FE transition is electrically switchable in these systems. In particular, it is accompanied by an indirect-direct band-gap transition for the ${\mathrm{CuInP}}_2{\mathrm{Se}}_6$ monolayer. Our study offers an effective approach to tune the FE and electronic properties of ${\mathrm{ABP}}_2{\mathrm{X}}_6$ thin films for applications in electronics and optoelectronics.

Magnetic damping is of great interest due to its importance in magnetization switching and dynamics. Here, we report the quantitative disentanglement of intrinsic and extrinsic Gilbert damping in epitaxial Fe thin films. Both intrinsic damping and two-magnon scattering (TMS) make significant contributions to the total Gilbert damping, leading to an enhanced total damping at low temperature. Our result suggests the correlation between interfacial magnetic anisotropy and TMS.

We investigate the spin excitation spectra in chiral and polar magnets by the linear spin-wave theory for an effective spin model with symmetric and antisymmetric long-range interactions. In one dimension, we obtain the analytic form of the dynamical spin structure factor for proper-screw and cycloidal helical spin states with uniform twists, which shows a gapless mode with strong intensity at the helical wave number. When introducing spin anisotropy in the symmetric interactions, we numerically show that the stable spin spirals become elliptically anisotropic with nonuniform twists and the spin excitation is gapped. In higher dimensions, we find that similar anisotropy stabilizes multiple-$Q$ spin states, such as vortex crystals and hedgehog lattices. We show that the anisotropy in these states manifests itself in the dynamical spin structure factor: a strong intensity in the transverse components to the wave number appears only when the helical wave vector and the corresponding easy axis are perpendicular to each other. Our findings could be useful not only to identify the spin structure but also to deduce the stabilization mechanism by inelastic neutron scattering measurements.

Tuning magnetic anisotropy can facilitate the practical application of two-dimensional ferromagnets. Thus, understanding the relationship between magnetic anisotropy and electronic structures is desirable. Here, using the first-principles calculations and the second-order perturbation analyses, we investigate the microscopic mechanism (or electronic origin) underlying the change of the magnetic anisotropic energy (MAE) of the ${\mathrm{MnS}}_2$ monolayer. The calculated results show that strain enables the MAE to transform between an in-plane and off-plane orientation, and interlayer coupling (IC) from heterostuctures induces it to change positively. Furthermore, the strain-induced change in the MAE is greater than its IC-induced change. The further analyses indicate that although the microscopic mechanism underlying the strain- and IC-induced changes of the MAE is distinct, the change in electron occupation of $\mathrm{Mn}\text{−}d$ orbitals dominates the two changes. This work confirms the dominance of electron occupation on the MAE change and thus enriches the microscopic understanding about a MAE change, providing a theoretical reference to tuning the MAE of 2D ferromagnetic semiconductors in the future.

We study the effect of anisotropy of spin-$S$ impurities on an antiferromagnetic SU(2) Heisenberg chain. The magnetic impurities are assumed to have an XXZ-type anisotropic exchange with their neighboring sites. First, using density-matrix renormalization group technique, we examine spin-spin correlation function, instability of Néel order, and local spin susceptibility in the presence of a single spin-$S$ impurity. Based on the results, we find that the types of spin-$S$ impurities are classified into two groups: (i) nonmagnetic and $S=1$ impurities enhance only short-range antiferromagnetic correlation and (ii) $S=\frac{1}{2}$ and $S>1$ impurities can, in contrast, stabilize a long-range Néel order in the disordered SU(2) Heisenberg chain. Then, we focus on the case of $S=\frac{1}{2}$ impurity as a representative of (ii) and investigate the evolution of some experimentally observable quantities such as magnetization, specific heat, and magnetic susceptibility, as a function of concentration and XXZ anisotropy strength of the impurity. We confirm that the Néel order is induced in the bulk spin chain in the presence of finite amount of easy-axis XXZ $S=\frac{1}{2}$ impurities. Furthermore, we recover some of the aforementioned features using cluster mean-field theory, which allows us to present results on experimentally accessible quantities at finite temperatures. Interestingly, in the presence of uniform magnetic field, the total magnetization exhibits a pseudogap behavior for low values of applied field. We also discuss the dependence of NMR spectrum on various XXZ impurities and identify that the spin state of Co impurity in ${\mathrm{SrCu}}_{0.99}{\mathrm{Co}}_{0.01}{\mathrm{O}}_2$ is $S=\frac{3}{2}$.

Quantum simulation experiments have started to explore regimes that are not accessible with exact numerical methods. To probe these systems and enable new physical insights, the need for measurement protocols arises that can bridge the gap to solid-state experiments, and at the same time make optimal use of the capabilities of quantum simulation experiments. Here we propose applying time-dependent photoemission spectroscopy, an established tool in solid-state systems, in cold atom quantum simulators. Concretely, we suggest combining the method with large magnetic field gradients, unattainable in experiments on real materials, to drive Bloch oscillations of spinons, the emergent quasiparticles of spin liquids. We show in exact diagonalization simulations of the one-dimensional $t\text{−}J$ model with a single hole that the spinons start to populate previously unoccupied states in an effective band structure, thus allowing us to visualize states invisible in the equilibrium spectrum. The dependence of the spectral function on the time after the pump pulse reveals collective interactions among spinons. In numerical simulations of small two-dimensional systems, spectral weight appears at the ground-state energy at momentum $\mathbf{q}=(\pi ,\pi )$, where the equilibrium spectral response is strongly suppressed up to higher energies, indicating a possible route toward solving the mystery of the Fermi arcs in the cuprate materials.

We study the topological phase transitions induced by Coulomb engineering in three triangular-lattice Hubbard models, $A{B}_2, A{C}_3$, and $B_2{C}_3$, each of which consists of two types of magnetic atoms with opposite magnetic moments. The energy bands are calculated using the Schwinger boson method. We find that a topological phase transition can be triggered by the second-order (three-site) virtual processes between the two types of magnetic atoms, the strengths of which are controlled by the on-site Coulomb interaction $U$. This class of topological phase transitions has rarely been studied and may be realized in a variety of real magnetic materials.

Rydberg chains provide an appealing platform for probing conformal field theories (CFTs) that capture universal behavior in a myriad of physical settings. Focusing on a Rydberg chain at the Ising transition separating charge density wave and disordered phases, we establish a detailed link between microscopics and low-energy physics emerging at criticality. We first construct lattice incarnations of primary fields in the underlying Ising CFT including chiral fermions, a nontrivial task given that the Rydberg chain Hamiltonian does not admit an exact fermionization. With this dictionary in hand, we compute correlations of microscopic Rydberg operators, paying special attention to finite, open chains of immediate experimental relevance. We further develop a method to quantify how second-neighbor Rydberg interactions tune the sign and strength of four-fermion couplings in the Ising CFT. Finally, we determine how the Ising fields evolve when four-fermion couplings drive an instability to Ising tricriticality. Our results pave the way to a thorough experimental characterization of Ising criticality in Rydberg arrays, and can inform the design of novel higher-dimensional phases based on coupled critical chains.

Time-dependent orbital-free density functional theory is an efficient ab initio method for calculating the electronic dynamics of large systems. In comparison to standard time-dependent density functional theory, it computes only a single electronic state regardless of system size, but it requires an additional time-dependent Pauli potential term. We propose a nonadiabatic and nonlocal Pauli potential whose main ingredients are the time-dependent particle and current densities. Our calculations of the optical spectra of metallic and semiconductor clusters indicate that nonlocal and nonadiabatic time-dependent orbital-free density functional theory performs accurately for metallic systems and semiquantitatively for semiconductors. This paper opens the door to wide applicability of time-dependent orbital-free density functional theory for nonequilibrium electron and electron-nuclear dynamics of complex materials.

We employ variational autoencoders to extract physical insight from a dataset of one-particle Anderson impurity model spectral functions. Autoencoders are trained to find a low-dimensional, latent space representation that faithfully characterizes each element of the training set, as measured by a reconstruction error. Variational autoencoders, a probabilistic generalization of standard autoencoders, further condition the learned latent space to promote highly interpretable features. In our study, we find that the learned latent variables strongly correlate with well known, but nontrivial, parameters that characterize emergent behaviors in the Anderson impurity model. In particular, one latent variable correlates with particle-hole asymmetry, while another is in near one-to-one correspondence with the Kondo temperature, a dynamically generated low-energy scale in the impurity model. Using symbolic regression, we model this variable as a function of the known bare physical input parameters and “rediscover” the nonperturbative formula for the Kondo temperature. The machine learning pipeline we develop suggests a general purpose approach, which opens opportunities to discover new domain knowledge in other physical systems.

Characterizing the delocalization transition in closed quantum systems with a many-body localized phase is a key open question in the field of nonequilibrium physics. We exploit the fact that localization of particles as realized in Anderson localization and standard many-body localization (MBL) implies Fock-space localization in single-particle basis sets characterized by a real-space index. Using a recently introduced quantitative measure for Fock-space localization computed from the density distributions, the occupation distance, we systematically study its scaling behavior across delocalization transitions and identify critical points from scaling collapses of numerical data. Excellent agreement with literature results is found for the critical disorder strengths of noninteracting fermions, such as the one-dimensional Aubry-André model and the three-dimensional Anderson model. We observe a distinctively different scaling behavior in the case of interacting fermions with random disorder consistent with a Kosterlitz-Thouless transition. Finally, we use our measure to extract the transition point as a function of filling for interacting fermions.