In-plane anomalous Hall effect (IPAHE) is an unconventional anomalous Hall effect (AHE) where the Hall current flows in the plane spanned by the magnetization or magnetic field and the electric field. Here, we predict a stable two-dimensional ferromagnetic monolayer ${\mathrm{Mn}}_3{\mathrm{Si}}_2{\mathrm{Te}}_6$ with collinear ordering of Mn moments in the basal plane. Moreover, we reveal that monolayer ${\mathrm{Mn}}_3{\mathrm{Si}}_2{\mathrm{Te}}_6$ possesses a substantial periodic IPAHE due to the threefold rotational symmetry, which can be switched by changing the magnetization orientation by external magnetic fields. In addition, we briefly discuss the impacts of moderate strains on the electronic states and the AHE, which lead to a near quantized Hall conductivity. Our work provides a potential platform for realizing a sizable and controllable IPAHE that greatly facilitates the application of energy-efficient spintronic devices.

The phenomenon of nonlinear transport has attracted tremendous interest within the condensed matter community. We present a theoretical framework for nonlinear transport based on the nonequilibrium retarded Green's function and examine the impact of disorder on nonlinear magnetotransport in Weyl semimetals (WSMs). It is demonstrated that bilinear magnetoconductivity can be induced in disordered WSMs by several mechanisms, including the impurity-induced tilt of the Weyl cones, Lorentz-force-induced normal orbital magnetic moment, and a chiral anomaly arising from the Berry-curvature-induced anomalous orbital magnetic moment. Additionally, we observe that the localization of Weyl fermions by impurity scattering will lead to resonant dips in both the chiral chemical potential and magnetoconductivity when the Fermi energy approaches the impurity resonance states. Our findings offer a theoretical proposition for modulating nonreciprocal transport in topological semimetals.

We study theoretically the generation of high harmonics by transition metal dichalcogenides disk-shaped quantum dots placed in an ultrashort optical pulse. The quantum dots are described within a massive Dirac type effective model with infinite mass boundary conditions. For a linearly polarized pulse, the cutoff frequency and the intensities of high-order harmonics increase with the quantum dot radius. For the frequency of the pulse below the system band gap, the radiation spectra show strong dependence on the quantum dot size, while for the large pulse frequency, i.e., above the band gap, both the intensities of high harmonics and cutoff frequency weakly depend on the quantum dot radius. For an elliptically polarized pulse, the intensities of harmonics monotonically decrease with the pulse ellipticity and become strongly suppressed for a circularly pulse. The cutoff frequency also monotonic decreases with the pulse ellipticity and shows weak dependence on the quantum dot material.

The nonequilibrium Green's function formalism provides a versatile and powerful framework for numerical studies of nonequilibrium phenomena in correlated many-body systems. For calculations starting from an equilibrium initial state, a standard approach consists of discretizing the Kadanoff-Baym contour and implementing a causal time-stepping scheme in which the self-energy of the system plays the role of a memory kernel. This approach becomes computationally expensive at long times, because of the convolution integrals and the large amount of computer memory needed to store the Green's functions. A recent idea for the compression of nonequilibrium Green's functions is the quantics tensor train representation. Here, we explore this approach by implementing equilibrium and nonequilibrium simulations of the two-dimensional Hubbard model with a second-order weak-coupling approximation to the self-energy. We show that calculations with compressed two-time functions are possible without any loss of accuracy, and that the quantics tensor train implementation shows a much improved scaling of the computational effort and memory demand with the length of the time contour.

We study the zero-temperature phase diagram of a symmetric electron-hole bilayer system by comparing the ground-state energies of two distinct limiting cases, characterized by an electron-hole plasma or an exciton gas, respectively. For the electron-hole plasma, the random-phase approximation is used; for the exciton gas, we consider three different approximations: the unscreened Coulomb interaction, the statically screened one, and the dynamically screened one under the plasmon-pole approximation. Our results suggest that the exciton gas is stable at small layer separation. However, static screening in general suppresses the formation of excitons, and dynamic screening gives different results depending on the representative energy scale we used in the plasmon-pole approximation. We conclude that energetic considerations alone are very sensitive to the approximation schemes, and the phase diagram of the system may depend crucially on exactly how the electron-hole attraction is treated in the theory. For very small and very large densities, however, all our approximations show the exciton gas to have lower energy than the plasma.

We investigate the first-order correction to the anomalous Hall conductivity of 2D massive Dirac fermions arising from electron-electron interactions. In a fully gapped system in the limit of zero temperature, we find that this correction vanishes, confirming the absence of perturbative corrections to the topological Hall conductivity. At finite temperature or chemical potential, we find that the total Hall response decays faster than in the noninteracting case, depending on the strength of electron-electron interactions. These features, which could potentially be observed experimentally, show the importance of two-body interactions for anomalous Hall transport.

Edge magnetoplasmons arise on a boundary of conducting layers in perpendicular magnetic field due to an interplay of electron cyclotron motion and Coulomb repulsion. Lateral electric field, which confines electrons inside the sample, drives their spiraling motion in magnetic field along the edge with the average drift velocity contributing to the total magnetoplasmon velocity. We revisit this classical picture by developing fully quantum theory of drift velocity starting from analysis of magnetic edge channels and their electrodynamic response. We derive the quantum-mechanical expression for the drift velocity, which arises in our theory as a characteristic of such response and can be calculated as the harmonic mean of group velocities of edge channels crossing the Fermi level. Using the Wiener-Hopf method to analytically solve the edge mode electrodynamic problem, we demonstrate that the edge channel response effectively enhances the bulk Hall response of the conducting layer and thus increases the edge magnetoplasmon velocity. In the long-wavelength limit of our model, the drift velocity is simply added to the total magnetoplasmon velocity, in agreement with the classical picture.

The ratio between the energies of optical and thermal ionization depends on the defect nature and the strength of the interaction of trapped electrons with phonons. Knowing this ratio for a certain type of defect allows one to predict, e.g., thermal emission energies from the optically measured values. We present the results of direct empirical extraction of the ratio between optical and thermal electron emission energies for ${\mathrm{HfO}}_2$ bulk electron traps combined with theoretical analysis of the physical mechanism of electron transitions from the trapped state to the mobility edge. We show that, by applying different excitation mechanisms, we affect the same deep traps inside the ${\mathrm{HfO}}_2$ band gap; i.e., these traps are both optically and thermally active and are likely to have similar nature. The extracted empirical optical/thermal ionization energy ratio of $2.2\pm 0.3$ is in good agreement with the polaronic nature of the probed electron traps, as shown by the results of theoretical calculations. Our results provide experimental and theoretical methodologies for consistently linking thermal and optical ionization energies of electron traps and describing their distributions in the band gap of amorphous oxides, and can help improve modeling frameworks for reliability issues related to oxide traps.

Antiferromagnetic (AFM) spintronics provides a route towards energy-efficient and ultrafast device applications. Achieving anomalous valley Hall effect (AVHE) in AFM monolayers is thus of considerable interest for both fundamental condensed-matter physics and device engineering. Here we propose a route to achieve an AVHE in A-type AFM insulator composed of vertically stacked monolayer quantum anomalous Hall insulators with strain and electric field modulations. Uniaxial strain and electric field generate valley polarization and spin splitting, respectively. Using first-principles calculations, ${\mathrm{Fe}}_2\mathrm{BrMgP}$ monolayer is predicted to be a prototype hosting valley-polarized quantum spin Hall insulators in which AVHE and quantum spin Hall effect are synergized in a single system. Our findings reveal a route to achieve multiple Hall effects in two-dimensional tetragonal AFM monolayers.

A finite spin life-time of conduction electrons may dominate Gilbert damping of two-dimensional metallic antiferromagnets or antiferromagnet/metal heterostructures. We investigate the Gilbert damping tensor for a typical low-energy model of a metallic antiferromagnet system with honeycomb magnetic lattice and Rashba spin-orbit coupling for conduction electrons. We distinguish three regimes of spin relaxation: exchange-dominated relaxation for weak spin-orbit coupling strength, Elliot-Yafet relaxation for moderate spin-orbit coupling, and Dyakonov-Perel relaxation for strong spin-orbit coupling. We show, however, that the latter regime takes place only for the in-plane Gilbert damping component. We also show that anisotropy of Gilbert damping persists for any finite spin-orbit interaction strength provided we consider no spatial variation of the Néel vector. Isotropic Gilbert damping is restored only if the electron spin-orbit length is larger than the magnon wavelength. Our theory applies to ${\mathrm{MnPS}}_3$ monolayer on Pt or to similar systems.

We report on crystal growth and physical properties of the quasi-one-dimensional compound ${\mathrm{Bi}}_{19}{\mathrm{S}}_{27}{\mathrm{I}}_3$ by combining crystal structure, electrical resistivity, magnetic properties, Seebeck coefficient, Hall coefficient as well as hydrostatic pressure effect up to 11.5 GPa. Unlike $p$-type ${\mathrm{Bi}}_{19}{\mathrm{S}}_{27}{\mathrm{I}}_3$ crystals, the maximum size of high-quality $n$-type ${\mathrm{Bi}}_{19}{\mathrm{S}}_{27}{\mathrm{I}}_3$ crystals can reach 2–3 mm by optimizing the chemical vapor transport method. The measurement results indicate that ${\mathrm{Bi}}_{19}{\mathrm{S}}_{27}{\mathrm{I}}_3$ is a diamagnetic semiconductor with two thermal activation energies, a large one $E_{\mathrm{g}1}\sim 0.81$ eV and a small one $E_{\mathrm{g}2}\sim 0.36$ eV, a huge room-temperature Seebeck coefficient of −1000 µV/K, and improved thermoelectric power factor $\sim 2.2\mu \mathrm{W}\mathrm{c}{\mathrm{m}}^{-1}{\mathrm{K}}^{-2}$ owing to the enhanced electrical conductivity. Under pressure, ${\mathrm{Bi}}_{19}{\mathrm{S}}_{27}{\mathrm{I}}_3$ undergoes a semiconductor-to-metal transition, and the thermal activation energy continuously decreases to almost zero near a critical pressure of 4.25 GPa. Accompanying this process, a density-wave-like transition emerges, characterized by the reversible jump observed in the temperature dependence of the resistivity. As the pressure further increases, the resistivity undergoes a crossover from a Fermi metal to a low-temperature upturn below a characteristic temperature, which decreases from 81 K at 4.5 GPa to 37 K at 11.5 GPa. The upturn in resistivity has a linear dependence on the logarithmic temperature, but does not saturate at low temperatures, which basically excludes a Kondo-like state and indicates the possibility of Anderson weak localization. High-pressure synchrotron x-ray diffraction confirms the absence of structural transition for $P<12.05$ GPa at room temperature, supporting pressure-induced electronic transition. Our density functional theory calculation on the assumption that the Bi1 occupies an average of $\sim \frac{1}{6}$ contradicts experimental electron bands, indirectly indicating that Bi1 should be partially ordered and has many vacancies in ${\mathrm{Bi}}_{19}{\mathrm{S}}_{27}{\mathrm{I}}_3$. Our results provide good examples for studying the mechanism of semiconductor metallization and exploring thermoelectric functional properties in low-dimensional materials.

In this paper, we theoretically investigate quantum phase transitions (QPTs) in a modified Kane-Mele (MKM) model with $C_{2z}$ and mirror symmetries. The phase diagram of the MKM model uncovers, in addition to the ${\mathbb{Z}}_2$ topological insulator phase, a second-order topological insulator (SOTI) phase and three distinct inversion-symmetry-breaking two-dimensional Weyl metal ($\mathcal{P}$-breaking 2D WM) phases in response to the interplay of the anisotropic nearest-neighbor hopping and Rashba spin-orbit coupling. During the QPTs among these phases, the edges states evolve topologically and manifest as helical edge states, quasiflat localized edge states, and Fermi arc edge states, respectively, in association with the merging and splitting of band-touching points. To achieve second-order topological corner states, we design a rhombus-shaped nanodisk, where two corner states emerge due to a filling anomaly resulting from the fixed Wannier configuration and $C_{2z}$ symmetry. In this paper, we not only propose an approach to realize the SOTI phase in the MKM model but also establish a platform for investigating the topological properties of the $\mathcal{P}$-breaking 2D WM.

Magnetic ordering in ${\mathrm{Cu}}_2{\mathrm{OSeO}}_3$ occurs without any detectable changes in the lattice symmetry but involves significant coupling with strain. The strain coupling effects in ${\text{Cu}}_2{\text{OSeO}}_3$ have been investigated with a focus on the skyrmion lattice by examining elastic and anelastic properties. Resonant ultrasound spectroscopy has been used to measure these properties of a ${\mathrm{Cu}}_2{\mathrm{OSeO}}_3$ single crystal as a function of temperature and magnetic field. On heating, the skyrmion phase has been characterized by slightly softer elasticity compared to the helical phase. However, there were no obvious anomalies in elastic and anelastic properties associated with the boundary of the stability field of the skyrmion lattice. Evolution of elastic properties with magnetic field, passing through the stability field of the skyrmion lattice, showed a characteristic pattern of a glassy state, where an equilibrium state is never reached. These imply that coupling of the skyrmions with strain is extremely weak in ${\mathrm{Cu}}_2{\mathrm{OSeO}}_3$, leading to glassy or liquidlike behavior of skyrmions. Three Debye-like loss peaks were observed near $\sim 40$, $\sim 50$, and $\sim 60\mathrm{K}$. The relaxation mechanism for the 40 K loss peak has been found to have a single relaxation time. Overlapping acoustic loss peaks in the temperature interval $\sim 50–62\mathrm{K}$ suggest that the magnetic transitions with variable temperature in this temperature range involve freezing of some dynamic aspect(s) of the magnetic structure with an activation energy of $\sim 0.1–0.15$ eV.

To increase the efficiency of the superconducting spin valve (SSV), special attention should be paid to the choice of ferromagnetic materials for the F1/F2/S SSV multilayer. Here, we report the preparation and superconducting properties of the SSV heterostructures where Pb is used as the superconducting S layer. In the magnetic part of the structure, we use the same starting material, the Heusler alloy ${\mathrm{Co}}_2{\mathrm{Cr}}_{1-x}{\mathrm{Fe}}_x{\mathrm{Al}}_y$, for both F1 and F2 layers. We utilize the tunability of the magnetic properties of this alloy, which, depending on the deposition conditions, forms either an almost fully spin-polarized half-metallic F1 layer or a weakly ferromagnetic F2 layer. We demonstrate that the combination of the distinct properties of these two layers boosts the generation of the long-range triplet component of the superconducting condensate in the fabricated SSV structures and yields superior values of the triplet spin-valve effect of more than 1 K and of the operational temperature window of the SSV up to 0.6 K.

Electronic and crystal structures of $\mathrm{YbCu}, {\mathrm{YbCu}}_2$, and ${\mathrm{YbCu}}_{6.5}$ under pressure were studied by high-resolution x-ray absorption spectroscopy (XAS) and x-ray diffraction. We found pressure-induced large valence transitions on the order of 0.3 in these Yb compounds. YbCu exhibited a first-order transition with a sudden change in the Yb valence at 4 GPa, at which a structural transition was not accompanied. With further increasing pressure, YbCu shows a structural transition from the FeB-type to the cubic CsCl-type structure around 10 GPa, where the Yb valence increases only gradually. In $\text{Yb}{\text{Cu}}_2$, a structural transition was not observed up to 11.8 GPa, while the Yb valence shows a rapid increase around 4–7 GPa, indicating a significant change in the electronic structure. Anomalous reentrant valence transition to a lower valence state was found at low pressures of 3–4 GPa in $\text{Yb}{\text{Cu}}_{6.5}$ without the structural phase transition. It is suggested that the anomaly in $\text{Yb}{\text{Cu}}_{6.5}$ is possibly correlated to the electronic structure of the Cu $d$ band. On the other hand, we found that these anomalous large changes in the Yb valences were linked to anomalies in crystal structures although there were no structural phase transitions. Complementary to the measurements of XAS at the Yb $L_3$ absorption edge, we also performed XAS at the Cu-$L_3$ absorption edge and measured the valence band spectra at ambient pressure.

We have explained the nonmagnetic state of ${\mathrm{TiF}}_3$, down to 10 K, in a strongly-correlated electron approach treating one $d$ electron of the ${\mathrm{Ti}}^{3+}$ ion as fully localized. We have derived the low-energy states, their energies in the meV energy scale, and their eigenfunctions, considering octahedral crystal-field, trigonal distortion, and spin-orbit interactions. The calculated ground state of the ${\mathrm{Ti}}^{3+}$ ion in ${\mathrm{TiF}}_3$ originates from the $e_g^{\pi }$ orbital doublet, contrary to the $a_{1g}$ orbital singlet reported on the basis of first-principles calculations. The ground Kramers-doublet state is almost nonmagnetic due to compensation of the spin and orbital momenta. Our approach reveals the fundamental importance of intra-atomic relativistic spin-orbit coupling for calculations of the magnetism and the electronic structure of $3d$ fluorides/oxides, pointing out that strong electron correlations are predominantly of a local on-site origin.

The half-filled $t_{2g}$ shell of the $t_{2g}^3$ configuration usually, in $LS$ coupling, hosts a $S=3/2$ ground state with quenched orbital moment. This state is not Jahn-Teller active. Sufficiently large spin-orbit coupling $\zeta$ has been predicted to change this picture by mixing in orbital moment, giving rise to a sizable Jahn-Teller distortion. In $5d^3{\mathrm{K}}_2{\mathrm{ReCl}}_6$ we study the electronic excitations using resonant inelastic x-ray scattering and optical spectroscopy. We observe on-site intra-$t_{2g}$ excitations below 2 eV and corresponding overtones with two intra-$t_{2g}$ excitations on adjacent sites, the Mott gap at 2.7 eV, $t_{2g}$-to-$e_g$ excitations above 3 eV, and charge-transfer excitations at still higher energy. The intra-$t_{2g}$ excitation energies are a sensitive measure of $\zeta$ and Hund's coupling $J_{\mathrm{H}}$. The sizable value of $\zeta \approx 0.29\mathrm{eV}$ places ${\mathrm{K}}_2{\mathrm{ReCl}}_6$ into the intermediate coupling regime, but $\zeta /J_{\mathrm{H}}\approx 0.6$ is not sufficiently large to drive a pronounced Jahn-Teller effect. We discuss the ground state wave function in a Kanamori picture and find that the $S=3/2$ multiplet still carries about 97% of the weight. However, the finite admixture of orbital moment allows for subtle effects. We discuss small temperature-induced changes of the optical data and find evidence for a lowering of the ground state by about 3 meV below the structural phase transitions.

Many-body interactions such as exciton-exciton interactions significantly affect the optical response of semiconductor nanostructures. These interactions can be rigorously modeled through microscopic calculations. However, these calculations can be computationally intensive and often lack physical insights. An alternative is to use phenomenological many-body-interaction models such as the modified optical Bloch equations and the anharmonic oscillator model. While both these models have separately been used to interpret experimental data, to the best of our knowledge, an explicit and direct correspondence between these models has not been established. Here, we show the empirical equivalence between these two complimentary models through two-dimensional coherent spectroscopy simulations. A quantitative correspondence between the parameters used to incorporate the exciton-exciton interactions in these two models are obtained. We also perform a quantitative comparison of these phenomenological models with experiments, which highlights their usefulness in interpreting experimental results.

Here we demonstrate the efficient photon-pair generation via spontaneous parametric down conversion from a semiconductor metasurface supporting dual quasibound states in the continuum (quasi-BICs). In a simple metasurface composed of AlGaAs elliptical nanocyclinders, the two high-$Q$ quasi-BIC resonances that coincide with the generated signal and idler frequencies significantly boost the local electric field. This leads to a substantial enhancement in the reverse classical nonlinear process of sum-frequency generation and subsequently the remarkably high generation rate of photon pairs under the quantum-classical correspondence principle. Within a narrowband wavelength regime around the quasi-BIC resonances, the rate of pair production is enhanced up to $\sim {10}^4$ Hz, two orders of magnitude larger than that in the Mie resonant AlGaAs nanoantennas. Moreover, the photon pair emission is mainly concentrated in the normal direction with respect to the metasurface, and shows a tunable rate with the $Q$ factor by engineering the rotation angle of nanocylinders. The presented work enables nanoscale sources of high-quality entangled photons which will find applications in advanced quantum imaging and communications.