Altermagnets, a recently identified class of magnetic materials, possess a spin-split Fermi surface that results in the so-called spin-splitter effect, enabling the generation of a spin current transverse to the injection direction and whose polarization lies along the Néel vector. In this study, we investigate how magnons interact with electrons in an altermagnetic metal. We find that while the electron-magnon interaction does not perturb the magnon dispersion, a charge current flowing in the material can induce a transverse magnon spin current, analogous to the electronic spin-splitter effect. This spin current possesses both electronic and magnonic characteristics, i.e., a chemical potential dependence and a strong temperature dependence. This effect realizes the efficient generation of spin currents via magnons without depending on the material's spin-orbit coupling.

Among the quasi-two-dimensional van der Waals magnetic systems, ${\mathrm{Fe}}_4\mathrm{Ge}{\mathrm{Te}}_2$ makes a profound impact due to its near-room-temperature ferromagnetic behavior and the complex magnetothermal phase diagram exhibiting multiple phase transformations, as observed from magnetization and magnetotransport measurements. A complete analysis of these phase transformations in light of electronic correlation and its impact on the underlying magnetic interactions remain unexplored in the existing literature. Using first-principles methodologies, incorporating the dynamical nature of electron correlation, we have analyzed the interplay of the direction of magnetization in an easy-plane and easy-axis manner with the underlying crystal symmetry, which reveals the opening of a pseudogap feature beyond the spin-reorientation transition temperature. The impact of dynamical correlation on the calculated magnetic circular dichroism and x-ray absorption spectrum of the $L$-edge of Fe atoms compare well with existing experimental observations. The calculated intersite Heisenberg exchange interactions display a complicated nature, depending upon the pairwise interactions among the two inequivalent Fe sites, indicating a Ruderman-Kittel-Kasuya-Yosida-like behavior of the magnetic interactions. We note the existence of significant anisotropic and antisymmetric exchange interactions, resulting in a chirality in the magnetic behavior of the system. Subsequent investigation of the dynamical aspects of magnetism in ${\mathrm{Fe}}_4\mathrm{Ge}{\mathrm{Te}}_2$ and the respective magnetothermal phase diagram reveals that the dynamical nature of spins and the decoupling of the magnetic properties for both sites of Fe is crucial to explain all the experimentally observed phase transformations.

Altermagnetism exhibits unique physical properties such as spin-momentum locking, anomalous Hall effect, nontrivial topological phase, and giant magnetoresistance. Among all the predicted candidates, several room-temperature altermagnets are suggested to host significant potential applications. $\mathrm{Ru}{\mathrm{O}}_2$ has been proposed as the most promising candidate. However, recently, there is intense debate about whether $\mathrm{Ru}{\mathrm{O}}_2$ exhibits magnetic order or not. Experiments by several different technologies claim the collinear magnetic order and spin-splitting-induced effects. However, very recent muon spin resonance (μSR) results reveal no magnetic order in $\mathrm{Ru}{\mathrm{O}}_2$, which indicates that the time-reversal symmetry is not broken. Direct observation of the high-resolution bulk band structure is absent to date but essential to investigate the electronic structure of $\mathrm{Ru}{\mathrm{O}}_2$. In this study, utilizing soft x-ray angle-resolved photoemission spectroscopy, we report systematic direct experimental observation of bulk band structure of $\mathrm{Ru}{\mathrm{O}}_2$.

We investigated spin-lattice coupling through detailed structural, magnetic, and lattice-dynamics studies of the $S=1$ quasi-one-dimensional lattice ${\mathrm{Ni}}_2{\mathrm{V}}_2{\mathrm{O}}_7$ compound by using x-ray diffraction, magnetic susceptibility $[\chi$ (T)], heat capacity $\left(C_P\right)$, x-ray absorption spectroscopy, x-ray photoelectron spectroscopy, and Raman spectroscopy. Further, the detailed study of lattice dynamics by first-principles calculation on ${\mathrm{Ni}}_2{\mathrm{V}}_2{\mathrm{O}}_7$ is presented. Our magnetic susceptibility $\chi$ (T), agreeing well with heat-capacity $\left(C_P\right)$ measurements, conclusively shows that the antiferromagnetic transitions occur at $T_{N1}\sim 6.7\mathrm{K}$ and $T_{N2}\sim 5.8\mathrm{K}$. However, a broad peak centered at around $T_{\text{max}}\sim 100\mathrm{K}$ in the $C_P/T$ data along with the divergence of the susceptibility from Curie-Weiss behavior indicates the formation of short-range magnetic correlations $\sim 100\mathrm{K}$, which is well above $T_{N1}$. The temperature-dependent x-ray diffraction analysis suggests Ni-Ni bond shortening and ${\mathrm{NiO}}_6$ octahedral distortion significantly below $\sim 100\mathrm{K}$. Further, from detailed lattice dynamics, two Raman modes showed anomalous lattice softening below $\sim 100\mathrm{K}$ which indicates the formation of the Ni atom dimerization as supported by ordering of the $J_1$ exchange interaction. All the other Raman modes soften below $\sim 30\mathrm{K}$. This suggests that the different exchange interactions order at different temperatures. This again supports the presence of short-range magnetic correlations well above $T_{N1}$ which is in good agreement with magnetization and heat-capacity data. Importantly, the spin-lattice coupling has been established in ${\mathrm{Ni}}_2{\mathrm{V}}_2{\mathrm{O}}_7$ below $\sim 100\mathrm{K}$ and the spin-lattice coupling constant $\left({\lambda }_{\text{sp}}\right)$ for various Raman modes has been deduced which shows multiferroic behavior below $T_{N1}$. The isostructural change observed in x-ray diffraction and Raman spectroscopy below 100 K provides structural origin for the multiferroic behavior shown by this compound.

The compounds LaPt(Ge, Si) and LaNiSi are exciting platforms to investigate the possibility of topological superconductivity due to their noncentrosymmetric (NCS) crystal structure and the presence of nodal-line Weyl electronic dispersion. Here, we investigate the effect of pressure on the superconducting and structural properties of LaPtGe through resistivity and x-ray-diffraction (XRD) studies. Our findings reveal a two-step increase of superconducting transition temperature $\left(T_c\right)$ from 3.06 K (0 GPa) to 3.94 K (6 GPa), with the rate of 0.071 K/GPa up to ∼3 GPa, and beyond that, it increases to a 2.5 times higher rate of 0.183 K/GPa up to 6 GPa. High-pressure XRD confirms the structural stability up to 7.82 GPa, though a change in lattice compression behavior is observed. The two distinct compressions of unit-cell parameters are observed with the bulk modulus of ∼144 GPa (below 3 GPa) and ∼162 GPa (above 3 GPa), derived using the Birch-Murnaghan equation of state. Magnetotransport studies up to 2.45 GPa show an increase in the upper critical field $\left[H_{c2}\left(0\right)\right]$ from 0.7 to 0.92 T, with a positive curvature of $H_{c2}\left(T\right)$, suggesting the possibility of multigap superconductivity in LaPtGe, similar to LaPtSi. Further, it indicates that the lattice compression effects primarily drive the enhancement of $T_c$, and it may be due to the modifications in the density of states at the Fermi level. Our results provide insights into pressure-tuned superconductivity mechanisms in NCS compounds.

Most properties of semiconductor devices are dominated by shallow impurities. However, deep defects often play an important role, for instance, in recombination processes or high field transport. While a variety of techniques are available to assess the density and energy levels of impurities, other properties, such as the recombination mechanisms of the defects, escape observation. We report on the direct measurement of hot electrons generated by trap-assisted Auger-Meitner recombination (TAAR) in GaN p-i-n diodes. By performing electron emission spectroscopy (EES) on diodes with surfaces activated to negative electron affinity by cesium, we observe the expected overflow electrons of p-i-n diodes under low current injection. However, when operating the devices at higher current densities, as low as ∼25 A/$\mathrm{c}{\mathrm{m}}^2$, we measure the emission of high-energy electrons. At variance with the observed hot electrons in light-emitting diodes (LEDs) using EES, the hot electrons generated in p-i-n diodes at our tested currents cannot be from eeh Auger-Meitner recombination due to the diodes' significantly lower carrier densities compared to those in LEDs. During our measurements, we observe the emission of accumulated electrons with energies ∼0.42 eV, ∼0.99 eV, ∼1.43 eV, and ∼2.32 eV above the conduction-band minimum (CBM) at various bias conditions, suggesting the existence of conduction-band features in GaN at these energies where electrons can be long-lived, such as satellite-valley minima and inflection points. We also measure incompletely relaxed hot electrons approaching energies 1.97 ± 0.13 eV and 2.94 ± 0.13 eV above the CBM, as the diodes are biased to high currents, suggesting at least some of the TAAR partaking defects have an energy level $\gtrsim 1.97$ eV and $\gtrsim 2.94$ eV away from either the conduction or valence band edges. Additionally, at our highest operating currents, we measure hot electrons with energies 3.28 ± 0.13 eV above the CBM, providing direct evidence of TAAR processes involving shallow impurities. This unexpected observation of TAAR in GaN p-i-n diodes spotlights the importance of further studies of defects in GaN and the necessity to incorporate the multi-phonon emission, radiative, and TAAR capture steps of defect-assisted recombination cycles into device modeling. Furthermore, this experiment demonstrates the applicability of the simplest semiconductor structures, p-i-n diodes, as a test bed to study the rich recombination physics of semiconductor materials.

We report the emergence of a square-shaped skyrmion lattice in multiorbital $f$-electron systems with easy-axis magnetic anisotropy on a centrosymmetric square lattice. By performing mean-field calculations for an effective localized model consisting of two Kramers doublets, we construct the low-temperature phase diagram in a static external magnetic field. Consequently, we find that a square-shaped skyrmion lattice with the skyrmion number of one appears in the intermediate-field region when the crystal field splitting between the two doublets is small. Furthermore, we identify another double-$Q$ state with a nonzero net scalar chirality at zero- and low-field regions, which is attributed to the help of the multiorbital degree of freedom. Our results offer another route to search for skyrmion-hosting materials in centrosymmetric $f$-electron tetragonal systems with multiorbital degrees of freedom, e.g., Ce-based compounds. This contrasts with conventional other $f$-electron systems hosting skyrmion lattices, such as Gd- and Eu-based compounds without orbital angular momentum.

We consider Bloch states of weak spacetime-periodic perturbations of homogeneous materials in one spatial dimension. The interplay of the periodicity of space and time leads to an infinitely degenerate dispersion relation in the free case. We consider a general perturbation term, and, as consequence of the infinite degeneracy, we show that the effective equations are given by the eigenvalue problem of an infinite matrix. Our method can be viewed as a time-modulated generalization of the nearly-free-electron model. Based on this result, we find that the infinite degeneracy may split into a family of nondegenerate bands. Our results are illustrated with numerical calculations, and we observe close agreement between the perturbation theory and the numerically computed full solution.

The exploration of spin configurations and magnetoresistance in van der Waals magnetic semiconductors, particularly in the realm of thin-layer structures, is of paramount significance for the development of two-dimensional spintronic nanodevices. In this Letter, we present detailed magnetotransport and photoluminescence studies on few-layer CrSBr flakes utilizing a vertical tunneling device configuration. Our investigations revealed complex magnetic states along the evolutionary path of few-layer CrSBr. We observed intermediate states exhibiting identical net magnetization demonstrate rectification properties, reminiscent of a diodelike behavior at positive and negative bias voltages. Notably, in devices with five-layer CrSBr, we detected an intriguing positive magnetoresistive state when subjected to an in-plane magnetic field along the $b$ axis. The implementation of the Mott two-current model successfully calculated the tunneling resistance of different magnetic states, thereby elucidating the spin configurations responsible for the observed transport phenomena. These insights not only provide different perspectives on the intricate spin textures of two-dimensional CrSBr, but also highlight the efficacy of tunneling measurements as a sensitive method for probing magnetic order in van der Waals materials.

We observe a twisted magnetocaloric effect (MCE) in a ferrimagnetic ${\mathrm{SmCaCoMnO}}_6$-antiferromagnetic ${\mathrm{Sm}}_{0.1}{\mathrm{Ca}}_{0.9}{\mathrm{MnO}}_3$ composite due to large exchange bias (EB) coupling (maximum EB $\sim 9.45$ kOe at 2 K with a bias field of $\sim 10$ kOe). The isothermal magnetization curves cross each other at high fields, and the crossover field increases with decreasing temperature. This leads to distinct moving peaks at the low temperature $(\le 15 \mathrm{K})$ in the nonmonotonic entropy change vs temperature curves, referred to as twisted MCE. The EB is also observed at low temperatures $(\le 15 \mathrm{K})$, due to the spin glass formation at the interface of ${\mathrm{SmCaCoMnO}}_6$ and ${\mathrm{Sm}}_{0.1}{\mathrm{Ca}}_{0.9}{\mathrm{MnO}}_3$. Similar materials systems without EB do not show the twisted MCE. Our proposed analytical model correlates the twisted MCE with the interacting anisotropies of spin glass and EB coupling at the interface. The coupled MCE and EB opens up new possibilities of simultaneous spintronic device applications and magnetocaloric refrigeration, leading to self-cooled electronics.

Vortex states in thin film disks host spin-wave modes that are geometrically quantized according to their radial and azimuthal indices. Previous studies have shown that hybridization between these modes and the vortex core results in a sizable frequency nonreciprocity between low-order clockwise and counterclockwise propagating azimuthal modes. Here, we present a computational study of these spin-wave modes in submicron disks in which the spatial extension of the vortex core becomes comparable to the wavelength of certain modes. In such cases, we find that the frequency nonreciprocity can be large even for higher-order radial and azimuthal indices, reaching several GHz and comparable to the mode frequencies themselves.

We study the phase ordering dynamics of the classical antiferromagnetic $J_1\text{−}{J}_2$ (nearest-neighbor and next-nearest-neighbor couplings) Heisenberg model on the square lattice in the strong frustration regime ($J_2/J_1>1/2$). While thermal fluctuations preclude any long-range magnetic order at finite temperatures, the system exhibits a long-range spin-driven nematic phase at low temperatures. The transition into the nematic phase is further shown to belong to the two-dimensional Ising universality class based on the critical exponents near the phase transition. Our large-scale stochastic Landau-Lifshitz-Gilbert simulations find a two-stage phase ordering when the system is quenched from a high-temperature paramagnetic state into the nematic phase. In the early stage, collinear alignments of spins lead to a locally saturated Ising-nematic order. Once domains of well-defined Ising order are developed, the late-stage relaxation is dominated by curvature-driven domain coarsening, as described by the Allen-Cahn equation. The characteristic size of Ising-nematic domains scales as the square root of time, similar to the kinetic Ising model described by the time-dependent Ginzburg-Landau theory. Our results confirm that the late-stage ordering kinetics of the spin-driven nematic, which is a vestigial order of the frustrated Heisenberg model, belongs to the dynamical universality class of a nonconserved Ising order. Interestingly, the system shows no violation of the superuniversality hypothesis under weak bond disorder. The dynamic scaling invariance is preserved in the presence of weak bond disorder. We also discuss possible applications of our results to materials for which vestigial Ising-nematic order is realized.

We investigate the dissipative quench dynamics in a family of two-band fermionic systems by linearly ramping the staggered on-site energy. In the Lindblad formalism, we present an analytical solution in the presence of uniform loss or loss difference on bipartite lattices, which tells that dissipation exponentially suppresses the Kibble-Zurek (KZ) scaling behavior and the quantum jump term of the dissipation is responsible for the anti-KZ (AKZ) behavior. Interestingly, we find two different scaling behaviors at the limit of loss difference. Both scaling behaviors arise from the gapless Liouvillian. But one is accompanied by impulse stage rendered by the criticality of the system, so that it is ascribed to the universal KZ scaling law. Another depends on the dissipation strength and there is no impulse stage in it. We also point out a convenient way to observe the two new scaling behaviors by counting the number of residual particles in the end, since it is immune to the influence of AKZ behavior. We illustrate our findings through the prototypical one-dimensional Rice-Mele model first. Then, in the one-dimensional Shockley model and the two-dimensional Haldane model for Chern insulators, we show that the two scaling behaviors can appear together or separately with appropriate quench protocols.

A bilayer formed by stacking two distinct materials creates a moiré lattice, which can serve as a platform for novel electronic phases. In this paper we study a unique example of such a system: the graphene-black phosphorous heterostructure (G/BP), which has been suggested to have an intricate band structure. Most notably, the valence band hosts a quasi-one-dimensional region in the Brillouin zone with high density of states, suggesting that various many-body electronic phases are likely to emerge. We derive an effective tight-binding model that reproduces this band structure, and explore the emergent broken-symmetry phases when interactions are introduced. Employing a mean-field analysis, we find that the favored ground state exhibits a striped spin density wave (SDW) order, characterized by either one of twofold degenerate wave vectors that are tunable by gating. Further exploring the phase diagram controlled by gate voltage and interaction strength, we find that the SDW-ordered state undergoes a metal to insulator transition via an intermediate metallic phase, which supports striped SDW correlations. Possible experimental signatures are discussed, in particular, a highly anisotropic dispersion of the collective excitations, which should be manifested in electric and thermal transport.

In this work, we present an empirical force field for hexagonal boron nitride (hBN) monolayers, derived via a bottom-up strategy from first-principles calculations. We aim to deliver a simple analytical force field for boron nitride which is efficient and applicable to large-scale simulations, without compromising its accuracy. The force field is developed with the goal to analytically reproducing the potential energy contributions arising from planar bond-stretching and bond-angle-bending deformations, as well as from out-of-plane torsional deformations, as obtained from periodic density functional theory calculations. Analytical anharmonic potential energy functions were employed to describe and parametrize the force field through a fitting process. The potential is applied for estimating in-plane (stiffness matrix and elastic constants) and out-of-plane (bending and Gaussian rigidity) mechanical properties of monolayer hBNs using both molecular mechanics calculations and analytical formulas. Additionally, we determined the elastic properties of more complex nanostructures, including hBN nanotubes and nanotori. By combining a continuum model for bent hollow beams with the fundamental definition of bending rigidity, we validate the model predictions against an analytical expression for the bending rigidity of large-diameter hBN nanotubes, expressed as a function of their radius and the in-plane Young modulus.

Recently, two-dimensional (2D) high-entropy transition metal dichalcogenides (TMDCs), which combine the characteristics of high-entropy and 2D materials, have been successfully synthesized in experiments and garnered considerable attention. However, research on point defects in 2D high-entropy TMDCs remains limited. Therefore, we focus on investigating the structure and formation energy of the common point defects of the 2D high-entropy $\left(\mathrm{WMoNbTaV}\right){\mathrm{S}}_2$ monolayer using first-principles calculations. By comparing the low-, medium-, and high-entropy TMDC materials, we found that among the point defects, the single sulfur vacancy $\left({\mathrm{V}}_{\mathrm{S}1}\right)$ and S-M antisite defects $\left({\mathrm{S}}_{\mathrm{M}}\right)$ with relatively small defect formation energies are most easily formed. From a single ordered structure to a multicomponent disordered structure, the distribution of defect formation energies broadens, especially for the five-component $\left(\mathrm{WMoNbTaV}\right){\mathrm{S}}_2$ monolayer. This is caused by the increase in the types of metallic elements, which leads to a greater complexity in the local chemical environments around the defects. Further examination of the effects of local chemical environments on vacancy formation energies revealed a correlation between the vacancy formation energies of multicomponent 2D TMDCs and those of their single-component counterparts. This study elucidates the influence of local chemical environments on the point defect properties of 2D high-entropy TMDCs, offering new insights and strategies for controlling defect generation in these systems.

Using the density matrix renormalization group algorithm, we map the ground-state phase diagram of a two-leg Rydberg ladder array with lattice spacings $a_x=2a_y$. We identify various density wave phases that spontaneously break the translational symmetry or the top-bottom reflection symmetry within the ladder. By increasing the laser detuning from zero, where the system is in a disordered phase that preserves all symmetries, we observe density wave orders with spontaneous breaking of the translational ${\mathbb{Z}}_p$ symmetries at intermediate detuning values, while the reflection symmetry is preserved. These orders exhibit nonzero bond orders with positive expectation values on every $p\mathrm{th}$ rung, thus labeled as ${\mathbb{Z}}_p^{+}$ phases. At larger detuning values, another spontaneous breaking of the reflection symmetry, which disrupted the bond orders on the rungs, occurs via an Ising phase transition. In these phases, either the top or the bottom site is occupied in a staggered way on every $p\mathrm{th}$ rung, breaking the translational ${\mathbb{Z}}_{2p}$ symmetry, thus labeled by ${\mathbb{Z}}_{2p}$ phases. We locate and characterize the 3-state Potts point and Ashkin-Teller point along the commensurate lines, as well as the direct chiral phase transitions between the disordered phase and the ${\mathbb{Z}}_p^{+}$ $(p=3,4)$ phases. Critical exponents $\nu$ and $z$ are calculated for both conformal and chiral phase transition points. We finally identify two types of floating phases in the phase diagram: one characterized by a quasi-long-range incommensurate bond-order wave, and the other by a quasi-long-range incommensurate wave of density differences in the rungs. Our work motivates further applications of Rydberg atom arrays in quantum simulation.

We use semiclassical Boltzmann transport theory to analytically study the electronic contribution to the linear thermoelectric response of anisotropic two-dimensional materials subjected to a perpendicular magnetic field. Conventional methods, such as the relaxation-time approximation within the Boltzmann formalism, often yield qualitatively incorrect results when applied to anisotropic mediums. On the other hand, the vector mean free path provides exact solutions for the linearized Boltzmann transport equation, and in principle, we can systematically evaluate it to any desired order in the external magnetic field. After laying out the general formalism of the magnetothermoelectric responses of anisotropic systems based on the vector mean free path approach, we study the longitudinal and Hall charge conductivities, as well as the Seebeck and Nernst coefficients of two representative anisotropic two-dimensional electronic systems: a semi-Dirac semimetal and a two-dimensional system of tilted massless Dirac fermions.

In recent years, the nonlinear anomalous thermal Hall effect has attracted substantial attention. In this paper, we carry out a theoretical exploration of the intrinsic anomalous thermal Hall and Nernst effect that is induced by the thermal Berry connection polarizability. This effect is independent of the relaxation time and can be present in antiferromagnets possessing $\mathcal{PT}$ symmetry. Additionally, we put forward a second-order intrinsic Wiedemann-Franz law, which represents the ratio of the second-order intrinsic thermal conductivity coefficient to the second-order intrinsic electrical conductivity coefficient. When analyzed within a four-band $\mathcal{PT}$ symmetric Dirac model, we observe that the second-order intrinsic thermal conductivity coefficient is linearly proportional to the second-order intrinsic electrical conductivity coefficient and the second-order intrinsic Wiedemann-Franz law is characterized by the chemical potential $\mu$ in the low-temperature regime. These findings provide significant implications for experimental verification.