The kagome-lattice Hubbard model attracts widespread interest due to its flat-band and Van Hove singularity features, which can give rise to unconventional magnetism. We employ determinant quantum Monte Carlo simulations to systematically investigate the uniform magnetic susceptibility across a range of on-site interactions and electron fillings on a two-dimensional kagome lattice. Beyond the Van Hove singularity, dominant ferromagnetic fluctuations emerge. Magnetic susceptibility grows markedly with increasing interaction strength and decreasing temperature, indicating that the Van Hove singularity acts as a critical point for the crossover of dominant magnetic fluctuations. Finite-size analysis further suggests the potential stabilization of a finite-temperature ferromagnetic phase. We also examine the sign problem to identify numerically reliable parameter regimes. These results provide valuable insights into controlling magnetic fluctuations in kagome systems and establish a computational framework for exploring flat-band physics in regimes characterized by novel quantum phases and competing orders.

The dependence of the magnetic fluctuations and the spin coherence time $T_2^{\mathrm{hyp}}$ of the lowest Stark states ${}^4{I}_{15/2}\left(Z_1\right)$ in ${}^{167}\mathrm{Er}^{3+}:{\mathrm{Y}}_2{\mathrm{SiO}}_5$ under zero magnetic field on Er concentration is numerically investigated in the range of 10 to 100 parts per million (ppm). We investigate two primary sources of magnetic fluctuation limiting spin coherence: a constant contribution from host Y nuclei and a concentration-dependent component from dipole-dipole interactions among Er ions. Due to these two components, the Er-concentration dependence of $T_2^{\mathrm{hyp}}$ at the zero first-order Zeeman (ZEFOZ) points saturates for crystals with Er concentration below 10 ppm and no extension of the $T_2^{\mathrm{hyp}}$ is expected without an external magnetic field. Under a magnetic field, the longest $T_2^{\mathrm{hyp}}$ at a particular ZEFOZ point is expected to be over 170 s (90 s) for site 1 (site 2), which is more than ${10}^4$ times longer than that at zero field for 10-ppm ${}^{167}\mathrm{Er}^{3+}:{\mathrm{Y}}_2{\mathrm{SiO}}_5$. Remarkably, these optimal ZEFOZ points form striking geometric patterns: a line for site 1 and a plane for site 2. This trend, which is favorable for experiments, can be explained by the anisotropy of the effective spin Hamiltonian parameters. Finally, the tolerance of the ZEFOZ point at each site with the longest $T_2^{\mathrm{hyp}}$ against the errors in the applied magnetic field vector is evaluated.

Non-Hermitian systems have attracted significant theoretical interest due to their extreme properties. However, realizations have mostly been limited to classical applications or artificial setups. In this study, we focus on the quantum nature inherent in bosonic Bogoliubov–de Gennes (BdG) systems, which from the perspective of spectral theory corresponds to non-Hermiticity. Based on this insight, we propose a steady-state skin effect in quantum condensed matter utilizing such BdG non-Hermiticity. Specifically, we introduce BdG quantum terms arising from parametric pumping to the edge states of an underlying bosonic Hermitian Chern insulator, thereby realizing non-Hermiticity without dissipation. This system design has the advantage of being largely independent of microscopic model details. Through analysis using nonequilibrium Green's functions, we find that under open boundary conditions, a steady state exhibiting the non-Hermitian skin effect is realized. The pronounced corner particle accumulation observed in this steady state shows quadrature anisotropy, which manifests the bosonic quantum nature. Our results bridge the gap between the fascinating mathematics of non-Hermitian matrices and practical quantum physical systems.

We develop a practical approach to electrically tuning the nonlinear photoresponse of two-dimensional semiconductors by explicitly incorporating a static out-of-plane electric field into the electronic ground state prior to optical excitation, as a gate bias. The method is implemented by dressing a Wannier-interpolated Hamiltonian with the field through its position matrix elements, which allows the gate bias to modify orbital hybridization and band dispersion beyond perturbative treatments. Within the independent-particle approximation, the resulting second-order (shift) conductivity is evaluated for both centrosymmetric and noncentrosymmetric layered systems. Applied to ${\mathrm{MoS}}_2$, the approach captures the emergence of a finite shift current in centrosymmetric bilayers and the tunability of intrinsic responses in polar structures. The shift conductivity rises linearly at small fields and saturates at higher intensities, reflecting the competition between the growing shift vector and the weakening interband coupling as resonant transitions move away from high-symmetry valleys. A Taylor expansion of the field-dressed conductivity connects this behavior to the third-order optical response, revealing a unified picture of field-induced nonlinearities. These results establish field dressing of Wannier Hamiltonians as a practical route to model and predict nonlinear photocurrents in layered materials.

The bulk photovoltaic effect (BPVE) refers to the direct current generation in a noncentrosymmetric material under illumination and can be applied to solar energy technology. BPVE includes injection and shift currents, driven by the change of velocity and displacement of wave packets during optical transitions, respectively. We focus on two-dimensional systems and derive the constraints on the conductivity tensors imposed by mirror-time ($\mathcal{MT}$) symmetry, which leads to orthogonal shift and injection current. For the Haldane model, we show that linearly polarized light can induce shift and injection currents. In contrast, circularly polarized light induces neither shift nor injection currents, as constrained by the three-fold rotation symmetry. Across the topological phase transition, the injection current does not change sign since the group velocity's sign remains unchanged. In contrast, shift current shows a sign flip, as a result of band inversion. Furthermore, we analyze the quantum geometric origins of the response, revealing that vortices in the symplectic connection lead to a suppression of the shift current magnitude in the topological regime. Experimentally, the predicted BPVE can be measured in the antiferromagnetic ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$ family, where the symmetry can be tuned by an external electric field, as well as in ultracold atomic systems.

We study the Josephson effect in junctions composed of two ferromagnetic insulator/diffusive superconductor bilayers separated by an insulating barrier. By computing the free energy of the system, we identify two distinct contributions: (i) The work performed by a current source to create a supercurrent through the junction, and (ii) an antiferromagnetic coupling between ferromagnetic insulators, mediated by the superconducting condensate across the insulating barrier. The competition between these contributions allows for switching between parallel and antiparallel configurations of the magnetizations of the ferromagnetic insulators. We explicitly show that the switching occurs at finite temperatures and for superconducting phase differences satisfying $\pi /2<\varphi <3\pi /2$. Importantly, this effect can be realized in ferromagnetic insulators with sufficiently large easy-plane anisotropy energy. Using realistic junction parameters, we demonstrate that the switching can be controlled by phase bias and triggered by half-flux-quantum voltage pulses or external magnetic field pulses on the microsecond timescale. These results provide a route towards controllable Josephson-based superconducting memory devices based on EuS/Al heterostructures.

The discovery of fractional Chern insulators (FCIs) unlocks exciting opportunities to explore emergent physical excitations arising from topological and geometric effects in novel phases of quantum matter. Here, we investigate the intraband neutral excitations, namely magnetorotons, in moiré FCIs within twisted ${\mathrm{MoTe}}_2$ by applying the generalized Girvin, MacDonald, and Platzman ansatz. Furthermore, we explore the quadrupole nature of magnetorotons in the long-wavelength limit and underscore the breakdown of volume-preserving symmetry due to the band fluctuations. Especially, we find that the band fluctuations brought by the lattice effects induce exotic behavior of magnetorotons in moiré FCI. Finally, we provide experimental proposals for detecting and advancing the study of intraband neutral excitations in moiré FCIs.

Structural anisotropy induces topological polaritons that have recently captivated current research exploring novel phenomena governed by non-Hermitian light-matter interaction, tunable by polarization of light. In this work we show polarization in Raman scattering, which is normally determined by the symmetry of vibrational modes, can be continuously controlled inside an optical microcavity. The evolution of polarization states emerging from the dynamics of a parity-time (PT)-symmetric system consists of two polarized Stokes modes in resonance with the polariton bands. The magnitude and sign of the ellipticity of polarization ellipse manifest three phases of a PT-symmetric system leading to the effect of handedness switching in Raman polarization as the external parameter is tuned across the exceptional point. Our work provides a distinct mechanism for the transformation of polarization states in anisotropic media under a strong coupling regime, enabling a different route for polarization control and laying the foundation for next-generation optical technologies.

The relaxation volume of point defects is known to be an important parameter for diffusion processes and interactions between defects in crystals. In this study, we investigate the effect of high pressure on the formation enthalpy and relaxation volume of point defects in magnesium oxide using first-principles calculations. Although energy corrections in charged systems are by now well established, the reliable evaluation of stress and pressure in charged periodic systems necessitates specific corrections to address the ill-defined electrostatic potential in first-principles codes. In this study, we extend a methodology based on the determination of the so-called hydrostatic absolute deformation potential applicable at high pressure. We show that, at ambient pressure, oxygen vacancies are associated with a positive relaxation volume, whereas the relaxation volume of magnesium vacancies is negative. With increasing pressure, all vacancy relaxation volumes decrease and become negative above 150 GPa. In addition, we demonstrate that the stability field of charged vacancies exhibits a pressure-dependent evolution, which correlates with the reduction of ionicity in MgO under pressure.

Magnetic topological materials are a class of compounds which can host massless electrons controlled by the magnetic order. One such compound is ${\mathrm{EuZn}}_2{\mathrm{Sb}}_2$, which has recently garnered interest due to its strong interplay between the Eu magnetism and charge carriers. However, the topology of the electronic band structure, which depends on the ground state magnetic configuration of the europium sublattice, has not been determined. Based on our ab initio calculations, we find that an in-plane and out-of-plane $A$-type antiferromagnetic (AFM) order generates a topological crystalline insulator and Dirac semimetal, respectively, whereas a ferromagnetic (FM) order stabilizes a Weyl semimetal. Our resonant x-ray elastic scattering measurements of single-crystal thin-film ${\mathrm{EuZn}}_2{\mathrm{Sb}}_2$ reveal both a sharp magnetic peak at $\mathit{Q}=\left(0,0,\frac{1}{2}\right)$ and broad $\mathit{Q}=(0,0,1)$ below $T_{\mathrm{N}}=12.9\mathrm{K}$, which is associated with an $A$-type AFM and FM order, respectively. Our measurements indicate that the FM and AFM layers are spatially separated along the crystal $c$ axis, with the former limited to the top three atomic layers. We propose that ${\mathrm{EuZn}}_2{\mathrm{Sb}}_2$ behaves as a Weyl semimetal in the surface FM layers, and as a topological crystalline insulator in the lower AFM layers.

Compounds containing ${\mathrm{Ag}}^{2+}$ ion with $4d^9$ configuration will cause significant Jahn-Teller distortions and orbital ordering. Such orbital order is closely related to the magnetic coupling, according to Goodenough-Kanamori rules. Our first-principles calculations reveal that the ground state of ${\mathrm{KAgF}}_3$ exhibits collinear A-type antiferromagnetic (A-AFM) ordering accompanied by C-type orbital ordering. In contrast, ${\mathrm{K}}_2{\mathrm{AgF}}_4$ adopts a collinear intralayer antiferromagnetic configuration coupled with ferromagnetic orbital ordering. The A-AFM ${\mathrm{KAgF}}_3$ presents distinct altermagnetic responses, including: (i) prominent anomalous transport effects, such as anomalous Hall conductivity (AHC), anomalous Nernst conductivity, and thermal anomalous Hall conductivity (TAHC); and (ii) strong magneto-optical responses, manifested through pronounced Kerr and Faraday effects. On the other hand, ${\mathrm{K}}_2{\mathrm{AgF}}_4$ behaves as a conventional collinear antiferromagnet preserving $\mathcal{PT}$ symmetry, hence precluding the emergence of an anomalous Hall response.

As an important member of metal nitrides, the rocksalt vanadium nitride (VN) shows an unusual coexistence of superconductivity and spin fluctuation, which provides a rare case for study of the interplay between these two exotic phenomena. However, due to the challenging sample synthesis its superconductive and magnetic properties have been sparsely explored, leading to many contradictory reports. Here we present a systematic study of VN based on high-quality single-crystal samples synthesized by a high-pressure method. Our results reveal that the material is a phonon-mediated type-II superconductor and exhibits weak magnetic response as potentially manifested by spin fluctuation that interacts weakly with superconductivity. Pressure enhances the electron-phonon coupling while it suppresses spin fluctuation, largely promoting its superconducting transition temperature ($T_c$) from 7.8 K at ambient pressure up to 12.8 K at $\sim 93\mathrm{GPa}$ with a 64% increase. The discovery of this work offers insights into how the superconductivity can be influenced by spin fluctuation.

Two-dimensional magnetic topological insulators, such as monolayer ${\text{V}}_2\text{W}{\text{S}}_4$, are promising platforms for realizing the quantum anomalous Hall (QAH) effect, with high Curie temperatures and large perpendicular magnetic anisotropy energies (MAEs). Using first-principles density functional theory calculations, we investigate the microscopic mechanism behind the large MAE in ${\text{V}}_2\text{W}{\text{S}}_4$. The MAE arises from two key contributions: a spin-orbit coupling (SOC)-driven term, originating from the strong SOC of W $5d$ orbitals, and a topology-assisted term near the band-inverted region. Both contributions favor out-of-plane magnetization, though they operate through different origins. The SOC-driven contribution arises from the intrinsic relativistic interaction, while the topology-assisted contribution is governed by the spin-orientation dependence of the electronic topology. Specifically, when the magnetization is in-plane, symmetry-protected nodal points near the Fermi level prevent the formation of a global gap. In contrast, out-of-plane magnetization lifts these degeneracies, opening a sizable topological gap with Chern number $C=-1$, leading to a greater energy separation between the two crossing bands and enhancing the MAE. Additionally, the energy and dispersion of these nodal points vary with the in-plane magnetization direction, introducing an additional anisotropy within the plane. As a result, the total MAE arises from the interplay between the SOC-driven and topology-assisted contributions, with the maximum MAE occurring when the system stabilizes the QAH insulating state with perpendicular magnetization.

${\mathrm{K}}_2{\mathrm{ReCl}}_6$ belongs to the antifluorite family and exhibits a sequence of structural transitions above the onset of magnetic order at $T_{\mathrm{N}}=12\mathrm{K}$. Because of its $5d^3$ electronic configuration in an octahedral coordination, the ground state is a pure spin state without orbital degeneracy within the $LS$ coupling scheme, but it can become Jahn-Teller active in the strong spin-orbit-coupling limit described by the $jj$ coupling [S. V. Streltsov and D. I. Khomskii, Phys. Rev. X 10, 031043 (2020)]. While the structural transitions in ${\mathrm{K}}_2{\mathrm{ReCl}}_6$ are understood in terms of octahedral rotation and tilting, the possible impact of a Jahn-Teller distortion remains an open issue. We report on comprehensive crystal-structure studies by means of powder neutron and single-crystal x-ray diffraction on ${\mathrm{K}}_2{\mathrm{ReCl}}_6$ and on ${\mathrm{K}}_2{\mathrm{SnCl}}_6$. The latter material is used as a reference, because it exhibits the same sequence of structural transitions as ${\mathrm{K}}_2{\mathrm{ReCl}}_6$ but possesses a filled $4d$ shell ruling out Jahn-Teller distortion. While the ${\mathrm{ReCl}}_6$ octahedron in ${\mathrm{K}}_2{\mathrm{ReCl}}_6$ presents sizable distortions at intermediate temperatures, there is no such distortion persisting to low temperatures excluding a sizable Jahn-Teller effect. Studies on polycrystalline samples of ${\mathrm{Rb}}_2{\mathrm{ReCl}}_6$ and ${\mathrm{Cs}}_2{\mathrm{ReCl}}_6$, in which the structural transitions are suppressed due to the larger alkaline ionic radius, also do not find any indications of Jahn-Teller distortion.

The accurate prediction of temperature-dependent magnetization dynamics is a fundamental challenge in computational magnetism. While atomistic spin dynamics simulations have emerged as a powerful tool for studying magnetic phenomena, their classical nature leads to significant deviations from experimental observations, particularly at low temperatures. Here we present a comprehensive implementation of quantum-corrected atomistic spin dynamics into the vampire software package, based on the open-system Landau-Lifshitz-Gilbert equation with a quantum thermostat. Our implementation incorporates memory effects along with colored noise derived from quantum-mechanical considerations that improve the description of the equilibrium magnetization. We demonstrate excellent quantitative agreement with experimental magnetization curves for nickel and gadolinium across the full temperature range. Our results establish that incorporating quantum environmental effects and colored noise substantially enhances the predictive capabilities of atomistic spin dynamics simulations, providing a robust framework for modeling temperature-dependent magnetic phenomena in localized moment magnetic systems.

The braiding of Majorana zero modes (MZMs) forms the fundamental building block for topological quantum computation. Braiding protocols which involve the physical exchange of MZMs are typically envisioned on a network of topological superconducting wires. An essential component of these protocols is the transport of MZMs, which can be performed by using electric gates to locally tune sections of the wire between topologically trivial and nontrivial phases. In this work, we numerically simulate this transport by tuning a single section of a superconducting wire which contains either disorder (uncorrelated and correlated) or noise. We focus on the impact of these additional effects on the diabatic error, which describes unwanted transitions between the ground state and excited states. We show that the behavior of the average diabatic error is predominantly controlled by the statistics of the minimum bulk energy gap which is suppressed in the presence of disorder. The increase in diabatic error can be several orders of magnitude and is most deleterious when the disorder correlation length is a finite fraction of the transport distance and negligible when these lengths are far apart. In the presence of noise, the diabatic error is significantly enhanced due to optical transitions which depend on the minimum bulk energy gap as well as the frequency modes present in the noise. The results presented here serve to further characterize the diabatic error in disordered and noisy settings, which are important considerations in practical implementations of physical braiding schemes.

The historical calculation of spectroscopic properties for trivalent lanthanide ions is a complex multistep process that has been prone to inaccuracies. In this work, we revise the parametric semi-empirical Hamiltonian and address long-standing discrepancies in the literature. We also resurface the distinctions between orthogonal and non-orthogonal operators, and use orthogonalized operators to provide an alternative parametric description. Based on experimental data available in the literature, an updated set of parameter values for the canonical case of lanthanide ions in ${\text{LaF}}_3$ is presented. Additionally, we provide calculations of spontaneous emission rates and oscillator strengths for magnetic dipole transitions in the ${\text{LaF}}_3$ crystal host. To ensure the replicability of our findings, we make available the open-source code qlanth, accompanied by a comprehensive set of electronic files to serve as an updated reference for future calculations.