We use density functional methods to identify the atomic configurations of H and D atoms trapped by O impurities embedded in bulk Nb. The O atoms are located at the octahedral position in the Nb body-centered cubic (BCC) lattice, and H (D) atoms tunnel between two degenerate tetrahedral sites separated by a mirror plane. Using nudged elastic band (NEB) methods, we calculate the double-well potential for O-H and O-D impurities and the wave functions and tunnel splittings for H and D atoms. Our results agree with those obtained from analysis of heat capacity and neutron scattering measurements on Nb with low concentrations of O-H and O-D.

The transition-metal chalcogenides ${\mathrm{Fe}}_2{\mathrm{P}}_2{\mathrm{S}}_6$ and ${\mathrm{Ni}}_2{\mathrm{P}}_2{\mathrm{S}}_6$ have been widely studied in recent years due to their remarkable properties, such as strong electron correlations and the coexistence of antiferromagnetism and Mott physics, which make these materials attractive for exploring fundamental and applied 2D physics. One powerful approach to tailor the magnetic order in these 2D van der Waals compounds is to exploit the strong magnetoelastic coupling, which is essential for multifunctional applications. In this work, the magnetoelastic coupling and the uniaxial pressure dependence of the antiferromagnetic ordering temperature $T_{\text{N}}$ are studied and quantified by high-resolution dilatometry. Using the Clausius-Clapeyron and Ehrenfest relations, we find that uniaxial pressure applied along the stacking axis ($c^{*}$) enhances the $T_{\text{N}}$ in both compounds, which is verified via direct measurements for ${\mathrm{Fe}}_2{\mathrm{P}}_2{\mathrm{S}}_6$, i.e., magnetic susceptibility under applied uniaxial pressure. Remarkably, despite the orbitally degenerate ${\mathrm{Fe}}_2{\mathrm{P}}_2{\mathrm{S}}_6$ exhibiting a much stronger magnetoelastic coupling compared to the nondegenerate ${\mathrm{Ni}}_2{\mathrm{P}}_2{\mathrm{S}}_6$, their uniaxial pressure dependence remains of the same order of magnitude, suggesting that it primarily arises from the van der Waals interactions between adjacent sulfur layers.

We microscopically construct an Abelian mutual Chern-Simons lattice gauge theory for magnetic field-tuned topological transitions in the Kitaev model and obtain a complete characterization of the phases, including their quasiparticles. At low fields for both ferro (FM) and antiferromagnetic (AFM) Kitaev interactions, we demonstrate non-Abelian Ising topological order (ITO), and explicitly construct the Majorana anyon as an intrinsic excitation—a twist defect in our Abelian gauge theory. For the AFM case, an Abelian chiral phase appears at intermediate fields with trivial topological order and fermionic bulk excitations. Remarkably, both the ITO phase and the intermediate phase have the same chiral central charge $c=1/2,$ implying no change in the quantized thermal Hall response across the transition. For the FM case, there is a direct transition from ITO to a partially polarized nontopological phase. Our paper completes the proof of Kitaev's original proposal of the low-field ITO, with $c=1/2$ going beyond his mean-field arguments by including the crucial effect of the gauge fluctuations, and provides a resolution of the debate surrounding the intermediate field phase in the AFM case.

Higher-order topological insulators, as a novel family of topological phases, are a hot frontier in condensed-matter physics and classical waves due to their adherence to unconventional bulk-boundary correspondence. A three-dimensional second-order topological insulator can support one-dimensional modes along its hinges (dubbed “hinge states”). Here, we present a simple and direct method to construct chiral hinge modes based on a Chern-insulator stack. We analyze the existence of the hinge modes through the nontrivial quadrupole indices and then design a photonic antiferromagnetic crystal to realize the specific flowing pattern of the hinge mode in our model. The experimental results align well with full-wave simulations, clearly demonstrating the existence of chiral hinge states. We also verify the robustness of these hinge states against defects in our photonic system.

In this Letter, we theoretically report a large family of two-dimensional (2D) transition metal sulfides $M_2\mathrm{S}$ with a MXene structure in the $2H$ and $1T$ phases, which we refer to as MSene. Twenty-four out of fifty-eight MSenes are proved to be stable. Notably, this family includes twelve superconducting (SC) materials, seven SC topological metals (SCTMs), four charge density wave (CDW) materials, and five magnetic materials including one ferromagnetic (FM) and four $A$-type antiferromagnetic (AFM) materials. For example, $2H-{\mathrm{Mo}}_2\mathrm{S}$ is a SCTM which exhibits a SC critical temperature ($T_c$) of 10.2 K and nontrivial topological properties; $1T-{\mathrm{Hf}}_2\mathrm{S}$ is a CDW material with the CDW originating from electron-phonon coupling. The CDW can be suppressed by compressive strain, leading to the emergence of superconductivity; $2H-{\mathrm{Cr}}_2\mathrm{S}$ and $1T-{\mathrm{Mn}}_2\mathrm{S}$ show FM and $A$-type AFM properties, respectively. Thus, the large family we predicted shows rich physical properties and significantly expands the repertoire of 2D materials. It serves as a platform for investigating the competition or coexistence of multiple orders such as SC, CDW, FM, AFM, and topological orders in 2D materials.

Materials composed of spin-1 antiferromagnetic (AFM) chains are known to adopt complex ground states that are sensitive to the single-ion-anisotropy (SIA) energy ($D$), and intrachain ($J_0$) and interchain ($J_{1,2}^{\prime }$) exchange energy scales. While theoretical and experimental studies have extended this model to include various other energy scales, the effect of the lack of a common SIA axis is not well explored. Here we investigate the magnetic properties of $\mathrm{Ni}\left(\mathrm{pyrimidine}\right){\left({\mathrm{H}}_2\mathrm{O}\right)}_2{\left({\mathrm{NO}}_3\right)}_2$, a chain compound where the tilting of Ni octahedra leads to a twofold alternation of the easy-axis directions along the chain. Muon-spin relaxation measurements indicate a transition to long-range order at $T_{\text{N}}=2.3\mathrm{K}$ and the magnetic structure is initially determined to be antiferromagnetic and collinear using elastic neutron diffraction experiments. Inelastic neutron scattering measurements were used to find $J_0=5.107\left(7\right)\mathrm{K}, D=2.79\left(1\right)\mathrm{K},J_1^{\prime }=0.00\left(5\right)\mathrm{K}, J_2^{\prime }=0.18\left(3\right)\mathrm{K}$, and a rhombic anisotropy energy $E=0.19\left(9\right)\mathrm{K}$. Mean-field modeling reveals that the ground state structure hosts spin canting of $\varphi \approx 6.5^{\circ }$, which is not detectable above the noise floor of the elastic neutron diffraction data. Monte Carlo simulation of the powder-averaged magnetization, $M\left(H\right)$, is then used to confirm these Hamiltonian parameters, while single-crystal $M\left(H\right)$ simulations provide insight into features observed in the data.

The search for new magnetic quantum materials with unique electronic properties has become the focal point in condensed matter physics. One effective way to study the unusual electronic properties is via Hall effect measurements, which stand as a pivotal experimental avenue for probing novel magnetotransport phenomena. We report the experimental investigation of the Hall effect in ${\mathrm{EuZn}}_2{\mathrm{As}}_2$, which is within the realm of emergent magnetic materials. Careful analysis of the Hall resistivity reveals the presence of the ordinary Hall resistivity $\left({\rho }_{xy}^0\right)$, anomalous Hall resistivity $({\rho }_{xy}^{\mathrm{AHE}})$, and residual Hall resistivity $(\mathrm{\Delta }{\rho }_{xy})$ above and below the antiferromagnetic transition temperature $T_N=19\mathrm{K}$. Both ${\rho }_{xy}^{\mathrm{AHE}}$ and $\mathrm{\Delta }{\rho }_{xy}$ are large and spread in a wide temperature and magnetic field range. Our results indicate unusually strong magnetic field and topological effects in ${\mathrm{EuZn}}_2{\mathrm{As}}_2$, despite its simple crystal and magnetic structure at zero field.

Spin current mediated synaptic devices have recently gained increasing attention for their potential in designing and developing artificial spiking neural network (SNN) circuits. The integration of these devices is hindered by the need for external magnetic fields, which introduces design complexity and limits their practical applicability compared to complementary metal-oxide-semiconductor technology. Here, we have demonstrated autospiking and autoresetting functionalities in spin-torque vortex oscillators (STVOs) through a vortex core switching mechanism, all under field-free conditions. This switching is mediated through the creation of a vortex-antivortex (V-AV) pair, followed by the annihilation of the antivortex with the initial vortex as it reaches its critical velocity. We further utilized the unconventional spin-orbit torque to precisely control the dynamic motion of the magnetic vortex via pure spin current injection and its impact on spike-timing characteristics and neuromorphic functionality. The designed STVOs-based SNNs demonstrate classification accuracy of 96% on the Modified National Institute of Standards and Testing (MNIST) handwritten digit dataset. The performance of the STVO-based SNN has also been evaluated on more complicated, computer vision dataset, such as Neuromorphic-MNIST and Fashion-MNIST, demonstrating its capability in classifying various types of datasets, with the added advantage of being a nanoscale, low-power device compared to standard “leaky-integrate-and-fire” model-based SNNs. This innovation of eliminating external magnetic fields for STVOs synaptic functionality is paving the way for more efficient neuromorphic spintronic devices for enhanced spatiotemporal processing and pattern recognition capabilities.

The anomalous transverse transport properties, including the anomalous Hall and Nernst effects, are crucial for probing the topological band features in magnetic materials. In this study, we present a comprehensive investigation of the magnetic, electrical, and thermal transport, as well as the electronic band structure, of the noncollinear ferromagnet ${\mathrm{PrMn}}_2{\mathrm{Ge}}_2$. Our findings reveal that ${\mathrm{PrMn}}_2{\mathrm{Ge}}_2$ exhibits prominent anomalous Hall and Nernst effects. Scaling analysis suggests that the anomalous Hall effect is predominantly influenced by the intrinsic Berry curvature contribution. Additionally, the large anomalous Nernst signal exceeds the magnetization scaling relations observed in conventional ferromagnets, and the calculated anomalous Nernst conductivity exhibits a $T\text{ln}T$ relation. These observations align with the behavior seen in other magnetic topological materials, further suggesting a substantial intrinsic Berry curvature effect. Complemented by detailed theoretical calculations, we clarify the electronic band properties and confirm a substantial net Berry curvature near the Fermi level in this compound. Our work elucidates that the large anomalous Hall and Nernst effects of ${\mathrm{PrMn}}_2{\mathrm{Ge}}_2$ are dominated by an intrinsic mechanism and presents it as a probable magnetic topological material with a significant Berry phase effect in the momentum space.

Multilayer moiré materials can exhibit topological electronic features yet are inherently quasiperiodic—leading to wave-function interference whose Anderson-localizing tendency can be mitigated by topology. We consider a quasiperiodic variant of the chiral Bistritzer-MacDonald model for twisted bilayer graphene with two incommensurate moiré potentials that serves as a toy model for twisted trilayer. We observe “filaments” linking magic angles with enhanced density of states and fractal wave functions that evade localization; states away from the filaments mimic fractal surface states of dirty topological superconductors. We demonstrate that topological quasiperiodicity can broadly enhance superconductivity without magic-angle fine-tuning.

In this paper, we present an exact formula for the phonon linewidths involving only dressed electron-phonon couplings and ensuring the positivity property. The formula is designed to account for both nonadiabatic and correlation effects, and it provides an alternative proof that density functional perturbation theory calculations of phonon linewidths are not affected by a double counting of screening diagrams. Furthermore, we extend the treatment to nonequilibrium scenarios and offer a rigorous justification for employing the phononic Boltzmann equation.

Our experimental (neutron diffraction, Mössbauer spectroscopy, magnetic susceptibility, specific heat) and numerical studies on the evolution of short- and long-range magnetic order in ${\gamma }_{\mathrm{II}}\text{−}{\mathrm{Li}}_2{\mathrm{FeSiO}}_4$ suggest a quasi-two-dimensional (2D) nature of magnetism. The experimental data obtained on single crystals imply long-range antiferromagnetic order below $T_{\mathrm{N}}=17$ K. A broad maximum in magnetic susceptibility $\chi$ at $T_{\mathrm{m}}\simeq 28$ K, observation of magnetic entropy changes up to 100 K, and anisotropy in $\chi$ are indicative of low-dimensional magnetism and suggest short-range magnetic correlations up to 200 K. Neutron diffraction shows that long-range antiferromagnetic order is characterized by the propagation vector $\mathbf{k}=(\frac{1}{2},0,\frac{1}{2})$. The ordered moment $\mu =2.50\left(2\right)$ ${\mu }_{\mathrm{B}}$/Fe, at $T=1.5$ K, is along the crystallographic $a$ axis. This is consistent with the observed static hyperfine field of $B_{\mathrm{hyp}}=14.8\left(3\right)\mathrm{T}$ by Mössbauer spectroscopy which indicates significant orbital contributions. The temperature dependence of $B_{\mathrm{hyp}}$ yields the critical exponent $\beta =0.116\left(12\right)$ which is in the regime of the 2D Ising behavior. $\mathrm{LSDA}+\mathrm{U}$ studies exploiting the experimental spin structure suggest dominating magnetic exchange coupling within the $ac$ layers (i.e., $J_3\simeq -6$ K and $J_6\simeq -2$ K) while interlayer coupling is much smaller and partly frustrated. This confirms the 2D nature of magnetism and is in full agreement with the experimental findings.

Chiral fermionic quasiparticles emerge in certain quantum condensed matter systems such as Weyl semimetals, topological insulators, and spin-orbit-coupled noncentrosymmetric metals. Here, a comprehensive theory of the chiral anomaly induced nonlinear Hall effect (CNLHE) is developed for three-dimensional chiral quasiparticles, advancing previous models by rigorously including momentum-dependent chirality-preserving and chirality-breaking scattering processes and global charge conservation. Focusing on two specific systems [Weyl semimetals (WSMs) and spin-orbit-coupled noncentrosymmetric metals (SOC-NCMs)], we uncover that the nonlinear Hall conductivity in WSMs shows nonmonotonic behavior with the Weyl cone tilt and experiences a “strong-sign reversal” with increasing internode scattering, diverging from earlier predictions. For SOC-NCMs, where nonlinear Hall conductivity has been less explored, we reveal that unlike WSM, the orbital magnetic moment alone can drive a large CNLHE with distinctive features: the CNLH conductivity remains consistently negative regardless of interband scattering intensity and exhibits a quadratic dependence on the magnetic field, contrasting the linear dependence in WSMs. Furthermore, we discover that in SOC-NCMs the spin Zeeman coupling of the magnetic field acts like an effective tilt term which can further enhance the CNLH current. These findings offer fresh insights into the nonlinear transport dynamics of chiral quasiparticles and can be verified in upcoming experiments on such materials.

In this study, we numerically investigate the transport properties of disordered topological insulators (TIs) with a domain wall (DW) structure. Three typical models are studied, including the magnetic TI, the Haldane model, and the Kane-Mele model, with Bloch or Néel walls. For the magnetic TI with a Bloch wall, it is found that weak disorder enhances the average conductance $\langle G\rangle$ and in the diffusive regime $\langle G\rangle$ exhibits a universal behavior independent of the DW configurations. Remarkably, when a gate voltage is applied, a plateau of $\langle G\rangle =N/2$ emerges in the diffusive regime, with $N$ being the number of edge modes. Within this plateau, the conductance distribution $P\left(G\right)$ is found to be uniform. In the localized regime, the variance of logarithmic conductance ${\sigma }^2(lnG)$ shows a universal linear scaling with its average $\langle lnG\rangle$ independent of DW configurations, gate voltages, and system sizes, which is different from the $2/3$ power law in normal metallic systems. The scaling behaviors of higher-order statistics, including the skewness, the kurtosis, and the third and fourth cumulants on $\langle lnG\rangle$ are also discussed. Furthermore, we show that the uniform conductance distribution of $\langle G\rangle$ = $N/2$ is a universal feature in all three model systems, which is attributed to the complete mixing of edge states from different domains at the domain wall. Finally, we map out the phase diagrams of uniform conductance distribution and analytically derive all statistical quantities associated with the uniformly distributed conductance from full-counting statistics.

The construction of tight-binding arrays for specific spectral emulations follows intuitive rules when the strength of couplings is engineered by adjusting distances between atoms. However, the nonorthogonality of adjacent orbitals introduces further corrections that might be mistaken as second-neighbor interactions. In this paper, we find a simple way to correct nonorthogonal orbitals by introducing a metric tensor in Hilbert space that renormalizes coupling constants, energies, and on-site potentials. This technique produces the expected results for previously engineered systems, such as finite oscillators in a nondilute regime. The renormalization of parameters constitute a way to circumvent the use of a Wannier transform. Armed with this formalism, we analyze the influence of nonvanishing orbital overlaps in typical models with and without phase transitions, such as Harper's model with irrational magnetic flux and the uncorrelated Anderson's model. The results show that previously existing transitions in the transmission are smoothed by the nonvanishing overlap. Also, spectral deformations and asymmetric bands in deformed tight-binding arrays are found. Applications to electromagnetic resonators and elastic beams are briefly discussed.

In a Hermitian photonic system, a Dirac point can split into a pair of exceptional points (EPs) or even form a ring of EPs when non-Hermiticity is introduced. Herein, we present a type of non-Hermitian Dirac point situated in the complex plane of eigenfrequency. When differential loss is present, the Dirac point exhibits dual behavior: first, it splits into a pair of EPs with opposite chirality in the band structure; second, it induces a pair of circularly polarized states (C points) with opposite handedness in the far-field radiation. Furthermore, the breaking of the corresponding mirror symmetries allows independent control of these Dirac-point-induced C points, enabling the merging of two C points and the generation of unidirectional guided resonances. Our results demonstrate a clear relationship between branch-point singularities and polarization singularities, providing a new mechanism for generating unidirectional emission. This can be beneficial for band engineering and polarization manipulation.

The use of the Green's function in quantum many-body theory often leads to nonlinear eigenvalue problems, as the Green's function needs to be defined in the energy domain. The $GW$ approximation method is one typical example. In this article, we introduce a method based on the FEAST eigenvalue algorithm for accurately solving the nonlinear eigenvalue $G_0{W}_0$ quasiparticle equation. Based on the FEAST contour integral method for nonlinear eigenvalue problems, the energy (eigenvalue) domain is extended to the complex plane. A hypercomplex number is introduced to the calculation of the $GW$ self-energy to carry the imaginary parts of both Green's functions and FEAST contour quadrature nodes. Calculation results for various molecules are presented and compared with the more conventional graphical solution approximation method and spectral functions method. It is confirmed that the highest occupied molecular orbital from the Kohn-Sham (and Hartree-Fock) equation is very close to that of $GW$, while the lowest unoccupied molecular orbital shows noticeable differences.

Single-orbital Hubbard models exhibit remarkably nontrivial correlation phenomena, even on nonfrustrated bipartite lattices. Some of these, like non-Fermi-liquid metal states, or the coexistence of heavy and light quasiparticles, are reminiscent of the properties of more complex multiorbital or Kondo-lattice systems. Here, we use basis transformations to map single-orbital models to effective multiorbital descriptions and clarify how a ferromagnetic Kondo-lattice-like behavior emerges in prototypical models with flat bands or van Hove singularities in the density of states: the Hubbard model on the diamond chain, square lattice, Lieb lattice, and honeycomb lattice. In particular, this mapping provides insights into mechanisms leading to the non-Fermi-liquid states and pseudogaps found in the correlated metal regime.

We study the Yang-Lee theory in quantum phase transitions from the perspective of quantum entanglement in one-dimensional many-body systems. We primarily focus on the distribution of Yang-Lee zeros and its associated Yang-Lee edge singularity of two prototypical models: the Su-Schrieffer-Heeger model and the XXZ spin chain. By taking the zero-temperature limit, we show how the Yang-Lee zeros approach the quantum phase transition points on the complex plane of parameters. To characterize the edge singularity induced by Yang-Lee zeros in a quantum phase transition, we introduce the entanglement entropy of the ground state to show that the edges of Yang-Lee zeros lead to the ground-state entanglement transition. We further show that our results are also applicable to the general noninteracting parity-time-symmetric Hamiltonians.