This paper studies the performance of time-dependent density-functional theory (TDDFT) for calculating the dielectric function of semiconductors and insulators at finite momentum transfer, comparing against the standard Bethe-Salpeter equation (BSE). Specifically, we consider a recently proposed hybrid approach that mixes dielectrically screened exact exchange with a semilocal functional, and we also introduce a hybrid functional featuring a truncated dielectric screening scheme. The computational effort of these hybrid TDDFT approaches is significantly less than that of the BSE, but they deliver comparable accuracy (for the same input band structures), as demonstrated for the semiconductors Si and GaN and the wide-band insulator LiF. This opens up possibilities for calculating exciton dispersions and electron energy loss functions efficiently and accurately for a wide range of materials.

Recent studies showed that topologically trivial insulators may have fractionally quantized corner charges due to the topological invariant called a filling anomaly. Such crystal shapes in three dimensions are restricted to vertex-transitive polyhedra, which are classified into spherical and cylindrical families. The previous works derived formulas of the fractional corner charge for the spherical family, which corresponds to the tetrahedral and cubic space groups (SGs). In this study, we derive all the corner charge formulas for the cylindrical family, which corresponds to the orthorhombic, tetragonal, hexagonal, and trigonal crystal shapes. We show that all the real-space formulas of the filling anomaly for the cylindrical SGs are universally determined by the total charges at the Wyckoff position $1a$. Moreover, we derive the $k$-space formulas of the corner charge for the cylindrical cases with time-reversal symmetry. From our results, we also show that ${\mathrm{CsLi}}_2{\mathrm{Cl}}_3, {\mathrm{KN}}_3$, and ${\mathrm{Li}}_3\text{N}$ are candidate materials with a quantized corner charge by using the ab initio calculations. Together with our previous work, we exhaust corner charge formulas for all the SGs and crystal shapes having quantized corner charges.

This study aims to clarify the physics of the transition that is observed in $\mathrm{P}{\mathrm{r}}_3\mathrm{C}{\mathrm{o}}_4\mathrm{S}{\mathrm{n}}_{13}$ (PCS) by specific heat at ∼138 K, which resembles the formation of a charge density wave (CDW). The work is performed using synchrotron radiation based scattering and spectroscopic techniques that include temperature-dependent x-ray diffraction (XRD), x-ray scattering (XRS), x-ray absorption near-edge structure (XANES)/extended x-ray absorption fine structure (EXAFS), resonant inelastic x-ray spectroscopy (RIXS), valence-band photoemission (VB-PES), and first-principles calculation. The appearance of satellite features in the XRD and XRS studies revealed a modulation of the structure along a new $q$ vector (0.5,0.5,0) below 120 K. Phonon calculations suggest softening of a phonon mode and a possible superlattice transition at low temperatures. The XANES spectra of the Sn K, Co $L_{3,2}$, and Pr $M_{5,4}$ edge revealed an increase of charge transfer (CT) at low temperatures as a result of an enhancement of $\mathrm{Co}3d\text{−}\mathrm{Sn}5p$/$\mathrm{Pr}4f$, and $\mathrm{Pr}4f\text{−}\mathrm{Sn}5p$ hybridizations. The EXAFS study at the Sn K edge further suggested local distortion of the Sn sites because of Sn-Sn bond elongation at low temperatures. The VB-PES study and theoretical calculations that are based on spin-polarized density functional theory together confirm the strength of hybridization and the lack of any significant change in the band gap of the PCS with temperatures. The RIXS spectra at the Co $L_3$ edge provide further evidence of the CT process, and the energy level of Co $3d$ states below the Fermi level were found to be static in the temperature range of 80 to 300 K, implying no change in the band gap of the PCS with temperatures. Based on the relevant results, the appearance of a clear kink around 138 K in the temperature-dependent specific heat measurements is caused by local lattice distortion at the Sn sites by a diffusionless mechanism, resulting in minimal alteration of the electronic structure through Sn-Sn bond elongation and widening of the Sn-Co-Sn bond angle in PCS. Thus, this study suggests that the system does not undergo a CDW transformation unlike many other similar materials.

In periodic crystalline systems, topological insulators are uniquely characterized by modern polarization concepts, analogous to Wannier centers. Here, we show that second-order topological insulators (SOTIs) arise throughout the full spectrum of a phase transition between degenerate photonic bands, due to the combination of nonsymmorphic glide symmetry and time-reversal symmetry in all-dielectric photonic crystals. We find that the hybridization of Wannier functions causes the center of the maximally localized Wannier function to lie at the unit cell origin rather than a site with nonzero net polarization. This mechanism leads to corner states originating from obstructed Wannier functions. By examining the local density of states, we confirm and characterize the second-order topology, revealing an additional type of corner state—type-III—which differs from the conventional type-I and type-II. These findings broaden the exploration of higher-order topological physics and hold promise for integrated and quantum photonics.

Scale-free localization emerging in non-Hermitian physics has recently garnered significant attention. In this work, we explore the interplay between scale-free localization and Anderson localization by investigating a unidirectional quasiperiodic model with generalized boundary conditions. We derive analytical expressions of the Lyapunov exponent from the bulk equations. Together with the boundary equation, we can determine properties of eigenstates and spectra and establish their exact relationships with the quasiperiodic potential strength and the boundary parameter. While eigenstates exhibit scale-free localization in the weak-disorder regime, they become localized in the strong-disorder regime. The scale-free and Anderson-localized states satisfy the boundary equation in distinct ways, leading to different localization properties and scaling behaviors. Generalizing our framework, we design a model with exact energy edges separating the scale-free and Anderson-localized states via the mosaic modulation of quasiperiodic potentials. Our models can be realized experimentally in electric circuits.

We construct a general theory of spin magnetization driven by chiral phonons under an adiabatic process, in which atoms rotate around their equilibrium positions with a low phonon frequency. Here the spin magnetization originates from the modulated electronic states with spin-orbital coupling by atomic rotations. Under the adiabatic approximation, the time-dependent spin magnetization can be calculated by a Berry-phase method. In this paper, we focus on its time average, which is evaluated by assuming that the phonon displacement is small. As a result, the time average of the spin magnetization is concisely formulated in the form of the Berry curvature defined in the phonon-displacement space as an intrinsic property of atomic rotations. Our formula for spin magnetization reflects the chiral nature of phonons, and is convenient for ab initio calculations.

We investigate the spin dynamics and low-energy magnetic excitations of an S = 3/2 spin zigzag chain system in the van der Waals material $\mathrm{AgCr}{\mathrm{P}}_2{\mathrm{S}}_6$ using electron spin resonance (ESR) and Raman spectroscopy. The temperature-dependent resonance intensity and $g$ factor reveal the development of critical correlations as the temperature approaches the ordering temperature. Additionally, the temperature-dependent linewidth of the magnetic resonance exhibits power-law behavior, characteristic of low-dimensional spin systems. The twofold angular dependence of the ESR linewidth and the significant magnetic Raman scattering in interchain ($bb$) polarization suggests coupling between the zigzag $\mathrm{C}{\mathrm{r}}^{3+}$ chains. Furthermore, anomalies in the ESR parameters and the Fano mode observed in Raman data around T* ≈ 120 K, together with an order-parameter-like shift of the magnetic spectral weight toward T*, indicate a change in the nature of the underlying magnetic excitations, induced by non-negligible next-nearest neighbor interactions and interchain couplings, which become effective below T*.

The antiferromagnetic (AFM) spintronics have aroused comprehensive interest due to the fast magnetic dynamic response and lack of stray fields. However, probing and manipulating AFM order has always been a scientific challenge due to the lack of macroscopic statistical magnetic moments and insensitivity to external fields. So, the discovery, control, and detection of AFM materials has important scientific and practical significance. In this paper, we propose a strategy to control and detect the AFM order through electrical and optical methods, and demonstrate its feasibility with a group of one-dimensional (1D) materials $\mathrm{Mo}{X}_3$($X=\mathrm{Cl}$, Br, I) as examples. The basic concept is to use the antisymmetric Compton profile to distinguish the ferrotoroidic order and antiferrotoroidic order in the 1D AFM spin chain based on the magnetoelectric multipole theory. $\mathrm{Mo}{X}_3$ is predicted to be an AFM semiconductor by the anisotropic Heisenberg model and Monte Carlo simulation. Moreover, under hole doping the magnetic ground state can switch between two different AFM states with different symmetry and short-range vortex order. More interesting, two different AFM orders, respectively, correspond to 1D ferrotoroidic and antiferrotoroidic states. Due to the existence of toroidal moment in ferrotoroidic state, the Compton scattering provides an intuitive detection of magnetic configurations by optical method. Through symmetry analysis and theoretical calculations, we show that the antisymmetric Compton profile serves as a detectable fingerprint that can be used to distinguish the symmetry-dependent toroidic phase transition or AFM orders. One-dimensional materials have potential application prospect in the development of miniaturization and high-density integration of devices while maintaining 1D electron transport. Therefore, the tunable and detectable AFM order endows 1D $\mathrm{Mo}{X}_3$ chain with great possibility for future applications in AFM spintronics.

We consider quantum spin chains with a hidden free fermionic structure, distinct from the Jordan-Wigner transformation and its generalizations. We express selected local operators with the hidden fermions. This way, we can exactly solve the real-time dynamics in various physical scenarios, including the computation of selected dynamical two-point functions, in continuous or discrete time. In the latter case, we build a quantum circuit that can be implemented on a quantum computer. With this, we extend the family of classically simulable quantum many-body processes.

The Fermi sea of a metal can host exotic quantum topology, which governs its conductance quantization and is characterized by the Euler characteristic (${\chi }_F$). In contrast to the well-known band topology, determined by the global features of wave functions, the topology of such metallic systems is intrinsically linked to the geometry of the Fermi sea. Consequently, probing and identifying ${\chi }_F$ in high-dimensional systems remains challenging. Here we propose a generic dimensional reduction theory for the Fermi sea topology in $d$-dimensional metallic systems, showing that ${\chi }_F$ can be determined by the features of so-called reduced critical points on Fermi surfaces. Moreover, we reveal that ${\chi }_F$ can be interpreted as a topological invariant of band topology through a mapping from metallic systems to gapped systems. Building on this result, we identify a broad class of topological superconductors (SCs) whose topological numbers are precisely determined by the ${\chi }_F$ of their normally filled bands. This provides an indirect method to capture ${\chi }_F$ by measuring the (pseudo)spin polarizations of these topological SCs. Our findings are expected to significantly advance research into Fermi sea topology.

We compute the second-order correction to the electronic dispersion relation of a free electron gas interacting with an effective electron-dislocation potential, derived from a modern quantized theory of dislocations. Our results demonstrate that dislocation patterning induces anisotropic flat bands in the electronic dispersion under specific strain fields and directions, referred to as “magic” parameters. These flat bands acquire nonzero curvature as the strain or direction deviates from these magic parameters.

We study the dynamics of critical spin fluctuations and hot electrons at the metallic antiferromagnetic quantum critical points with $Z_2$ and O(2) spin symmetries, building on earlier works on the O(3) symmetric theory. The interacting theories in $2+1$ dimensions are approached from $3+1$-dimensional theories in the $\epsilon$ expansion that tunes the codimension of Fermi surface as a control parameter. The low-energy physics of the $Z_2$ and O(2) theories qualitatively differ from each other and also from that of the O(3) theory. The difference is caused by higher-order quantum corrections beyond the one-loop order that are important even to the leading order in $\epsilon$. The naive loop expansion breaks down due to dynamical quenching of kinetic energy: the speed of the collective mode ($c$) and the Fermi velocity perpendicular to the magnetic ordering vector ($v$) become vanishingly small at low energies. What sets the three theories apart is the hierarchy that emerges between the quenched kinetic terms. At the infrared fixed point, $c/v$ becomes 0, 1, and $\infty$ in the $Z_2$, O(2), and O(3) theories, respectively. At intermediate energy scales, the slow renormalization group (RG) flows of $c$ and $v$ toward their fixed point values create approximate scale invariance controlled by approximate marginal parameters. The manifold of those quasifixed points and the RG flow therein determines crossovers from scaling behaviors with transient critical exponents at intermediate energy scales to the universal scaling in the low-energy limit. If the symmetry group is viewed as a tuning parameter, then the O(2) theory corresponds to a multicritical point which has one additional quasimarginal parameter than the other two theories.

Two-dimensional (2D) semiconductors, known for their small size and high responsiveness, are key to advancing nanoscale nonlinear optical devices. 2D III–VI semiconductors like InSe have enabled transistors with speeds exceeding silicon limits. Recently, the 2D semiconductor $\alpha \text{-I}{\mathrm{n}}_2\mathrm{S}{\mathrm{e}}_3$ has achieved large single crystals, with its noncentrosymmetric nature and ferroelectric polarization showing promise for nonlinear optical applications. Here, we investigate its Janus structures, formed by metal substitution in $\alpha \text{-I}{\mathrm{n}}_2\mathrm{S}{\mathrm{e}}_3$ to produce $\mathrm{InGaS}{\mathrm{e}}_3$ and $\mathrm{GaInS}{\mathrm{e}}_3$, and examine their enhanced nonlinear optical properties. We find that the metal substitution significantly boosts the second-harmonic generation (SHG) response in $\alpha \text{-I}{\mathrm{n}}_2\mathrm{S}{\mathrm{e}}_3$, with the second-order susceptibility of $\mathrm{GaInS}{\mathrm{e}}_3$ reaching 256.9 pm/V at 444 nm, over eight times that of commercial ${\mathrm{LiNbO}}_3$. This enhancement is attributed to changes in the conduction band near the Fermi level due to metal substitution. Polarization-resolved SHG reveals that the vertical and parallel components of $\mathrm{InGaS}{\mathrm{e}}_3$ differ by nearly 100 times, a significant anisotropy absent in $\alpha \text{-I}{\mathrm{n}}_2\mathrm{S}{\mathrm{e}}_3$ and $\mathrm{GaInS}{\mathrm{e}}_3$. These findings highlight the potential of metal-substituted $\mathrm{InGaS}{\mathrm{e}}_3$ as a strong, anisotropic nonlinear optical material for compact photonic chips and other devices.

Nanowire quantum dots, i.e., heterostructures consisting of an axial insertion of low-band-gap semiconductor within a large-band-gap semiconductor nanowire, attract interest due to their emerging applications in the field of quantum communication technology. Here, we report on the fabrication of ZnTe/(Zn,Mg)Te nanowire quantum dots by molecular beam epitaxy and on a detailed investigation of the optical emission from individual structures by means of a combined study involving cathodoluminescence and micro-photoluminescence. A distinct dependence of the biexciton binding energy, defined as the spectral distance between the exciton and biexciton emission lines on the height of ZnTe insertions along the axis, is observed. With increasing dot height, not only does the biexciton binding-energy value decrease distinctly, but also its character changes from binding to antibinding. The explanation of this effect relies on the appearance of a piezoelectric field along the nanowire axis, leading to a pronounced separation of electrons and holes. The change from a bound to an unbound character of biexciton energy can be well reproduced by theoretical calculations, which indicates an important contribution of excited hole states to this effect in the case of relatively large quantum dots.

We demonstrate that a disordered magnetic Weyl semimetal may be mapped onto a two-dimensional array of coupled replicated Hubbard chains, where the Hubbard $U$ is directly related to the variance of the disorder potential. This is a three-dimensional generalization of a similar mapping of the two-dimensional quantum Hall plateau transition to a one-dimensional Hubbard chain. We demonstrate that this mapping leads to the conclusion that the Weyl semimetal becomes a diffusive metal with a nonzero density of states at arbitrarily weak disorder, in agreement with recent work. We also discuss the absence of localization in strongly disordered Weyl semimetals from the viewpoint of this mapping.

The presence of Rashba-like surface states (RSS) in the electronic structure of topological insulators (TIs) has been a longstanding topic of interest due to their significant impact on electronic and spin structures. In this study, we investigate the interaction between topological and Rashba-like surface states (TSS and RSS) in ${\mathrm{Mn}}_{1-x}{\mathrm{Sn}}_x{\mathrm{Bi}}_2{\mathrm{Te}}_4$ systems using density functional theory (DFT) calculations and high-resolution angle-resolved photoemission spectroscopy (ARPES). Our findings reveal that increasing Sn concentration shifts RSS downward in energy, enhancing their influence on the electronic structure near the Fermi level. ARPES validates these predictions, capturing the evolution of RSS and their hybridization with TSS. Orbital analysis shows RSS are localized within the first three Te-Bi-Te trilayers, dominated by $\text{Bi-}p$ orbitals, with evidence of the orbital Rashba effect enhancing spin-momentum locking. At higher Sn concentrations, RSS penetrate deeper into the crystal, driven by Sn-$p$-orbital contributions. These results position ${\mathrm{Mn}}_{1-x}{\mathrm{Sn}}_x{\mathrm{Bi}}_2{\mathrm{Te}}_4$ as a tunable platform for tailoring electronic properties in spintronic and quantum technologies.

In our quest to unravel the topological properties of nodal points in three-dimensional semimetals, one hallmark property which warrants our attention is the chiral anomaly. In the Brillouin zone (BZ), the sign of the Berry-curvature field's monopole charge is referred to as the chirality $\left(\chi \right)$ of the node, leading to the notion of chiral quasiparticles sourcing chiral currents, induced by internode scatterings proportional to the chiral anomaly. Here, we derive the generic form of the chiral conductivity when we have multifold nodes. Since the sum of all the monopole charges in the BZ is constrained to vanish, the nodes appear in pairs of $\chi =\pm 1$. Hence, the presence of band-crossing degeneracies of order higher than 2 make it possible to have two distinct scenarios: the pair of conjugate nodes in question comprise bands of (1) the same pseudospin variety and exhibiting Berry-curvature profiles differing by an overall factor of $\chi$, or (2) two distinct pseudospin representations. Covering these two possibilities, we apply our derived formula to semimetals harboring triple-point (threefold-degenerate) and Rarita-Schwinger-Weyl (fourfold-degenerate) nodes, and show the resulting expressions for the conductivity featuring the chiral anomaly.

Discrete time crystals (DTCs) exhibit a special nonequilibrium phase of matter in periodically driven many-body systems with spontaneous breaking of time translational symmetry. The presence of decoherence generally enhances thermalization and destroys the coherence required for the existence of DTCs. In this paper, we devise a mechanism for establishing a stable DTC with period-doubling oscillations in an open quantum system that is governed by a properly tailored non-Hermitian Hamiltonian. We find a specific class of nonreciprocal couplings in our non-Hermitian dynamics which prevents thermalization through eigenstate ordering. This choice of non-Hermitian dynamics significantly enhances the stability of the DTC against imperfect pulses. Through a comprehensive analysis, we determine the phase diagram of the system in terms of pulse imperfection.

We propose a systematic recurrence method for the energy spectra of non-Hermitian systems under open boundary conditions based on the recurrence relations of their characteristic polynomials. Our formalism exhibits better accuracy and performance on multiband non-Hermitian systems than numerical diagonalization or the non-Bloch band theory. It also provides a targeted and efficient formulation for the non-Hermitian edge spectra. As demonstrations, we derive general expressions for both the bulk and edge spectra of multiband non-Hermitian models with nearest-neighbor hopping and under open boundary conditions, such as the non-Hermitian Su-Schrieffer-Heeger and Rice-Mele models and the non-Hermitian Hofstadter butterfly–two-dimensional lattice models in the presence of nonreciprocity and perpendicular magnetic fields, which is only made possible by the significantly lower complexity of the recurrence method. In addition, we use the recurrence method to study non-Hermitian edge physics, including the size-parity effect and the stability of the topological edge modes against boundary perturbations. Our recurrence method offers a favorable formalism to the intriguing physics of non-Hermitian systems under open boundary conditions.

We investigate the dynamics of the quantum Ising model on two-dimensional square lattices up to 16 $\times$ 16 spins. In the ordered phase, the model exhibits dynamically constrained dynamics, leading to confinement of elementary excitations and slow thermalization. After demonstrating the signatures of confinement, we probe the dynamics of interfaces in the constrained regime through sudden quenches of product states with domains of opposite magnetization. We find that the nature of excitations can be captured by perturbation theory throughout the confining regime and identify the crossover to the deconfining regime. We systematically explore the effect of the transverse field on the modes propagating along flat interfaces and investigate the crossover from resonant to diffusive melting of a square of flipped spins embedded in a larger lattice.