Ferrovalley (FV) materials are promising for practical applications in valley electronics due to their spontaneous valley polarization, and the control of the anomalous valley Hall effect (AVHE) by external means is crucial for the development of valley electronic devices. This work presents a novel two-dimensional FV non-Janus $\mathrm{Fe}X\text{I}$ ($X$ = Cl, Br). Compared to the half-metallicity of Janus $\mathrm{Fe}X\text{I}$, non-Janus $\mathrm{Fe}X\text{I}$ exhibits intrinsic FV semiconducting properties with a large valley splitting of 134 meV for FeClI and 129 meV for FeBrI due to the strong spin-orbit coupling. The AVHE can be realized by selective excitation of carriers at the valleys. However, most of the reported methods for controlling AVHE are either volatile or irreversible. To achieve nonvolatile reversible AVHE, we propose two models of multiferroic van der Waals heterojunctions (vdWHs) by introducing a ferroelectric (FE) substrate: FV/FE and FV/FE/FV. Based on the principle of band alignment, ${\mathrm{In}}_2{\mathrm{S}}_3$ and ${\mathrm{In}}_2{\mathrm{Se}}_3$ are selected as the FE substrates for FeClI and FeBrI, respectively. By modulating the direction of FE polarization, reversible switching of AVHE is achieved in FV/FE vdWHs, and layer-dependent Hall voltage is further obtained in FV/FE/FV vdWHs. Our findings validate the feasibility and effectiveness of FE modulation of AVHE and provide new insights into the design of valley electronics.

The phenomenon of oxygen adsorption induced surface restructuring is widespread across various metal-oxygen systems, yet its impact on initiating bulk oxide formation remains largely unexplored. Through in situ atomic-resolution electron microscopy observations of surface oxidation of Cu(110) and $\mathrm{C}{\mathrm{u}}_{85}\mathrm{A}{\mathrm{u}}_{15}$(110), we unveil intermittent oxide-film growth modulated by oxygen-induced surface restructuring. This modulation is evidenced by repeated pinning of the $\mathrm{C}{\mathrm{u}}_2\mathrm{O}$ growth front at isolated Cu columns of the $c$(6×2)-O reconstruction, owing to required long-range diffusion of Cu and O atoms to the $\mathrm{C}{\mathrm{u}}_2\mathrm{O}$ growth front. We reveal that Cu vacancies, generated at the $\mathrm{C}{\mathrm{u}}_2\mathrm{O}$ growth front, are injected into the $\mathrm{C}{\mathrm{u}}_2\mathrm{O}$/Cu interface, inducing hill and valley undulation of the $\mathrm{C}{\mathrm{u}}_2\mathrm{O}$ film. In contrast, atomic vacancies produced during the $\mathrm{C}{\mathrm{u}}_{85}\mathrm{A}{\mathrm{u}}_{15}$(110) oxidation preferentially migrate into interfaces between Au-rich and Au-poor regions in the bulk, resulting in a flat and adherent $\mathrm{C}{\mathrm{u}}_2\mathrm{O}$ film. These findings demonstrate the critical role of oxygen-induced surface restructuring in modulating oxide film growth kinetics and the manipulability of the fate of injected vacancies by alloying, thereby offering insights applicable to a broader range of metal-oxygen systems for fine-tuning oxidation kinetics and enhancing oxide/metal interfacial adhesion.

The scattering of an electromagnetic field oscillating in a complex-frequency plane is known to capture and release energy within a lossless cavity, yielding virtual loss and virtual gain, respectively. In this context, we use electromagnetic with complex frequency to optimize both the longitudinal beam shifts (Goos-Hänchen shift) and transversal beam shifts (spin-Hall shift) in the transmitted field through the slab and scattered field by the sphere. The results demonstrate a significantly large gradient in the transmission amplitude due to the occurrence of scattering poles in the lower half plane of the complex frequency. In addition, the complex frequency allows the suppression of both forward and backward scattering by the spherical cavity. Consequently, the scattered plane undergoes a substantial shift, leading to an enhanced Goos-Hänchen shift and spin-Hall shift in both planar and spherical cavities. Importantly, this scheme relies on the incident-beam profile without the need for engineering materials parameters of the cavity. As a result, it surpasses previous methods that rely on materials engineering to optimize the beam shifts. To underscore the broad applicability and versatility of the current formalism, we have chosen to implement it using the commercial software COMSOL and found perfect agreement with the analytical results.

Classical magnets exhibit exotic ground-state properties such as spin liquids and fractionalization, promising a manifestation of superposition and projective symmetry construction in classical theory. While system-specific spin-ice or soft-spin models exist, a formal theory for general classical magnets remains elusive. Here, we introduce a generic symmetry group construction built from a vector field in a plaquette of classical spins, demonstrating how classical spins superpose in irreducible representations (irreps) of the symmetry group. The corresponding probability amplitudes serve as order parameters and local spins as fragmented excitations. The formalism offers a many-body vector field representation of diverse ground states, including spin liquids and fragmented phases described as degenerate ensembles of irreps. We apply the theory to a frustrated square kagome lattice, where spin-ice or soft spin rules are inapt, to describe spin liquids and fragmented phases, all validated through irreps ensembles and unbiased Monte Carlo simulation. Our generic theory sheds light on previously unknown aspects of spin-liquid phases and fragmentation and broadens their applications to other branches of field theory.

A helical structure shows a naturally fluctuating behavior in charge distribution, analogous, to some extent, to the charge density wave. It is an open question whether superconductivity can be induced by suppressing the helical structure in a largely gapped system. Here, we report the superconductivity in chiral $\alpha$-HgS with a band gap of 1.9 eV after a pressure-driven helical-nonhelical transition. The maximum critical temperature ($T_c$) reaches 11 K at 25.4 GPa, and the $T_c$-critical magnetic field ($B_{c2}$) relation exhibits multiband features. Furthermore, an isostructural-like transition, together with a transition of direct-indirect band gaps, is presented due to the reduced distance of the helical chains near 8 GPa, at which the second harmonic generation shows a strong response. Phonon softening plays a key role in the stability of the helical structure and the emergence of superconductivity. This Letter may inspire the exploration of superconductivity and other new physics in other helical/chiral systems and will extend our understanding of the versatile behavior in such kinds of materials.

Magnetic catalysis is a known proposal for inducing dynamical axionic gapped phases by means of external magnetic fields from a Weyl or Dirac semimetal phase. At finite Fermi level, the phase transition is of first-order type and the magnetic field needs to reach a critical value for the transition to take place. Using the theory of bubble nucleation, we predict the order parameter features a hysteretic behavior as a function of the external magnetic field. We also analyze the experimental consequences of this hysteretic behavior in several observables like magnetoconductivity, magnetic susceptibility, and nonlinear optical coefficients. This hysteretic behavior might serve as a fingerprint of magnetic catalysis in condensed matter systems.

We study shot noise in a two-dimensional, disordered marginal Fermi liquid (MFL) driven out of equilibrium. We consider electrons with Planckian dissipation on the Fermi surface coupled to quantum-critical bosons in the presence of disorder. In the noninteracting and strong electron-boson drag limits, MFL effects disappear and our theory reproduces known results. For the case where the bosons remain in equilibrium, inelastic scattering strongly suppresses the noise by dissipating the injected energy. Interestingly, we find that MFL effects do play a role in this regime, and give a weak enhancement on top of the otherwise strong suppression. Our results suggest that shot noise can be strongly suppressed in quantum-critical systems, and this scenario may be relevant to recent measurements in a heavy-fermion strange metal.

Bound states in the continuum (BICs) with an infinite quality factor ($Q$ factor) and lifetime are demonstrated as far-field polarization singularities in momentum space. Unidirectional guided resonances (UGRs) are only a $V$ point (vortex polarization singularity) in the upper or lower half space to block the radiation channel. In photonic systems, the evolution between BICs and UGRs can be achieved by adjusting the structure parameters. However, the coexistence of BICs and UGRs has not been realized so far. Here, we propose a dielectric corrugated structure and find that the far-field radiation of the structure can be controlled by changing the phase difference between the upper and lower surfaces. The coexistence of BICs and UGRs can be achieved at a specified phase difference. Interestingly, as the $V$ points evolve with the phase difference, a high $Q$ factor platform is realized. Our results provide a means to achieve the high $Q$ factor platform and the coexistence of BICs and UGRs in momentum space, which promotes greatly the study of far-field radiation manipulation and high $Q$ factor resonance.

The ferroelectric topological crystalline insulator SnTe is a promising material for ferronics and spintronics studies and applications because of the strong spin-orbit coupling and surface states protected by the mirror symmetry of the crystal. In this paper, we report ferromagnetic resonance-driven spin-pumping experiments in several bilayers and trilayers having one layer of SnTe with thickness varying in the range 3–30 nm and different ferromagnetic layers used to inject the spin-pumped spin current. In all samples x-ray-diffraction spectra strongly indicate that the SnTe films grown are topological crystalline insulators and consequently have preserved topological surface states. The spin-pumping studies reveal that the dominant spin to charge conversion mechanism in SnTe is the inverse Rashba-Edelstein effect (IREE), evidencing that the topological surface states are preserved. From the data in all samples, we obtain for the IREE parameter values in the range 1.1–2.8 pm, which are comparable to the ones measured in graphene, $\mathrm{B}{\mathrm{i}}_2\mathrm{S}{\mathrm{e}}_3$, and others topological insulators.

Sliding ferroelectricity was originally proposed to elucidate the out-of-plane polarization generated by a specific stacking arrangement of nonpolar van der Waals layers. However, the concept of sliding ferroelectricity can be generalized to more geometries. Here, the ${\mathrm{NbI}}_4$ bulk is theoretically demonstrated as a quasi-one-dimensional sliding ferroelectric material, which exhibits a polarization of $0.11\mu \mathrm{C}/{\mathrm{cm}}^2$ perpendicular to the Nb's chains. The most possible ferroelectric switching path is found to be via the interchain sliding along the chain direction, while other paths such as Peierls dimerization of Nb pairs may also work. Moreover, its polarization can be augmented for $82\%$ by hydrostatic pressure up to $10\mathrm{GPa}$, beyond which ${\mathrm{NbI}}_4$ becomes a polar metal. In addition, negative longitudinal piezoelectricity is also predicted.

Quantum spin Hall insulators hosting edge spin currents hold great potential for low-power spintronic devices. In this paper, we present a universal approach to achieve a high and near-quantized spin Hall conductance plateau within a sizable bulk gap. Using a nonmagnetic four-band model Hamiltonian, we demonstrate that an even-spin Chern (ESC) insulator can be accessed by tuning the sign of spin-orbit coupling (SOC) within a crystal symmetry-enforced orbital doublet. With the assistance of a high-spin Chern number of $C_S=-2$ and spin $U\left(1\right)$ quasisymmetry, this orbital doublet driven ESC phase is endowed with the near double quantized spin Hall conductance. We identify 12 crystallographic point groups supporting such a sign-tunable SOC. Furthermore, we apply our theory to realistic examples, and show the phase transition from a trivial insulator governed by positive SOC in the ${\mathrm{RuI}}_3$ monolayer to an ESC insulator dominated by negative SOC in the ${\mathrm{RuBr}}_3$ monolayer. This orbital doublet driven ESC insulator, ${\mathrm{RuBr}}_3$, showcases nontrivial characteristics including helical edge states, near double quantized spin Hall conductance, and robust corner states. Our work provides different pathways in the pursuit of the long-sought quantum spin Hall insulators.

The simplest Weyl semimetal with broken time-reversal symmetry consists of a pair of Weyl nodes located at wave vectors ${\mathbf{K}}_{\tau }=\tau \mathbf{b}$ in momentum space with $\tau =\pm 1$ the node index and chirality. The electronic dispersion in a small wave-vector region near each node is linear and isotropic. In a magnetic field $\mathbf{B}=B\widehat{\mathbf{z}}$, this band structure is modified into a series of positive and negative energy Landau levels $n=\pm 1,\pm 2,...$ which disperse along the direction of the magnetic field, and a chiral Landau level $n=0$, with a linear dispersion given by $e_{\tau ,n=0}\left(k_z\right)=-\tau \hslash v_F{k}_z,$ where $k_z$ is the component of the electron wave vector $\mathbf{k}$ along the direction of the magnetic field and $v_F$ is the Fermi velocity. In the extreme quantum limit and for a small doping, the Fermi level is in the chiral levels near the Dirac point. It has been shown before that, when Coulomb interaction is considered, a Weyl semimetal may be unstable towards the formation of a condensate of internodal electron-hole pairs which gives rise in real space to an excitonic charge-density wave. This new state of matter is usually studied by using a short-range interaction between the electrons. In this paper we use the full long-range Coulomb interaction and the self-consistent Hartree-Fock approximation to generate the condensed state. We study its stability with respect to a change in the Fermi velocity, doping, and strength of the Coulomb interaction and also consider the situation where the Weyl nodes have a higher Chern number $C=2,3$ and more complex excitonic states are possible. We derive the response functions and collective excitations of the excitonic state working in the generalized random-phase approximation (GRPA). We show that, in the mean-field gap induced by the internodal coherence, there is, in the excitonic response function, a series of bound electron-hole states (excitons) with a binding energy that decreases until the renormalized Hartree-Fock energy gap is reached. In addition, there is a collective mode gapped at exactly the plasmon frequency. By contrast, the plasmon mode is the only excitation present in the density and current response functions. Despite the U(1) symmetry of the excitonic state, there is no gapless mode in the GRPA excitonic response. Indeed, the gapless mode present in the proper excitonic response function is pushed to the plasmon frequency by the long-range Coulomb interaction.

The phase diagrams and the nature of the phase transitions in multicomponent gauge theories with an Abelian gauge field are important topics with various physical applications. While an early renormalization-group-based study indicated that the direct transition from a fully ordered to a fully disordered state is continuous for $N=1$ and $N>183$, recently it was demonstrated that the transition is discontinuous for $N=2$. We quantitatively study the dependence on $N$ of the degree of discontinuity of this transition. Our results suggest that the transition is discontinuous at least up to $N=7$. Furthermore, we demonstrate that, at increased coupling strength, the phase transitions of the neutral and charged sectors of the model split, which for $N>2$ yields a new phase with composite order. The transition from the composite-order phase to the fully disordered phase is then also discontinuous, at least for $N=3$ and $N=4$. Via a duality argument, this indicates that van der Waals–type interaction between directed loops may be responsible for the discontinuous phase transitions in these models.

We study an effective field theory of a vortex lattice in a two-dimensional neutral rotating superfluid. Utilizing particle-vortex dualities, we explore its formulation in terms of a $U\left(1\right)$ gauge theory coupled to elasticity that at low energies reduces to a compact Lifshitz theory augmented with a Berry phase term encoding the vortex dynamics in the presence of a superflow. Utilizing elasticity- and Lifshitz-gauge-theory dualities, we derive dual formulations of the vortex lattice in terms of a traceless symmetric scalar-charge theory and demonstrate low-energy equivalence of our dual gauge theory to its elasticity-gauge theory dual. We further discuss a multipole symmetry of the vortex lattice and the multipole one-form symmetries of its dual gauge theory. We also study its topological crystalline defects, where the multipole one-form symmetry plays a prominent role. It classifies the defects, explains their restricted mobility, and characterizes descendant vortex phases, which includes a vortex-supersolid phase. Using the dual gauge theory, we also develop a mean-field theory for the quantum melting transition from a vortex crystal to a vortex supersolid.

Phosphorene and its components are highly reactive to oxygen when exposed to ambient conditions due to the presence of lone pairs of electrons on phosphorus atoms. Functionalization serves as a solution to prevent the chemical degradation of these materials. In this paper, we investigate the impact of relatively strong covalent or noncovalent functionalization on phosphorene [monolayer black phosphorus (BP)], few-layer BP, and bulk BP. We use an effective tight-binding Hamiltonian that corresponds to one orbital per site, wherein covalent functionalization is simulated by atomic vacancies, and noncovalent functionalization is simulated by Anderson disorder. We demonstrate that these two types of functionalization act differently on the electronic structure and quantum diffusion, particularly affecting the gap and mobility characteristics, especially with a high degree of functionalization. However, we also show that the mobility gap is not significantly modified by the two types of defects. We also analyze the electron-hole asymmetry that is more important for multilayer and bulk BP.

Recently, quantum anomalous Hall state has been observed in moiré transition metal dichalcogenide bilayers. Its topological physics can be explained by a band-inverted charge transfer insulator model, in which the topological phase transition occurs in the presence of the band inversion. Starting from an effective three-band low-energy model, we investigate the Landau levels and the magneto-optical conductivity of a band-inverted charge transfer insulator on the honeycomb lattice. We derive the real and imaginary parts of the longitudinal conductivity and Hall conductivity using Kubo formalism. We find that the magneto-optical conductivity indicates a discontinuity at the point of band inversion in the low-frequency regime, which can serve as a probe for band topology. It is shown that the charge transfer gap, chemical potential, and magnetic field have a sensitive effect on the magneto-optical conductivity. The unique band structure also changes the peaks in the imaginary part of the Hall conductivity into two distinct contributions of opposite signs. We also study the relationship of the band-inversion signature and transport properties and highlight its distinct features that can be probed experimentally.

We study the Casimir-Lifshitz force (CLF) between a gold plate and a graphene-covered dielectric grating. Using a scattering matrix (S-matrix) approach derived from the Fourier modal method (FMM), we find a significant enhancement in the CLF as compared to a mere dielectric slab coated with graphene, over a wide range of temperatures. Additionally, we demonstrate that the CLF depends strongly on the chemical potential of graphene, with maximal effects observed at lower filling fractions. Finally, we analyze the Casimir force gradient between a gold sphere and a graphene-coated dielectric grating, highlighting potential avenues for experimental measurements.