Ferroelectricity has been one major focus in modern fundamental research and technological applications. We consider the physical origin of improper ferroelectricity in Mott insulating materials. Beyond the well-known Katsura-Nagaosa-Balatsky inverse Dzyaloshinskii-Moriya mechanism for noncollinearly ordered magnets, we point out the induction of electric polarizations in multipolar-ordered Mott insulators. Using the multiflavor representation for the multipolar magnetic moments, we can show the crossover or transition from the pure inverse Dzyaloshinskii-Moriya mechanism to the pure multipolar origin for the ferroelectricity and also incorporate the intermediate regime with the mixture of both origins. We expect our results to inspire a reexamination of ferroelectricity in the multipolar-ordered magnets.

Altermagnets have dual properties similar to ferromagnets and antiferromagnets like spin splitting and zero net magnetization, and their potentially novel physical effects have aroused the interest of researchers. In this work, we report the observation of a large x-ray magnetic circular dichroism (XMCD) signal (≈0.6), which is comparable to the value in ferromagnets, in altermagnetic CrSb with collinear antiferromagnetic structure at room temperature. The XMCD signal is anisotropic and it persists even at a magnetic field of 5 T, which distinguishes the altermagnetic contribution to XMCD from the weak ferromagnetism. The anisotropic XMCD signal can be explained through symmetry analysis of CrSb with altermagnetic characteristics. Our results highlight the distinct XMCD phenomenon in altermagnetic CrSb with time-reversal-symmetry breaking, and its potential application for optical spectroscopy and microscopy at room temperature.

Majorana and trivial Andreev bound states are predicted to appear in superconductor-semiconductor hybrid systems, but their identification is still a challenging task. Here, we consider superconducting junctions with Rashba spin-orbit coupling and explore the signatures of Majorana and trivial Andreev bound states in the emergent superconducting correlations when the systems are subjected to an external Zeeman field. We first show that robust zero-energy Andreev bound states naturally appear because of confinement and helicity when the normal sector of the junction becomes helical. These Andreev states can evolve into Majorana states, developing alike oscillations around zero energy as a function of Zeeman field. Unlike Majorana states located at both ends, helical Andreev states are located at the interface. We then demonstrate that the emergent superconducting correlations are locally composed of even-frequency spin-singlet even-parity and odd-frequency spin-triplet even-parity pair amplitudes, which coexist due to the interplay of spin-orbit coupling, Zeeman field, and spatial translation invariance breaking. In the helical regime, trivial Andreev states enhance odd-frequency spin-triplet pairing, which decays in the superconductor and has a homogeneous long-range profile in the normal region. At zero frequency, however, odd-frequency spin-triplet pairing vanishes in the helical regime. In the topological phase, Majorana states enhance odd-frequency spin-triplet pairing, producing a long-range homogeneous leakage into the normal region. Interestingly, we discover that when Majorana states are truly zero-energy modes, odd-frequency pairing develops a divergent low-frequency profile, which we interpret as the unambiguous self-conjugated Majorana signature. Our results help understand Majorana and trivial Andreev states from a superconducting correlations perspective in Majorana devices.

We investigate the effect of dissipation on the dynamics of edge modes in the monitored Su-Schrieffer-Heeger model. Our study considers both a linear observable and a nonlinear entanglement measure, namely the two-point correlation function and the disconnected entanglement entropy, as diagnostic tools. While dissipation inevitably alters the entanglement properties observed in the closed system, statistical analysis of quantum trajectories reveals that by protecting the chain's edges from dissipation, it is possible to recover characteristic features analogous to those found in the unitary limit. This highlights the fundamental role of spatial dissipation patterns in shaping the dynamics of edge modes in monitored systems.

We extend Kitaev's real-space formulation of the first Chern number to the second Chern number and establish a computational framework for its evaluation. To test its validity, we apply the derived formula to the disordered Wilson-Dirac model and analyze its ability to capture topological properties in the presence of disorder. Our results demonstrate that the real-space approach provides a viable method for characterizing higher-dimensional topological phases beyond momentum-space formulations.

A ($2+1$)D topologically ordered phase with U(1) symmetry may or may not have a symmetric gapped edge state, even if both thermal and electric Hall conductivity are vanishing. It has recently been discovered that there are “higher” versions of Hall conductivity valid for fermionic fractional quantum Hall (FQH) states that obstruct symmetry-preserving gapped edge states beyond thermal and electric Hall conductivity. In this paper, we show that one can extract higher Hall conductivity from a single wave function of an FQH state, by evaluating the expectation value of the “partial rotation” unitary, which is a combination of partial spatial rotation and a U(1) phase rotation. This result is verified numerically with the fermionic Laughlin state with $\nu =1/3$ and $1/5$, as well as the non-Abelian Moore-Read state. Together with topological entanglement entropy, we prove that the expectation values of the partial rotation completely determine if a bosonic/fermionic Abelian topological order with U(1) symmetry has a symmetry-preserving gappable edge state or not. We also show that thermal and electric Hall conductivity of Abelian topological order can be extracted by partial rotations. Even in non-Abelian FQH states, partial rotation provides the Lieb-Schultz-Mattis type theorem constraining the low-energy spectrum of the bulk-boundary system. The generalization of higher Hall conductivity to the case with Lie group symmetry is also presented.

We develop a theory of nonlinear low-magnetic-field magnetotransport on a helical edge of a quantum spin Hall insulator due to the edge state coupling to bulk midgap states. We focus on the part of the nonlinear I-V characteristic that is odd in the applied magnetic field, and quadratic in the applied bias voltage. This part of the I-V characteristic corresponds to the resistance of the sample being dependent on the relative orientation of the current and an external magnetic field, hence representing a type of edge magnetochiral anisotropy. We identify a mechanism of the magnetochiral anisotropy related to the Hubbard interaction on the midgap state, which leads to the dependence of the scattering characteristics on the current flowing on the edge. This in turn leads to bias-voltage-dependent resistance or, equivalently, conductance, hence a nonlinear I-V. We compare the developed theory to the experiments on monolayer ${\mathrm{WTe}}_2$, and find good agreement with the developed theory.

In this work, we investigate electronic properties of two-dimensional (2D) stoichiometric ladder- and zigzag-patterned inversion domain boundaries (IDBs) in wurtzite III-nitrides based on first-principles calculations performed on GaN. From the atomic configuration, band structure, and differential charge distribution of these two IDBs, we demonstrate that the wurtzite zigzag IDBs have specific atomic structures composed of alternating III-III and V-V zigzag bond chains, giving rise to intersecting conventional metallic states, electroneutrality, and consequently, mid-band-gap Fermi energy level positioning. The analysis of the band structure including spin-orbit coupling and the determination of the effective masses show that IDBs lead to alternating one-dimensional (1D) electron and hole accumulation paths along the zigzag chains, while the band structure of the GaN bulk matrix is only weakly impacted. Moreover, anisotropic 1D transport is shown to be promoted selectively through atomic rows of the wurtzite zigzag IDBs. This finding may open opportunities for next-generation of electronic devices.

We investigated the optical and excitonic properties of holey graphyne (HGY), which is a recently synthesized two-dimensional (2D) carbon allotrope, using first-principles calculations. The potential of HGY for high-temperature excitonic Bose-Einstein condensation (BEC) is highlighted. The unique 2D porous structure and band-edge wave-function symmetry of HGY lead to strong Coulomb interactions and symmetry-forbidden optical transition, resulting in the formation of long-lived dark excitons. The lowest-energy dark exciton in HGY has a large binding energy of 0.63 eV and can be well described by the screened hydrogenic model. By analyzing the constraints on exciton density and temperature necessary for BEC, a phase diagram for the electron-hole system in HGY is constructed, and a maximum BEC transition temperature of 503 K is predicted. Our findings thus reveal the great possibility of achieving above-room-temperature excitonic BEC in 2D carbon materials.

One-dimensional (1D) van der Waals heterostructures (vdWHs) based on transition-metal dichalcogenides (TMDs) show great promise for next-generation optoelectronic devices, often surpassing their 2D counterparts. As a critical determinant of optoelectronic performance, exciton dynamics in 2D TMD vdWHs have been extensively studied, whereas they remain largely unexplored in 1D TMD vdWHs. In this study, we investigate the exciton dynamics in 1D $\mathrm{W}X{}_2/\mathrm{Mo}Y{}_2$ ($X, Y$ = S, Se) vdWHs using ab initio nonadiabatic molecular dynamics simulations. The high curvature in 1D vdWHs significantly modifies their band gaps and band alignment compared to their 2D counterparts, which possess zero curvature, resulting in carrier recombination three orders of magnitude faster than in 2D vdWHs. Notably, 1D ${\mathrm{WS}}_2/{\mathrm{MoSe}}_2$ exhibits phonon-assisted ultrafast intertube carrier transfer at 300 K, with timescale of 20 fs. The ultrafast electron transfer is dominated by the adiabatic (AD) mechanism, whereas the ultrafast hole transfer arises from comparable contributions of AD and nonadiabatic mechanisms. By analyzing phonon excitation and electron-phonon coupling at 100 K and 300 K, we reveal distinct intertube carrier transfer behaviors and their temperature-dependent variations across four 1D vdWHs. These atomic-level insights provide a critical foundation for the design of advanced optoelectronic devices utilizing 1D TMD vdWHs.

We demonstrate that the endurance and compatibility of organic ferroelectric films can be impressively enhanced by a strategy of inhibiting charge injection and high-energy ions diffusion through inserting a large-band-gap ${\mathrm{Al}}_2{\mathrm{O}}_3$ interfacial layer between the organic ferroelectric films and electrodes. The $\mathrm{Pt}/{\mathrm{Al}}_2{\mathrm{O}}_3/\mathrm{P}\left(\mathrm{VDF}\text{−}\mathrm{TrFE}\right)/{\mathrm{Al}}_2{\mathrm{O}}_3/\mathrm{Al}$ capacitor exhibited remarkable fatigue-recovery characteristics with a durability of up to ${10}^{11}$ cycles, which is attributed to the sandwiched ${\mathrm{Al}}_2{\mathrm{O}}_3$ layer, suppressing the charges injection and slowing down the dehydrofluorination process within the P(VDF-TrFE) films. Additionally, the ${\mathrm{Al}}_2{\mathrm{O}}_3$ layer effectively impedes the damage of ion diffusion during the following film deposition, improving the compatibility of P(VDF-TrFE) films with other metal electrodes and semiconductor materials. A ZnO-based memory transistor modulated by ${\mathrm{Al}}_2{\mathrm{O}}_3/\mathrm{P}\left(\mathrm{VDF}\text{−}\mathrm{TrFE}\right)/{\mathrm{Al}}_2{\mathrm{O}}_3$ back gate was successfully fabricated which shows a robust on/off ratio of ∼100 over ${10}^{11}$ cycles.

Another polar phase was discovered in lead hafnate ${\mathrm{PbHfO}}_3$ single crystal below 270 K. The transition was manifested through complex permittivity anomalies with a thermal hysteresis of approximately 30 K. While pristine ${\mathrm{PbHfO}}_3$ crystal was shown to have modest pyroelectric signals of 20–235 $\mathrm{nC}\mathrm{c}{\mathrm{m}}^{–2}$, DC field poling at 10 $\mathrm{kV}\mathrm{c}{\mathrm{m}}^{–1}$ induced a huge polarization of 60 $\mu \mathrm{C}\mathrm{c}{\mathrm{m}}^{–2}$, dramatically larger than values observed in lead zirconate ${\mathrm{PbZrO}}_3$. The polar nature of this transition was validated through multiple experimental techniques. Similarly to dielectric measurements, the pyroelectric effect showed distinct thermal behavior between cooling and heating cycles. The distinct change in the polarization gap occurring in the polarization–electric field function was observed below the transition. Second harmonic generation measurements have displayed signals sensitive to applied DC fields around the transition point. It was suggested that the mechanism behind this polar transition involves complex interactions within both $\mathrm{P}{\mathrm{b}}^{2+}$ and ${{\mathrm{O}}^2}^{-}$ substructures. The possible symmetry of this low-temperature polar phase was considered based on the absence of the piezoelectric coefficients $d_{31}$ and $d_{33}$.

We demonstrate that random flux is able to drive nontrivial topological phase transitions, in particular between weak topological insulators (WTIs) and Chern insulators (CIs), illustrated on an anisotropic Wilson-Dirac model in two dimensions. Remarkably, an intriguing topological transition sequence $\mathrm{WTIs}\to \mathrm{CIs}\to \mathrm{WTIs}$ occurs with the reentrance to a WTI but of different weak topology, which is unattainable with chemical potential disorder. The involvement of anisotropy and weak topology in such a transition gives rise to emergent quasicritical points, where eigen states are extended in one spatial direction but localized in the other one. This new quantum criticality lies outside the conventional quantum Hall universality class. We provide a comprehensive characterization of the random-flux-induced phase transitions and quantum criticality from both bulk and boundary perspectives. Our results describe a qualitatively new disorder effect based on the interplay of random flux with topological phases of matter.

The optimal transport problem seeks to minimize the total transportation cost between two distributions, thus providing a measure of distance between them. In this work, we study the optimal transport of the eigenspectrum of one-dimensional non-Hermitian models as the spectrum deforms on the complex plane under a varying imaginary gauge field. Notably, according to the non-Bloch band theory, the deforming spectrum continuously connects the eigenspectra of the original non-Hermitian model (with vanishing gauge field) under different boundary conditions. It follows that the optimal spectral transport should contain key information of the model. Characterizing the optimal spectral transport through the Wasserstein metric, we show that, indeed, important features of the non-Hermitian model, such as the (auxiliary) generalized Brillouin zone, the non-Bloch exceptional point, and topological phase transition, can be determined from the Wasserstein-metric calculation. We confirm our conclusions using concrete examples. Our work highlights the key role of spectral geometry in non-Hermitian physics and offers a practical and convenient access to the properties of non-Hermitian models.

One of the cornerstones of spintronics is the application of a spin-transfer torque to a nanomagnet, driving the magnetization of the nanomagnet into a steady-state precession and realizing a spin-torque oscillator. Such a steady state, sustained by a balance between driving and dissipation, could be a textbook example of a nonequilibrium situation. Nevertheless, most theoretical descriptions of spin-torque oscillators simply assume local equilibrium. Here, based on a simple model, we investigate the relevance of nonequilibrium effects in spin-torque oscillators. We use a nonequilibrium Keldysh description, which allows us to treat the effects of spin relaxation, and find that, in the absence of spin relaxation, persistent precessions of the magnetization are not allowed, when magnetic anisotropies are absent. However, introducing spin relaxation enables persistent precessions, where the strength of spin relaxation has both quantitative and qualitative effects on the magnetization dynamics. In the presence of magnetic anisotropy, we find that persistent precessions are allowed even when spin relaxation is absent, but spin relaxation causes a nonlinear relation between the oscillator power and the applied voltage bias. Finally, we consider an alternative spin relaxation mechanism and study the resulting magnetization precessions, highlighting the importance of understanding the exact nature of the relaxation in nanomagnets.

We present a detailed investigation of single-crystal samples of the magnetic compound ${\mathrm{Gd}}_2{\mathrm{AlSi}}_3$, which crystallizes in the $\alpha \text{−}{\mathrm{ThSi}}_2$-type tetragonal structure. We report the temperature and magnetic field dependence of the magnetic susceptibility, magnetization, heat capacity, electrical resistivity, and magnetoresistance for magnetic fields applied along both the tetragonal $c$-axis and in the basal $ab$ plane. X-ray diffraction measurements confirm a centrosymmetric, $I{4}_1/amd$ space group of the crystal structure. Despite single-site occupancy of the Gd position in this tetragonal structure, we identify two successive antiferromagnetic phase transitions at Neél temperatures 32 K and 23 K via magnetic susceptibility, heat capacity, and transport measurements, as well as a complex magnetic interaction with a magnetic anisotropy that plays an important role in the direction-dependent transport response. Our identification of multiple magnetic phases in ${\mathrm{Gd}}_2{\mathrm{AlSi}}_3$, where Gd is the only magnetic species, helps to elucidate the field-induced skyrmionic behavior in the Gd-based intermetallic compounds.

After the decade-long exhaustive study of binary high-$T_{\mathrm{c}}$ superconducting hydrides, the frontier of this stimulating research field has recently shifted to ternary hydrides with much expanded conformational space in search of coveted room-temperature superconductors. This task, however, presents a formidable challenge due to enormous demands on computational resources. Here, we devise an efficient high-throughput approach using keen material insights and a self-built database to screen for robust ternary hydrides in clathrate structures, which were proven to host the highest $T_{\mathrm{c}}$ in binary hydrides, and to estimate $T_{\mathrm{c}}$ by a reliable empirical formula. This approach has made it possible to uncover a diverse set of complex multiple-hydrogen-cage ternary hydrides hosting near or above-room-temperature $T_{\mathrm{c}}$, which are beyond the reach of prevailing structure search methods. Using self-built databases and empirical models, in this study, we open a fresh avenue to enable and accelerate the discovery of promising room-temperature superconductors among unprecedented complex clathrate hydrides.

In the search for superconductivity with a high critical temperature $T_c$, new materials with light elements are expected to favor electron-phonon coupling at higher frequencies, thus achieving a higher $T_c$. Due to their typically stiff bonds and the small atomic masses, carbon intercalated compounds are among the prototypes of light-element superconductors. Here, we report on the electronic and superconducting properties of ${\mathrm{Sc}}_{20}{\mathrm{BC}}_{27}$, a member of the carbon-based layered compounds. By using mostly muon-spin spectroscopy $\left(\mu \mathrm{SR}\right)$, we investigate the ${\mathrm{Sc}}_{20}{\mathrm{BC}}_{27}$ superconductivity as a function of temperature and hydrostatic pressure. The experimental data are compatible with a single $s$-wave-gap superconductor in the clean limit. In particular, we find that the magnetic penetration depth at ambient pressure, $\lambda =512\left(3\right)\mathrm{nm}$, decreases only slightly with increasing pressure (up to 16.7 kbar). The superconducting gap $\mathrm{\Delta }\left(0\right)=1.73\left(3\right)\mathrm{meV}$ is fully isotropic at ambient pressure, but it becomes anisotropic under hydrostatic-pressure conditions. At the same time, similar to other carbon-based superconductors, $T_c$ increases with applied pressure.

In the BCS theory of superconductivity, an instability towards pairing develops at arbitrarily weak dimensionless coupling $\lambda$ due to a divergence of logarithmic perturbative series for the pairing susceptibility (Cooper logarithms) at $T_c\sim {\omega }_0{e}^{-1/\lambda }$, where ${\omega }_0$ is an energy cutoff. On the contrary, in many models of superconductivity out of a non-Fermi-liquid, the Cooper logarithm is absent, and superconductivity emerges only when $\lambda$ exceeds a certain threshold. We argue that there are situations when there is no threshold, and at weak coupling the formula for $T_c$ is BCS-like, yet the origin of the pairing instability is fundamentally different from that in the BCS scenario. As an example, we revisit superconductivity in a metal at the onset of $(\pi ,\pi )$ spin-density-wave order. Earlier studies of this problem found no threshold and a BCS-like expression for $T_c$ at weak coupling. We argue that, despite this, the pairing is not caused by the Cooper logarithm, and in many respects it is qualitatively similar to that in non-Fermi-liquids.

Motivated by the orientation-dependent properties of $d$-wave superconductors (SCs), we investigate Andreev bound states (ABSs) and Josephson current in $s$-wave SC/altermagnet/$d$-wave SC (S/AM/D) and $d$-wave SC/altermagnet/$d$-wave SC (D/AM/D) junctions. The asymmetric S/AM/D junction exhibits a nodeless ABS spectrum with distinct spin states, arising from AM manipulation. In contrast, a symmetric D/AM/D junction with $\pm {45}^{\circ }$-oriented order parameters exhibits a nodal ABS spectrum, characterized by spin-split sinusoidal curves. This behavior arises from the angular dependence of the $d$-wave pairing potential, which introduces a $\pi$ shift in the ABS spectrum of the D/AM/D junction compared to that of conventional S/AM/S junctions. Additionally, the positions of the ABS nodes in the D/AM/D junction can be controlled by adjusting the length and strength of the AM layer. Analysis of the free energy and Josephson current reveals additional extrema in the free energy at intermediate phase differences, leading to skewness and a nonsinusoidal behavior in the current-phase relationship of both configurations. These zero-supercurrent phase differences can be tuned by adjusting the properties of the AM layer such as AM strength and length. These findings provide insights into AM-based Josephson junctions.