Altermagnets have recently been recognized as a distinct class of magnetic materials characterized by alternative spin-split electronic structures without net magnetization. Despite intensive studies on their single-particle spintronic and valleytronic properties, many-electron interactions and optical responses of altermagnets remain less explored. In this work, we employ many-body perturbation theory to investigate excited states and their strain tunability. Using monolayer ${\mathrm{Mn}}_2{\mathrm{WS}}_4$ as a representative candidate, we uncover a novel spin–valley-dependent excitonic selection rule in two-dimensional altermagnetic Lieb-lattices. In addition to strongly bound excitons, we find that linearly polarized light selectively excites valley-spin-polarized excitons. Importantly, these lowest-energy excitons are optically bright, making them experimentally accessible. Moreover, due to the interplay between altermagnetic spin symmetry and electronic orbital character, we predict that applying uniaxial strain can lift valley degeneracy and enable the selective excitation of spin-polarized excitons—an effect not achievable in previously studied transition-metal dichalcogenides. These generalized spin-valley-locked excitonic states and their strain tunability can offer a robust mechanism for fourfold symmetric altermagnets to encode, store, and read valley and spin information.

In a one-dimensional phthalocyanine molecular conductor $\text{TPP}{\left[\text{Fe}\left(\text{Pc}\right){\left(\text{CN}\right)}_2\right]}_2$, giant negative magnetoresistance was observed at low temperatures, where the resistivity increases. To clarify the origin of this magnetoresistance, we investigated the low-temperature insulating state, revealing a diffuse x-ray streak midway between the Bragg peaks. This suggests charge order driven by Coulomb repulsion among $\pi$-conduction electrons. The intensity increases below the temperature where the short-range antiferromagnetic order of $d$ local moments develops, which stabilizes the charge order via a Hund-like intramolecular $\pi \text{−}d$ interaction.

A density functional theory (DFT) framework is presented that links functional derivatives of free-energy functionals to nonlinear static density response functions in quantum many-body systems. Within this framework, explicit expressions are derived for various higher-order response functions of systems that are homogeneous on average, including the first theoretical result for the cubic response at the first harmonic ${\chi }_0^{(1,3)}\left(\mathbf{q}\right)$. Specifically, our framework includes hitherto neglected mode-coupling effects that are important for the nonlinear density response even in the presence of a single harmonic perturbation. We compare these predictions for ${\chi }_0^{(1,3)}\left(\mathbf{q}\right)$ to new Kohn-Sham DFT simulations, leading to excellent agreement between theory and numerical results. Exact analytical expressions are also obtained for the long-wavelength limits of the ideal quadratic and cubic response functions. Particular emphasis is placed on the connections between the third- and fourth-order functional derivatives of the noninteracting free-energy functional $F_s\left[n\right]$ and the ideal quadratic and cubic response functions of the uniform electron gas, respectively. These relations provide exact constraints that may prove useful for the future construction of improved approximations to $F_s\left[n\right]$, in particular, for warm dense matter applications at finite temperatures. Here, we use this framework to assess several commonly employed approximations to $F_s\left[n\right]$ through orbital-free DFT simulations of the harmonically perturbed ideal electron gas. The results are compared with Kohn-Sham DFT calculations across temperatures ranging from the ground state to the warm dense regime. Additionally, we analyze in detail the temperature- and wave number-dependent nonmonotonic behavior of the ideal quadratic and cubic response functions.

The algebraic or ring structure of anyons, called the fusion rule, is one of the most fundamental research interests in contemporary studies on topological orders (TOs) and the corresponding conformal field theories (CFTs). Recently, the algebraic structure realized as generalized symmetry, including noninvertible and categorical symmetry, captured attention in the fields. Such non-Abelian anyonic objects appear in a bulk CFT or chiral CFT (CCFT), but it has been known that the construction of a CCFT contains theoretical difficulties in general. In this work, we propose the fusion rules in $Z_N$ extended CCFT and bulk CFT and the corresponding TOs. We explicitly construct a nontrivial expression of subalgebra structure in the fusion rule of a bulk CFT. We name this subalgebra bulk semion. This corresponds to the fusion rule of the CCFT and categorical symmetry of the TO or $Z_N$ graded symmetry topological field theory. This gives a bulk-edge correspondence based on the symmetry analysis and corresponds to an anyon algebraic expression of topological holography in the recent literature. The recent topological holography is expected to apply to systems in general space-time dimensions. Moreover, we present a concise way of unifying duality (or fractional supersymmetry), generalized or categorical symmetry, and Lagrangian subalgebra. Our method is potentially useful to formulate and study general TOs, fundamentally only from the data of bulk CFTs or vice versa, and gives a clue in understanding CCFT (or ancillary CFT more generally).

We introduce a second-quantized field theory for Chern insulators in which the Hamiltonian features a static vector potential that has the periodicity of the crystal's lattice and spontaneously breaks time-reversal symmetry in the system's ground state. Such a vector potential generates a magnetic field at the microscopic level that may be thought of as arising from local moments associated with one or more magnetic ions in each unit cell. Considering spinor electrons, we study the Chern invariants characterizing the topology of the occupied valence bands of Chern insulators in both two and three dimensions – the Chern number and the Chern vector, respectively – and we derive expressions for these topological invariants that are globally defined across the Brillouin zone and involve the full band structure of the system. We also study the long-wavelength response of a Chern insulator to electromagnetic fields at finite frequency, generalizing the quantum anomalous Hall effect in the static limit to the optical regime.

The Hall effect in the superconducting mixed state serves as a crucial probe of vortex dynamics. Here, we report a systematic magnetotransport study on the vortex Hall effect in exfoliated FeSe flakes with varying thickness. In flakes of intermediate thickness (e.g., 159 nm), the Hall resistivity ${\rho }_{xy}$ in the superconducting mixed state exhibits a double sign reversal as a function of temperature, characterized by a pronounced peak-dip structure. This behavior evolves with magnetic field and current, and is qualitatively consistent with the predictions of vortex dynamics theory (Wang, Dong, and Ting theory). However, the complex evolution of the sign reversal across different thicknesses cannot be explained by existing models. We propose a unified physical picture that incorporates Magnus force, equivalent Lorentz force, and carrier-type effects to explain the sign reversals and their thickness-dependent evolution. Finally, we discuss the connection between the unconventional Hall response and the microscopic origin of vortex pinning in FeSe. Our results advance the understanding of vortex transport and the anomalous Hall effect in iron-based superconductors.

Exceptional points (${\mathrm{EP}}_{\mathrm{S}}$) can enhance sensitivity for weak signal detection, but noise typically degrades their performance. Here, contrary to this conventional wisdom, we theoretically introduce and experimentally demonstrate a noise-assisted sensing paradigm leveraging stochastic resonances near a driven-dissipative nonlinear fourth-order exceptional point (${\mathrm{EP}}_4$). Using a minimal two-site model with asymmetric dissipation and single-site driving, we realize an ${\mathrm{EP}}_4$ exhibiting fourfold degeneracy in steady-state eigenamplitudes under fixed-frequency driving. Crucially, this ${\mathrm{EP}}_4$ coexists with third-order ${\mathrm{EP}}_{\mathrm{S}}$ and second-order exceptional lines, partitioning a two-parameter space into mono-, bi-, and tristable regions. We demonstrate that noise induces bi- and multistable stochastic resonances, synergizing with the quartic-root dispersion around ${\mathrm{EP}}_4$ to dramatically amplify weak signals and enhance signal-to-noise ratios. Experimental validation with nonlinear electrical circuits confirms these predictions. Our work establishes driven-dissipative nonlinear higher-order ${\mathrm{EP}}_{\mathrm{S}}$ with eigenamplitude degeneracy as a potent platform for noise-enhanced sensing, harnessing nonlinear multistability and EP physics to transcend conventional noise limitations.

We demonstrate a graphene nanoribbon (GNR) metagrating on a rectangular dielectric ridge that exhibits plasmonically induced transparency (PIT) through the excitation of both odd- and even-order dark modes. The metagrating integrates a top bright mode GNR coupled directly to light and a sidewall dark mode GNR isolated from direct excitation. Increasing the dark mode width generates sequentially ordered dark mode based PIT effects. Taking the PIT effects formed by the fundamental bright mode coupled to the first six-order dark modes as examples, we employ Fabry-Pérot theory, Lorentzian oscillator modeling, and finite-difference time-domain (FDTD) simulations to reveal the underlying physics: The resonant phase remains fixed near $\pi /2$ at the bright mode distal edge but transitions continuously from $-\pi /2$ to $+\pi /2$ at its bright mode facing edge. Consequently, the corresponding phase shift is nearly invariant at the distal edge but varies substantially at the facing edge. This transitional resonant phase—together with the resulting asymmetric phase shift—permits noninteger mode orders which correspond to the conversion from the high-frequency mode of the $j\mathrm{th}$-order PIT to the low-frequency mode of ($j+1$)th-order PIT under variation of the sidewall GNR width while high absorption is maintained. Finally, using refractive index sensing as an example, we demonstrate that the high-order modes of this PIT device outperform conventional gratings and other lower-order PIT devices in key metrics, including the quality factor and figure of merit.

Quantum tunneling allows electrons to be transferred between two regions separated by an energetically forbidden barrier. Performing a position measurement that finds a particle in the barrier forces the tunneling electrons to transition from having a classically forbidden energy to an energy above the barrier height. We exploit this effect to define quantum tunneling engines that can use the unconditioned detection of virtually occupied states as a resource for power generation and cooling. Leveraging energy exchange with the detector, we show that the device can operate in a hybrid regime, enabling simultaneous cooling and power generation. Furthermore, we demonstrate measurement-assisted autonomous refrigeration and checkpoint cooling driven purely by a thermal bias, without the need for an applied potential. We also find a purification by noise effect when the measurement drives the system into a stationary dark state. These results underscore the intriguing dual role of measurement as a thermodynamic resource and a dark state generator.

Competing exchange interactions in frustrated three-dimensional spin lattices offer a promising route to realize rich physical phenomena, including spin liquids with exotic low-energy excitations. In this work, we focus on the three-dimensional frustrated magnet ${\mathrm{MgCrGaO}}_4$, where ${\mathrm{Cr}}^{3+}$ ions occupy a pyrochlore-like network. Due to intrinsic disorder and partial occupation of the pyrochlore sites by magnetic ions, the actual structure highlights the famous closed loops of the pyrochlore lattice. Our thermodynamic, electron spin resonance (ESR), muon spin relaxation ($\mu \mathrm{SR}$), and inelastic neutron scattering (INS) techniques detect no signature of magnetic order or spin freezing down to 57 mK, despite a sizable exchange interaction ($J=58\text{K}$) between ${\mathrm{Cr}}^{3+}$ moments. Below the characteristic exchange energy scale, all probes reveal the emergence of antiferromagnetic short-range spin correlations, corroborated by magnetic diffuse scattering in the wave vector dependence of low-energy magnetic excitations centered on $Q=1.5{\text{Å}}^{-1}$ in inelastic neutron scattering experiments. The low-temperature specific heat follows a near-quadratic dependence suggesting the presence of nontrivial low-energy excitations. These results establish ${\mathrm{MgCrGaO}}_4$ as a rare three-dimensional spin liquid with exotic excitations, offering a strong impetus for the experimental realization of spin liquids in higher-dimensional frustrated quantum magnets.

Spin Hall magnetoresistance (SMR), the variation in resistance of a heavy metal (HM) with the magnetization orientation of an adjacent ferromagnet (FM), has been extensively studied as a powerful probe of surface magnetic moments in a variety of magnetic materials. While conventional SMR relies on the angular rotation of an assumed rigid magnetization of fixed magnitude, we report a SMR arising from magnetic-field modulation of spin fluctuations in the FM, with its magnetization direction kept fixed parallel to the spin Hall accumulation in the HM. In contrast to conventional SMR, which scales with the magnetization and vanishes near the Curie temperature ($T_c$), such “longitudinal” SMR, though suppressed at low temperatures, becomes critically enhanced at $T_c$, reaching a magnitude comparable to conventional SMR amplitudes. Our work highlights the overlooked importance of the magnetization-magnitude degree of freedom in spin transport and opens a promising avenue for electrically detecting enhanced spin fluctuations in magnetic systems.

The recent experimental observation of fractional quantum anomalous Hall (FQAH) states in rhombohedral multilayer graphene has attracted significant attention. One of the most intriguing observations is that the FQAH states at various fractional fillings give way to IQAH states as the temperature is lowered. In this work, we propose a mechanism for the appearance of FQAH states within a finite temperature range in a toy model. The model consists of a flat Chern band and impurities, and we analyze the effects of impurities on the system's behavior at finite temperatures. Because a doped FQAH state has a large entropy due to the abundant quasiparticle excitations, we believe that the crossover may arise from the competition between the energy penalty for thermal excitations and the increase in entropy. We support our theoretical argument with numerical calculations using exact diagonalization. Our results suggest that impurities may play a crucial role in the crossover from the FQAH to IQAH states in rhombohedral pentalayer graphene.

Out-of-time ordered correlators are a probe of how the information of an initial perturbation is effectively scrambled under unitary time evolution, widely used to study quantum chaos. They have also been used to demonstrate that information is trapped in the zero-dimensional edge modes of topological insulators and superconductors, and does not become scrambled. Here we study scrambling in two-dimensional topological models. In the bulk the butterfly velocity, the speed at which the out-of-time ordered correlator spreads, gains a directional dependence from the underlying lattice. Furthermore when there are chiral or helical edge modes present these cause a form of dynamical scarring. The information about an initial perturbation on the boundary of the system travels around the edge, carried by the edge modes, but is not scrambled over very long time scales. The direction and speed of the scars are given by the velocities of the linearly dispersing edge modes. We further show that these scars do not interact, passing through each other. We back up these results with analytical and numerical calculations on exemplary models.

We present a combined x-ray and polarized neutron scattering study on chiral, polar and magnetoeletric ${\mathrm{NiCo}}_2{\mathrm{TeO}}_6$ single crystals with different net crystal chiralities as characterized by optical methods. The rastering images taken with a micrometer-sized polarized x-ray beam unambiguously reveal the coupling of crystal and magnetic chiralities in a heterochiral sample. Spherical neutron polarimetry experiments on a different, structurally enantiopure sample—as evidenced by polarized x-ray scattering—reveal a homochiral magnetic spiral, which proves the stabilization of a single helical handedness despite the intertwined structural helices in the ferri-chiral crystal structure.

Antiskyrmions (ASks) exhibit an electrically tunable antiskyrmion Hall effect, enabling zero-Hall-angle propagation compared to skyrmions. However, the fundamental mechanism governing ASks formation in two-dimensional systems is rarely explored. Here, we demonstrate that the ${MX}_2$ (${\mathrm{NbO}}_2, {\mathrm{NbS}}_2, {\mathrm{NbSe}}_2, {\mathrm{MoS}}_2$, and ${\mathrm{MoSe}}_2$) monolayers with $D_{2d}$ symmetry host an intrinsic sizable anisotropic Dzyaloshinskii-Moriya interaction (DMI) enabling ASks stabilization. Crucially, in 2D ${\mathrm{MoS}}_2$, biaxial strain dramatically tunes the anisotropic DMI strength ($|d_{\parallel }|/J_1=0.07–0.16$) and the magnetic frustration ratio ($|J_2|/J_1=0.18–0.5$), cooperatively driving sequential magnetic phase transitions from an antiskyrmion lattice to an ASks-wormlike-domain mixture and ultimately to distorted spin spiral states. A comprehensive investigation of magnetic parameters further reveals that frustration reduces the critical DMI threshold for ASks formation by 70% compared to frustration-free systems while minimizing the antiskyrmion diameter to sub-2 nm, greatly expanding their stability window. This study elucidates the fundamental mechanisms and provides valuable information for achieving multichiral topological spin configurations in low-dimensional magnets.

Inspired by the successful synthesis of two-dimensional (2D) ${\mathrm{MoSi}}_2{\mathrm{N}}_4$ and ${\mathrm{WSi}}_2{\mathrm{N}}_4$ [Y. L. Hong et al., Science 369, 670 (2020)], which demonstrates that Si-N layers can act as effective modification layers on 2D materials, we investigate superconductivity and charge density waves (CDWs) of hexagonal boron nitride $\left(h\text{−}\mathrm{BN}\right)$ passivated with a Si-N layer based on first-principles calculations. Our results demonstrate that $h$-BN undergoes a transition from an insulator to a phonon-mediated superconductor with the transition temperature $\left(T_c\right)$ of 14.3 K when it is functionalized with a Si-N layer on one side. The electron-phonon coupling (EPC) of ${\mathrm{BN}}_2\mathrm{Si}$ is primarily attributed to the coupling between electrons in N-$p_z$ orbitals and the low-frequency in-plane vibrational modes of Si and N atoms, as well as higher-frequency out-of-plane vibration modes of N and B atoms. By applying $5\%$ biaxial tensile strain, the EPC constant $\lambda$ increases from 0.66 to 1.82, resulting in a significantly enhanced $T_c$ of 34.9 K. However, when the biaxial tensile strain reaches $6\%$, an obvious soft mode emerges in the lowest acoustic branch of phonon curves, indicating the presence of a CDW. The origin of the instabilities is analyzed based on Lindhard electron susceptibility and the phonon linewidths, revealing that the CDW is driven by both Fermi-surface nesting and EPC. Thus, the predicted ${\mathrm{BN}}_2\mathrm{Si}$ presents a promising platform for exploring 2D superconductivity and CDWs.

The dynamics of a cycloidal spin structure driven by an ac magnetic field is theoretically studied in the weak-field limit. A specific model Hamiltonian describing the cycloidal spin structure in a ferromagnetic thin film is constructed, and its dynamics is analyzed using the collective-coordinate approach within the Lagrangian formalism. We demonstrate that the cycloidal spin structure exhibits a unidirectional gliding motion under an ac magnetic field applied along a specific direction, and an expression for the average velocity is derived as a function of the magnitude, the direction, and the frequency of the ac magnetic field. We compare our theoretical predictions with the results of micromagnetic simulations and identify two resonance frequencies determined by the eigenenergies of the excitation modes. Furthermore, evaluating spin motive forces induced by the dynamics reveals a substantial dc voltage, which may be exploited in energy-harvesting devices utilizing ambient electromagnetic radiation.