We report nonreciprocal charge transport in epitaxial monoclinic ${\mathrm{SrIrO}}_3$ ($m$-SIO) thin films, a nonsymmorphic symmetry-protected Dirac semimetal with strong spin-orbit coupling. Second-harmonic magnetotransport measurements reveal both bilinear magnetoelectric resistance and a nonlinear planar Hall effect, providing clear evidence of spin-momentum locked states in $m$-SIO. Systematic angular-dependent measurements suggest a predominantly in-plane spin vector oriented perpendicular to the electron momentum, consistent with Rashba-type spin-momentum locking on the Fermi surface. Quantitative analysis of the bilinear magnetoelectric resistance based on a simplified Rashba model yields a large effective Rashba parameter, suggesting that the Rashba-type surface states are closely related to the strongly spin-orbit coupled electronic structure of the nonsymmorphic Dirac bulk states. Our results provide a transport route to probe Rashba-type spin-momentum locking in $m$-SIO and identify $m$-SIO as a promising platform for oxide spin-orbitronics.

We address the problem of magnetic field induced corner states in quantum spin Hall insulators beyond the particle hole symmetric limit. Starting from a realistic low-energy model for zinc-blende semiconductor quantum wells, we derive the effective edge Hamiltonian in the form of a Dirac Hamiltonian with two magnetic field dependent mass terms, whose structure depends on the crystallographic orientation of the edge and of the magnetic-field orientation. Our analytical results show that magnetic field induced corner states are most naturally understood as in-gap bound states of the effective edge theory, controlled by the relative configuration of the edge mass vectors rather than, in general, as higher-order topological corner modes protected by a stable bulk invariant. We demonstrate that, although mirror-graded winding numbers can be defined and quantized for certain crystallographic configurations, the existence of magnetic field induced corner states is not restricted to regimes in which these bulk invariants are well defined. Finally, we argue that even without higher-order topological protection these corner states may remain spectrally robust under weak perturbations as isolated in-gap quasiparticle excitations.

We present a systematic study of the evolution of the band structure in the Fe-based superconductor family ${\mathrm{BaFe}}_{2-x}{M}_x{\mathrm{As}}_2$ ($M$ = Co, Cu, Cr, Ru, and Mn) using polarization-dependent angle-resolved photoemission spectroscopy (ARPES). Low-substituted samples, with comparable spin-density wave transition temperatures ($T_{\text{SDW}}$), were chosen to enable controlled comparisons with respect to the parent compound. The sizes of the central $\alpha$ and $\beta$ hole pockets remain largely unchanged across different substitutions, while the $\gamma$ pocket exhibits no clear correlation with either $T_{\text{SDW}}$ or the As height relative to the Fe plane. The evolution of both hole and electron pockets does not follow a rigid-band shift scenario and shows no systematic dependence on the nominal electron or hole character of the substituting element. However, subtle trends are observed: a modest increase of the electron pocket with planar character that correlates with $T_{\text{SDW}}$ suppression, while its contraction appears linked to an increase in the As height relative to the Fe planes. Our results suggest that the suppression of $T_{\text{SDW}}$ is primarily driven by changes in the Fe–As bond length, with the effect being more pronounced in electronic states with planar character. These findings provide insight into the electronic structure of ${\mathrm{BaFe}}_{2-x}{M}_x{\mathrm{As}}_2$.

Inducing spin-sublattice inequivalence through correlation-driven orbital order, rather than crystallographic anisotropy, has been proposed as a route to altermagnetism (AM). However, in two-orbital systems, it is blocked by the GKA rules, which favor ferromagnetic superexchange for orderly occupied orbitals. Through first-principles calculations on ${\mathrm{KCoF}}_3$ and ${\mathrm{KNiF}}_3\text{−}{\mathrm{KCuF}}_3$ superlattice, we show that the limitation can be bypassed by employing two coupled orbital subsets that separately supply the occupation-ordered orbitals and the dominant antiferromagnetic exchange contribution, establishing a minimal orbital condition for intrinsic spin-orbital AM, and reveal the distortion-insensitive finite spin splitting in ${\mathrm{KCoF}}_3$, which is distinct from extrinsic AM induced by crystallographic anisotropy. These results clarify the microscopic mechanism of spin-orbital AM and delineate a general route for realizing unconventional magnetism in correlated materials.

There is a close theoretical connection between topological Floquet physics and cavity quantum electrodynamics (QED), yet this connection has not been realized experimentally due to complicated cavity QED models that often arise. We propose a simple, experimentally viable protocol to realize nonadiabatic topological photon pumping mediated by a single qubit, which we dub the anomalous Floquet photon pump. For both quantized photons and external drive, the system exhibits a nontrivial topological phase across a broad range of parameter space. Transitions out of the topological phase result from frequency-space delocalization. Finally, we argue that the protocol can be implemented in existing experiments via driven qubit nonlinearities, with topological pumping witnessed in reconstruction of the cavity Wigner distribution functions.

Accurate simulations of the Hubbard model are crucial to understanding strongly correlated phenomena, in which small energy differences between competing orders demand high numerical precision. In this work, neural quantum states are used to probe the strongly coupled and underdoped regime of the square-lattice Hubbard model. We systematically compare the hidden fermion determinant state and the Jastrow-Backflow Ansatz, parametrized by a vision transformer, finding that in practice, their accuracies are similar. We also test different symmetrization strategies, finding that output averaging yields the lowest energies, although it becomes costly for larger system sizes. On cylindrical systems, we consistently observe filled stripes. On the torus, our calculations display features consistent with a doped Mott insulator, including antiferromagnetic correlations and suppressed density fluctuations. Our results demonstrate both the promise and current challenges of neural quantum states for correlated fermions.

Among the extensive studies of fractional quantum anomalous Hall (FQAH) states, there recently has appeared a growing interest in the topological states with coexisting charge density wave (CDW) orders. Such states are referred to as Hall crystals. However, compared to those with integer Hall conductivities, the FQAH crystal (FQAHC) is still elusive, even at the level of a microscopic model. In this work, we numerically study a topological flat-band model on a triangular lattice with spinless fermions. At fractional filling of the Chern band, the nearest-neighbor interaction leads to a commensurate and topologically trivial CDW state. Interestingly, the folded miniband above the CDW gap is nontrivial, and we focus on the doping of it without any projection. A series of (F)QAHC states at (fractional) integer fillings of this miniband is discovered and some FQAHC state might even exist in less “ideal” conditions. The ground-state degeneracies of such (F)QAHC states are enlarged by the CDW degeneracy, and the Hall conductivities—determined by the fillings of the miniband—are different from the fillings of the original Chern band. We also study the thermodynamics of an FQAHC state and find a compressible CDW phase at intermediate temperatures, which might serve as a precursor of a lower-temperature FQAHC phase. Moreover, we numerically demonstrate that such a generic scheme of doping a CDW-folded topological miniband could be applied to bosonic systems, broadening the platforms of Hall-crystal physics and motivating its exploration in quantum moiré and cold-atom systems.

In strongly correlated transition metal dichalcogenides, an intricate interplay of polaronic distortions, stacking arrangement, and electronic correlations determines the nature of the insulating state. Transient changes in pristine and Mo-doped $1T\text{−}{\mathrm{TaS}}_2$ are measured by femtosecond time-resolved photoelectron spectroscopy and modeled using nonequilibrium dynamical mean-field theory complemented by density functional theory. Inspired by vibrational spectroscopy we analyze real-time oscillatory spectral changes induced by the charge density wave amplitude mode through modulations of electronic interactions. We distinguish effects induced by electron doping and structural disorder which can modify the layer stacking and conclude that doped crystals might feature an enhanced fraction of monolayers. Our work establishes substitutional doping as a pathway to engineer the dynamic electronic structure in correlated transition metal dichalcogenides.

Long-lived spin-helix states facilitate the study of nonequilibrium dynamics in quantum magnets. We consider the decay of transverse spin helices in antiferromagnetic spin-$S$ XXZ chains with single-ion anisostropy. The spin-helix decay is observable in the time evolution of the local magnetization that we calculate numerically for the system in the thermodynamic limit using infinite time-evolving block decimation simulations. Although the single-ion anisotropy prevents helix states from being eigenstates of the Hamiltonian, they still can be long-lived for appropriately chosen wave numbers. In the case of easy-axis exchange anisotropy, the single-ion anisotropy may even stabilize the helices. Within a spin-wave approximation, we obtain a condition giving an estimate for the most stable wave number $Q$ that agrees qualitatively with our numerical results.

In this work, we investigate the interaction between the tip of an atomic force microscope and single Ho and Gd adatoms adsorbed on Cu(111). Force-distance spectroscopy shows a pronounced long-range repulsive force that dominates at tip-sample separations larger than 700 pm and precedes the onset of measurable van der Waals attraction. In constant-height imaging, this interaction gives rise to a characteristic ring-shaped contrast. The measured force-distance curves and image features are quantitatively reproduced by a model that only includes electrostatic, van der Waals, and Pauli terms. Measurements performed with CO-terminated and bare metal tips show that the long-range repulsion is governed primarily by the electrostatic interaction between the dipole moment of the metallic tip apex and the adatom. Comparison with Fe adatoms demonstrates that the observed long-range rings are distinct from short-range chemical contrast.

We systematically investigate the magneto-optical transport and the associated Faraday and Kerr rotations in 8-Pmmn monolayer borophene subjected to a perpendicular magnetic field and off-resonant Floquet driving. The interplay between Floquet band engineering, Landau quantization, and optical selection rules leads to distinct features in the magneto-optical response. In particular, the real part of the longitudinal magneto-optical conductivity exhibits a series of resonance peaks arising from optically allowed intra- and inter-Landau-level transitions, whereas the Hall conductivity is dominated by a single pronounced resonance peak associated with the lowest cyclotron-like transition. Consistently, the complex reflectivity spectra for right- and left-circularly polarized light display analogous structures: their real parts contain multiple resonant peaks reflecting the longitudinal response, while their imaginary parts show a single dominant resonance peak with opposite phases originating from the Hall contribution. As a consequence of the resulting circular birefringence and dichroism, the Faraday and Kerr rotation angles exhibit similar resonance characters but opposite spectral phases, reflecting the different electromagnetic boundary conditions of transmission and reflection geometries. These findings indicate that the combined effects of magnetic-field-induced Landau quantization and Floquet band engineering provide an efficient pathway to control magneto-optical responses in borophene, revealing its potential for tunable two-dimensional magneto-optical and optoelectronic devices.

Altermagnets (AMs) are an emergent class of magnetic materials that combine properties of ferromagnets and antiferromagnets, exhibiting spin-polarized Fermi surfaces and zero net magnetic moment due to combined time-reversal and crystal symmetry. Here, we construct a Kondo-lattice model on a two-dimensional (2D) square Lieb lattice to investigate the topological properties of AMs. We identify a type-II quantum spin Hall state characterized by spin-polarized counterpropagating edge states. Breaking the $C_{4z}\mathcal{T}$ symmetry, which connects magnetic sublattices, induces a transition to a quantum anomalous Hall state. We further establish a strain-induced mechanism to control these topological phase transitions and present the corresponding phase diagram. Finally, we demonstrate the predicted transitions in monolayer CrO, a realistic altermagnetic candidate, using first-principles calculations. Our findings highlight the potential of 2D AMs as a versatile platform for topological spintronics, enabling strain-tunable helical and chiral edge states within a single system.

We introduce a linear-scaling real-space methodology to compute time-resolved electrical responses of materials driven far from equilibrium, with energy relaxation and disorder treated on an equal footing. Applying this approach to gapped monolayer and AB-stacked (Bernal) bilayer graphene, when driven by a circularly polarized optical pulse, we observe the generation (suppression) of a finite Hall conductivity when the system is trivial (topological). This Hall signal oscillates during optical driving and remains sizable after the light is switched off before relaxing toward equilibrium. Remarkably, this dynamical Hall response is robust in the presence of realistic descriptions of disorder, suggesting that disorder and relaxation dynamics can be leveraged as design parameters rather than as limitations. More broadly, our new methodology enables the investigation of electrical responses in driven, complex disordered quantum materials and highlights how engineered energy-transfer pathways can enable ultrafast optoelectronic functionality.

The valley degree of freedom in tetragonal systems, governed by crystalline symmetry rather than time-reversal symmetry, offers a distinct paradigm for valleytronics compared to hexagonal systems. Combining symmetry-adapted tight-binding and first-principles calculations, we investigate strain-engineered valley physics in tetragonal monolayers featuring ${\mathit{C}}_{4z}$ and ${\mathit{S}}_{4z}$ symmetries. We unveil that the intrinsic valley-layer coupling within ${\mathit{S}}_{4z}$ symmetric systems, together with a giant bipolar out-of-plane piezoelectric response, enables significant and reversible valley polarization under bipolar in-plane uniaxial strain in the ${\text{Hf}}_2{\text{SiCO}}_2$ monolayer. This phenomenon arises from the interplay between strain-modified hopping integrals and the resulting crystal field potential, distinguishing it from traditional electric-field-driven mechanisms. Furthermore, the symmetry-allowed valley-contrasting linear dichroism under applied strain is notably robust. Our findings highlight strain-direction selectivity as a powerful strategy for achieving and controlling valley polarization, providing a general theoretical framework for mechanovalleytronics in tetragonal systems.

Understanding the kinetic pathways connecting carbon polymorphs at multimegabar pressures remains a major unsolved problem in high-pressure physics. Here, we provide insights into the long-standing question of BC8 formation and stability by combining a state-of-the-art SNAP machine-learning interatomic potential with enhanced sampling via metadynamics, enabling direct access to transition mechanisms far beyond the reach of standard molecular dynamics. Our simulations show that carbon phase transformations are intrinsically complex, proceeding through multiple intermediate disordered and crystalline states governed by nontrivial kinetic ordering. We determine the upper pressure limit for BC8 formation and reveal that hexagonal diamond transforms to BC8 faster than cubic diamond—an unexpected and experimentally testable prediction. We also identify a $P222$ carbon phase that becomes competitive with diamond and simple cubic above 1.8 TPa, and we demonstrate that BC8 may be quenched to ambient conditions at moderate temperatures. Together, these results establish a general and transferable framework for resolving kinetic pathways in solid-solid phase transitions and provide physical insights into carbon's complex high-pressure landscape.

Topological edge states, immune to backscattering, present challenges in confining them to specific positions within a waveguide. Topological photonic cavities have been shown to localize edge states along their propagation path, opening unique opportunities for manipulating and collecting electromagnetic waves. In this Letter, we realize a geometry-dependent underwater topological ultrasound cavity, created using a valley sonic crystal waveguide terminated by a rigid reflector. The strong confinement of sound wave energy in the oblique reflector surface is experimentally observed through ultrasound frequency- and time-domain measurements, which is due to the extended time delay of the valley polarization flipping. We further demonstrate that the valley-flipping time at the reflector surface is dependent on the edge configuration of the valley sonic crystals and the orientation of the reflector. Our Letter introduces a strategy for designing topological acoustic cavities and achieving ultrasonic wave confinement in underwater environments.

Optical spectroscopy provides a powerful, contact-free probe of topological quantum states, yet exact constraints on antisymmetric Hall absorption remain much less well developed than their longitudinal counterparts. Motivated by earlier Hall-conductivity sum rules, we formulate the corresponding first-frequency-moment constraint for the antisymmetric optical conductivity, whose imaginary part governs chirality-dependent absorption. We then demonstrate this sum rule in two classes of time-reversal-breaking topological systems. For a zero-field moiré continuum model hosting topological bands, the moment vanishes exactly, implying that any low-frequency anomalous Hall absorption must be compensated by a higher-frequency spectral weight of the opposite sign. For a Hofstadter model under a uniform magnetic field, the same moment takes a universal value fixed by the magnetic flux density, independent of microscopic model details. By linking low- and high-frequency spectral contributions, this optical Hall absorption sum rule provides a rigorous framework for quantifying circular dichroism constraints and diagnosing Landau-level mixing. Our results show how a known Hall spectral constraint acquires experimentally relevant content in modern interacting topological materials.

Fractional quantum Hall (FQH) systems are strongly interacting electron systems with topological order. These systems are characterized by novel ground states and fractionally charged and neutral excitations. The neutral excitations are dominated by a low-energy collective magnetoroton mode. Here, we derive and use a quasi-one-dimensional model to investigate the ultrafast nonequilibrium dynamics of a laser-driven FQH system within a two-Landau-level approximation. As opposed to traditional and synthetic bilayers, our model accounts for interactions where electrons can scatter from one Landau level to another. By performing exact time evolution of the system, we create an out-of-equilibrium state following the laser pulse that exhibits a rich set of nonequilibrium phenomena. Our calculations show the presence of nontrivial excited modes. One of these modes is electromagnetically active and represents density oscillations of the magnetoplasmon mode. Another mode is identified by evaluating the overlap between the initial ground state and the nonequilibrium state. This mode is analogous to the chiral-graviton mode for FQH systems recently measured in experiments [Liang et al., Nature (London) 628, 78 (2024)]. Our results show that a linearly polarized pulse field can excite the graviton mode when inter-Landau-level scattering occurs.

We report high-resolution polar Kerr effect measurements on ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$ and ${\mathrm{ScV}}_6{\mathrm{Sn}}_6\mathrm{single}$ crystals in search of signatures of the spontaneous polar Kerr effect (PKE) below the charge order transitions of these materials. Utilizing two separate zero-area loop Sagnac interferometers operating at 1550- and 830-nm wavelengths, we studied the temperature dependence of a possible PKE after training with the magnetic field. While a finite-field Kerr measurement yielded optical rotation expected from the Pauli susceptibility of the itinerant carriers, no signal was detected at zero field to within the noise floor limit of the apparatus of below $\sim 100$ nanorad. Simultaneous coherent reflection measurements confirm the sharpness of the charge order transition in the same optical volume as the Kerr measurements. The application of strain to reveal a hidden flux-ordered magnetic state did not result in a finite Kerr effect.