${\mathrm{NaRuO}}_2$ forms a delafossitelike structure that contains triangular sublattices of edge-sharing ${\mathrm{RuO}}_6$ octahedra. It shows no evidence of magnetic order down to 100 mK and persistent spin fluctuations, suggestive of a quantum disordered magnetic ground state. In order to characterize the physical regime from which this disordered state arises, we use resonant inelastic x-ray scattering (RIXS) and x-ray absorption spectroscopy (XAS) at the $\mathrm{Ru}\text{−}{L}_{2,3}$ edge, along with pulsed high-field magnetization to characterize both the local electronic structure and the magnetic interactions. We infer significant spin-orbit coupling from XAS measurements. With RIXS, we observe intraorbital excitations, which are damped due to the near itinerancy of the band and suggestive of strong charge fluctuations. Corroborated by fits of the high-field magnetization to a random singlet model, we propose a picture of a nearly itinerant system with strong magnetic and charge fluctuations that destabilize long-range magnetic order.

Coherent phonons are phase-locked lattice vibrations that provide a sensitive probe of nonequilibrium dynamics in materials. Existing first-principles equation-of-motion (EOM) approaches have successfully described coherent phonons in metals and semimetals, but their extension to semiconductors requires treating the long-lived electron-hole plasma created by above-gap excitation. Here we extend the first-principles EOM framework to photoexcited semiconductors by incorporating the residual plasma force that shifts the lattice equilibrium together with the plasma-induced renormalization of the electronic structure. For monolayer ${\mathrm{MoTe}}_2$, the calculated carrier-density-dependent Te displacement agrees with constrained density-functional perturbation theory only when the renormalized band structure is included. We then show that increasing the photoexcited carrier density enhances both the initial total force and the plasma-shifted equilibrium position of the $A_{1g}$ mode, producing larger coherent lattice oscillations. This density-controlled response appears in both the atomic displacement and the coherent-phonon-induced band renormalization, providing complementary signatures for ultrafast reflectivity and time- and angle-resolved photoemission spectroscopy.

Boosting superconductivity by metallic reservoirs is the essence of Kivelson's bilayer proposal. One layer provides pairing to the electrons, while the weakly coupled metal provides additional phase coherence to those pairs by mediating extended-range pair-pair coupling. Demonstrating significant and unambiguous performance gains with strong-coupling methods for such setups had been difficult. In the present work, we study these systems doped away from half filling, corresponding to a partially spin-polarized 1D Anderson or Kondo lattice. We show that this breaks the coexistence of dominant superconducting and density-density correlations decisively in favor or the former. Consequently, we provide evidence that in this doped regime, superconducting near-long-range order is not precluded by a small charge gap in the thermodynamic limit, as we have recently shown to be the case at half filling. We study the complex manner in which the enhancement of superconductivity in the pairing layer depends on the parameters of the metal, and especially that both pairing-limited and stiffness-limited regimes may appear in these systems. In addition to superconducting bilayers, our results are relevant, via a particle-hole transformation, for heavy-fermion Kondo-lattice materials in magnetic fields, as we provide previously lacking insight on the competition between antiferromagnetic and easy-plane magnetism, as well as a route for comprehensive indirect tests of Kivelson's bilayer proposal well beyond previous capabilities.

Two-dimensional (2D) low-symmetry semiconductors exhibit intrinsic in-plane anisotropy that is attractive for directional nanoelectronic and optoelectronic applications, yet high-performance candidates remain limited. Using density functional theory, we systematically investigate the structural stability and electronic anisotropy of monolayer (1L) and bilayer black arsenene (b-As) with $\mathit{AA}, \mathit{AB}, \mathit{AC}$, and $A\delta$ stacking configurations. Relative energy calculations identify $\mathit{AB}$ stacking as the ground-state bilayer structure, while $\mathit{AA}, \mathit{AC}$, and $A\delta$ are metastable but dynamically stable within the considered configurations. All structures retain semiconducting character, with stacking-dependent band dispersions and band gap magnitudes. Tensile strain along the armchair direction efficiently modulates the band gap, with the $\mathit{AB}$ bilayer exhibiting the strongest response, showing a reduction of up to 0.9 eV at $6\%$ strain, indicative of pronounced anisotropic deformation potentials. A perpendicular electric field (EF; $\pm 0.5\mathrm{V}/\AA$) produces negligible band gap variation in the 1L but induces substantial and continuous band gap tuning in bilayers due to interlayer potential asymmetry, most prominently in the $\mathit{AB}$ stacking, which maintains a robust and continuously tunable band gap across the entire field range. Twist-induced interlayer coupling further alters the electronic anisotropy: A ${90}^{\circ }$ twisted bilayer exhibits nearly isotropic in-plane band dispersion, while external EFs restore anisotropic features by breaking the symmetry between rotated layers. These results demonstrate that stacking order, strain, twist angle, and electrostatic gating provide independent and effective control parameters for engineering the magnitude and directional character of the band structure in b-As, establishing it as a mechanically and electrically tunable anisotropic 2D semiconductor.

Unconventional odd-parity magnets, characterized by an odd-parity momentum-dependent spin splitting, offer a fundamentally distinct paradigm for noncollinear spintronics. Here, we theoretically investigate the Ruderman-Kittel-Kasuya-Yosida indirect exchange in two-dimensional odd-parity magnets subjected to Rashba spin-orbit coupling. Using an analytical real-space Green's function formalism, we uncover a parity-driven spatial decoupling in the magnetic response. Because of the odd-parity exchange field, the out-of-plane Ising interaction is structurally insulated from the macroscopic magnetic modulation, oscillating isotropically at the shifted Fermi wave vector. Conversely, the in-plane Heisenberg components exhibit pronounced, directionally tunable spatial beating. Beyond collinear exchange, the hybridized bands generate a highly tunable, three-component Dzyaloshinskii-Moriya interaction alongside symmetric off-diagonal anisotropies. We reveal that the out-of-plane chiral twisting is driven by the massive, nonrelativistic momentum shift, while the in-plane chiral components are relativistic. Furthermore, the competition between the nodal geometry and the Rashba gap drives an anomalous, dimension-reducing crossover, in which the in-plane chiral components follow a one-dimensional-like $1/R$ spatial decay along the nodal lines over an extended intermediate-distance window before ultimately recovering the conventional two-dimensional $1/R^2$ asymptote. We demonstrate that these findings hold universally across the odd-parity class by explicitly generalizing our framework to $f$-wave magnets. Finally, numerical tight-binding calculations verify the striking robustness of this parity-driven decoupling against both scalar and random magnetic exchange disorder. These findings establish odd-parity magnets as promising platforms for engineering robust, directionally tunable noncollinear spin textures.

We study the genuine multipartite entanglement (GME) in a disordered system and its potential for detecting the transition from the thermalized to the localized phase. We demonstrate that the quenched average GME can approach its near-maximum value in the ergodic phase of a disordered quantum spin model. In contrast, GME vanishes in the many-body localized (MBL) phase, both in equilibrium and in the long-time dynamical regime, indicating a lack of multipartite entanglement in the localized regime. To establish this, we analyze the disordered Heisenberg spin chain subjected to a random magnetic field along with the two- and three-body Dzyaloshinskii-Moriya (DM) interactions. We show that the behavior of GME, in both the mid-spectrum of the Hamiltonian and the dynamically evolved states from an initial Néel configuration, serves as a reliable indicator of the critical disorder strength required for the ergodic-to-MBL transition. The identified transition point aligns well with standard indicators such as the gap ratio and the correlation length. Moreover, we find that the presence of DM interactions, particularly the three-body interaction, significantly enlarges the thermal phase and delays the onset of localization with an extended correlation length. This shift in the transition point is consistently reflected in both static and dynamical analyses, reinforcing GME as a robust probe for MBL transitions.

Two-dimensional materials have enabled unprecedented control over material properties through interlayer engineering, where mechanisms such as sliding can directly modify symmetry configurations and functionalities. Based on first-principles calculations, we demonstrate that bilayer $\mathrm{VP}{X}_3$ $(X=\mathrm{S},\mathrm{Se},\mathrm{Te})$ provides an ideal platform in which interlayer sliding simultaneously activates and couples sliding ferroelectricity (FE) and altermagnetism (AM). Here, a switchable out-of-plane FE polarization emerges from the sliding-induced breaking of inversion symmetry, whereas momentum-dependent spin splitting arises from crystal symmetries, without spin-orbit coupling. Crucially, the interlayer sliding displacement that reverses FE polarization also fully inverts the spin polarization in the AMs, equivalent to a 180° spin reversal. Remarkably, this magnetoelectric (ME) coupling is directly manifested as clear sign reversals in the magneto-optical Kerr rotation. Furthermore, interlayer compression enhances both the FE polarization and AM band splitting in a nearly linear manner, demonstrating strain-tunable multiferroic behavior. Our results establish $\mathrm{VP}{X}_3$ bilayers as a prototypical system for achieving deterministic control over coupled ferroic orders via atomic-scale layer manipulation, opening avenues for designing spintronic and ME devices.

Measurements of resistive breakdown in electronic systems under a DC electric field have shown that the threshold fields for the insulator-to-metal transition are significantly lower than predicted by single-electron excitation scenarios, such as the Landau-Zener theory. In this work, we propose an alternate mechanism of destabilizing ordered insulators under a DC electric field by fluctuations of the order parameters through the Goldstone mode excitations. The low-energy bosonic excitations receive energy from accelerated electrons and thus destroy the spontaneous symmetry breaking. Using the Keldysh Green's function formalism, we numerically confirm that the Goldstone mode remains well-defined in the nonequilibrium steady state, while its nonequilibrium excitations are sensitive to the electric field. The effective temperature of the Goldstone mode increases much more rapidly than the electronic effective temperature, with the bosonic threshold field significantly smaller than the electronic one, which suggests that collective phase dynamics may further reduce the transition field to the experimental range via a purely electronic mechanism.

Determining the Fermi surface (FS) volume in underdoped cuprates is crucial for understanding the nature of the strongly correlated pseudogap phase. Conventional quantum oscillation techniques, typically used for this purpose, are inapplicable in this high-temperature regime due to thermal and disorder-induced smearing of Landau levels. We propose Sondheimer oscillations (SO), semiclassical oscillations of in-plane magnetoresistivity in thin films, as a robust alternative probe of FS reconstruction. SO arise from the commensuration between the cyclotron radius and film thickness, do not rely on Landau quantization, and remain observable at moderate fields and elevated temperatures where quantum oscillations are suppressed. Their frequencies depend solely on the FS parameters (e.g., curvature), and not on specific details of scattering mechanisms. SO are also sensitive to the coherence of interlayer tunneling, allow contributions from individual FS pockets to be distinguished in the frequency domain, and naturally include the Yamaji angle effect (if present in the system) as a prominent feature in the frequency spectrum. We compute SO spectra as a function of the magnetic field orientation for three representative scenarios: (i) an unreconstructed large FS, (ii) a spin density wave reconstructed FS with volume $p/4$, and (iii) a fractionalized Fermi liquid (${\mathrm{FL}}^{*}$) with pocket volume $p/8$ (here $p$ is the hole doping). We show that the SO spectrum offers a wealth of universal features that could be used to differentiate between these scenarios. In particular, we highlight a FS geometry-dependent phase shift between oscillations in longitudinal and transverse conductivities, characterize how the FS curvature can be extracted from SO if the film orientation is perpendicular to the crystallographic $c$ axis, and analyze the evolution of the SO spectrum with doping.

Femtosecond time-resolved optical absorption spectroscopy of gallium arsenide was performed using an x-ray free-electron laser (XFEL) to excite the K-shell electrons. The transient absorption spectra show a distinct increase in absorption extending to approximately $50\mathrm{meV}$ below the band gap within $1.4\mathrm{ps}$ after XFEL irradiation, suggesting the formation of localized states associated with core-hole generation. The transient feature subsequently relaxes on a few-picosecond timescale through many-body processes, accompanied by Pauli blocking arising from the redistribution of hot carriers within the valence and conduction bands. By comparison with near-infrared laser excitation, we found that x-ray excitation is characterized by the formation of transient absorption states below the band gap and reduced carrier accumulation at the conduction-band minimum. These results provide valuable insights into x-ray-induced electronic structure modifications and carrier dynamics in semiconductors on the picosecond timescale, where deep core holes and many-body effects play crucial roles.

We propose extended $s$-wave altermagnets ($s\mathrm{AMs}$) as a class of magnetic states that are fully gapped, spin compensated, and feature spin-polarized bands. $s\mathrm{AMs}$ are formed through valley-exchange symmetries, which act as momentum-space translations beyond standard crystallographic spin-group classifications. Using an effective two-valley model, we demonstrate that $s\mathrm{AMs}$ exhibit isotropic spin splitting, enable spin-selective transport in tailored heterostructures, and give rise to descendant pair density wave order. From a microscopic $s\mathrm{AM}$ minimal model, we develop the guiding principles to identify $s\mathrm{AMs}$ in quantum magnets.

Fluctuations in disordered matter play a central role in determining material properties and physical responses. Recent studies have identified an exotic class of systems known as structurally hyperuniform materials, in which large-scale density fluctuations are anomalously suppressed through special spatial organization of particles, phases, or microstructural features. Here we demonstrate that ordinary, structurally nonhyperuniform disordered materials can nevertheless support hyperuniform physical source fields, including scalar, vector, or tensor observables such as incompatibility, bound charge, and bound current. These sources can also regularize the response fields they drive. We develop a general theoretical framework in which a physical field is generated from a more primitive parent field through a local physical operator. In Fourier space, the spectrum of the derived field is determined by the product of the parent-field spectrum and the Fourier symbol of the operator. When the operator has a small-wave-number zero of order $m$, the derived source spectrum is suppressed by $k^{2m}$ relative to the parent spectrum in the coupled spectral channels. For a parent spectrum scaling as $k^{\alpha }$, hyperuniformity is guaranteed if $\alpha +2m>0$, provided the assumed infrared scaling holds; sufficiently infrared-divergent parent fluctuations need not yield a hyperuniform source. We demonstrate this mechanism in elastic, electrostatic, and magnetostatic settings, showing that operator-generated incompatibility, bound charge, and bound-current fields can become hyperuniform even when their parent eigenstrain, polarization, or magnetization fields remain conventionally disordered. These findings identify a general, explicitly conditional route to field hyperuniformity through local physical constraints and distinguish hyperuniform source generation from regularization of the resulting response fields.

Using first-principles calculations, we investigate how homogeneous strain and crystallographic surface orientation modify the vibrational and superconducting properties of niobium. For bulk Nb, tensile strain strongly softens the phonon spectrum and enhances the electron-phonon coupling, increasing the superconducting transition temperature within the harmonic approximation from 9.5 K at equilibrium to 14.6 K at $\sim 6\%$ lattice expansion. Anharmonic corrections are small near equilibrium but become increasingly significant under tensile strain and considerably reduce this enhancement at large expansion. For ultrathin Nb slabs with low-index (001), (110), and (111) surface orientations, harmonic phonon calculations exhibit imaginary modes, showing that anharmonic lattice effects are essential. To treat these effects efficiently, we train Nb-specific machine-learning interatomic potentials on bulk and slab first-principles configurations and use them to accelerate stochastic self-consistent harmonic approximation calculations, thereby obtaining anharmonically renormalized phonon modes that are combined with density-functional perturbation theory electron–phonon matrix elements to construct the Eliashberg spectral function. Among the three clean free-standing slabs examined, Nb(001) exhibits the strongest electron-phonon coupling and the highest calculated transition temperature of 10.1 K, while Nb(110) and Nb(111) show progressively reduced pairing strength. Finally, by analyzing the Eliashberg spectral function and the functional derivative $\delta T_{\text{c}}/\delta {\alpha }^{2}F\left(\omega \right)$, we identify the phonon energy ranges most effective for superconducting pairing. Our results show that strain, surface termination, and anharmonic phonon renormalization provide complementary and interrelated microscopic routes for tuning superconductivity in Nb.

We construct a generalized time-dependent Ginzburg-Landau model to demonstrate the possibility of inducing $p$- and $d$-wave components in an originally pure $s$-wave centrosymmetric superconductor via microwave radiation. In this framework, specializing to $O_h$ point-group symmetry, we introduce gradient terms that couple the $s$-wave superconducting order parameter with other symmetry-allowed components. The singlet-to-singlet gradient terms are quadratic in spatial derivatives, while, in the presence of spin-orbit coupling, linear-in-derivatives terms coupling singlet and triplet order parameters are also permitted. Through the minimal substitution procedure, these terms enable coupling between different superconducting order parameters via the vector potential, thereby leading to the generation of $p$-wave, $d$-wave, and other symmetry-allowed components. Such a manipulation of the superconducting state locally via a microwave beam could be considered as one more facet of the concept of quantum printing.

Hafnia-based ferroelectric films switch at room-temperature coercive fields of about $1\mathrm{MV}{\mathrm{cm}}^{-1}$, an order of magnitude below the fields predicted for homogeneous polarization reversal. The predicted switching fields can exceed the dielectric breakdown strength, although polarization reversal is experimentally observed before breakdown. In this work, we use first-principles calculations and symmetry-reduced atom mappings to compare switching in the polar $Pca{2}_1$ and $Pmn{2}_1$ structures of ${\mathrm{HfO}}_2$ and ${\mathrm{Hf}}_{0.5}{\mathrm{Zr}}_{0.5}{\mathrm{O}}_2$ (HZO). Systematic comparison of the matched pathways shows that Zr substitution lowers nearly all sampled $Pca{2}_1$ barriers and produces a particularly low-barrier $Pmn{2}_1 {180}^{\circ }$ reversal route. The comparison identifies a favorable $Pmn{2}_1 {180}^{\circ }$ reversal whose barrier is substantially reduced in HZO. For a field normal to the predominantly exposed $\left\{111\right\}$ facets, the estimated room-temperature critical field for this route, obtained using phenomenological fourth-order Landau-Ginzburg-Devonshire temperature scaling, decreases from 2.57 $\mathrm{MV}{\mathrm{cm}}^{-1}$ in ${\mathrm{HfO}}_2$ to 1.02 $\mathrm{MV}{\mathrm{cm}}^{-1}$ in HZO. These results clarify how Zr substitution lowers the switching field. The identified reversal pathway helps explain the discrepancy between conventional homogeneous-switching predictions and low-field measurements.

We investigate the emergence of a scarred discrete time crystal (SDTC) phase in a periodically driven dimerized spin chain. While generic interacting Floquet systems are expected to thermalize according to the eigenstate thermalization hypothesis (ETH), we demonstrate that this system hosts quantum many-body scars (QMBS) that induce a regime of weak ergodicity breaking. Through an analysis of Floquet level statistics, entanglement entropy, and eigenstate fidelity, we identify a manifold of low-entanglement states characterized by semi-Poisson statistics embedded within an otherwise thermal spectrum. These scarred states support robust subharmonic oscillations with period doubling, signaling the spontaneous breaking of discrete time-translation symmetry. We show that the SDTC response is robust against a variety of initial state configurations, demonstrating its stability beyond fine-tuned conditions. A finite-size scaling analysis reveals that the time-crystalline lifetime grows with system size within the range accessible to our exact-diagonalization calculations. However, drawing on the general phenomenology of approximate many-body scars, we expect that hybridization between Floquet scars and the thermal continuum will eventually curtail this growth, causing the lifetime to saturate at system sizes beyond our current numerical reach. This characterizes the SDTC as a long-lived metastable dynamical regime rather than a strictly stable thermodynamic phase, providing a comprehensive framework for understanding the interplay between periodic driving and constrained many-body dynamics in disorder-free systems.

Two-dimensional (2D) multiferroics offer an appealing platform for electrically and magnetically reconfigurable nonlinear photoresponses. In particular, the bulk photovoltaic (BPV) effect, as a second-order nonlinear optical response rooted in Bloch wave quantum geometry, is highly sensitive to crystalline and magnetic symmetries. Therefore, the BPV effect could serve as a sensitive method to capture subtle symmetries in multiferroic systems with fruitful ferroic order switching. Here, taking a monolayer multiferroic $\mathrm{CuCr}{\mathrm{P}}_2{\mathrm{S}}_6$ as a representative example, we systematically investigate the BPV effect across the paraelectric-ferroelectric-antiferroelectric phase transition by combining first-principles calculations with magnetic group symmetry analysis. We explore the influence of phase transition, polarization reversal, multiferroicity, magnetic order, magnetocrystalline anisotropy, and its emergent altermagnetism on the BPV responses. Our results suggest that the BPV effect can effectively distinguish distinct polarization and magnetic states, establishing monolayer $\mathrm{CuCr}{\mathrm{P}}_2{\mathrm{S}}_6$ as a promising 2D multiferroic platform for electrically and magnetically reconfigurable BPV and nonlinear optoelectronic devices.

We report on current-induced spin–orbit torques and field-free magnetization switching in a two-dimensional van der Waals heterostructure consisting of ${\text{WTe}}_2$ as the spin source and ${\mathrm{Fe}}_3{\mathrm{GeTe}}_2$ as the magnetic layer. Harmonic magneto-optical Kerr effect measurements enable the disentanglement of torque components arising from out-of-plane and in-plane spin accumulations generated by charge transport in ${\text{WTe}}_2$. Our analysis reveals a dominant contribution from out-of-plane spins, yielding a field-like torque efficiency of approximately $8.8\%$, and a damping-like torque efficiency arising from in-plane spins of $1.8\%$ at $150\mathrm{K}$. We observe deterministic bipolar field-free switching of the perpendicular magnetization of ${\mathrm{Fe}}_3{\mathrm{GeTe}}_2$ up to the Curie temperature, consistent with the strong torque generated by out-of-plane spins. The switching efficiencies extracted from current-induced magnetization reversal and hysteresis loop shifts exceed the intrinsic torque efficiencies measured by harmonic magneto-optical Kerr effect by more than one order of magnitude. The switching efficiencies scale with the temperature-dependent coercivity of the ${\mathrm{Fe}}_3{\mathrm{GeTe}}_2$ flake, indicating that they reflect a combination of thermal effects assisting magnetization reversal and different torque components. Our results provide a quantitative assessment of unconventional spin–orbit torques in ${\mathrm{Fe}}_3{\mathrm{GeTe}}_2/{\text{WTe}}_2$ and emphasize the importance of carefully interpreting torque efficiencies derived from switching-based measurements, particularly in van der Waals systems with Curie temperatures near or below room temperature.