We propose a novel protocol to realize discrete time-crystal (DTC) order in clean, periodically driven spin-$1/2$ chains. In each drive cycle, a global spin flip is followed by a two-tone flat-band segment. This flat-band segment engineers a fully degenerate Floquet quasienergy spectrum, suppresses thermalization, and stabilizes a robust period-doubled subharmonic response. Using exact time evolution, we identify a pronounced subharmonic peak at half the drive frequency in the Fourier spectrum of the order parameter, thereby providing clear evidence for the emergence of stable DTC. The resulting phase is insensitive to system size, interaction strength, and interaction range; however, it remains sensitive to spin-rotation errors (${\varepsilon }_r$), which can destabilize the subharmonic response. Compared with disorder-induced many-body localized (MBL) and disorder-free dynamically many-body localized (DMBL) DTCs, we find that the exact flat-band protocol offers a broader tunability of drive parameters, whereas MBL and DMBL-based DTCs are more resistant to ${\varepsilon }_r$. In particular, the ${\varepsilon }_r$ sensitivity can be suppressed by incorporating additional spin-spin interactions that have modest deviations from the ideal flat-band protocol. This manifests itself in a robust DTC response over a finite window of spin-coupling strengths and drive frequencies. Our results establish flat-band driving as a versatile and experimentally relevant route to DTC order in disorder-free spin systems and motivate further exploration of nonequilibrium phases.

We report on antiferromagnetic resonance experiments in bulk tetragonal NaMnAs—a room-temperature antiferromagnetic semiconductor. Our results corroborate previous ab initio studies which propose that NaMnAs is an easy-axis antiferromagnet with the Néel vector oriented along the tetragonal axis. At $B=0$, we find a single antiferromagnetic resonance line at 7 meV and associate it with a doubly degenerate ($k=0$) magnon mode. Its energy softens considerably with increasing $T$, but remains clearly visible in the data up to room temperature. From the experimental data, we estimate the single-ion anisotropy of the Mn ions in NaMnAs in the range 0.1–0.2 meV, a value that is relatively large compared to other manganese-based antiferromagnets.

The critical behavior of two-dimensional Josephson-junction network with the positional disorder is studied via Monte Carlo simulations. We consider a two-band model in which each superconducting grain is randomly displaced from its ideal position on a square lattice and the effective strength of the disorder is controlled by tuning the magnitude of transverse magnetic field. With the finite-size scaling analysis, we determine the phase diagram of this system as a function of disorder and temperature. In the weak disorder regime, our numerical results demonstrate the emergence of multiple Berezinskii-Kosterlitz-Thouless-type phase transitions from the normal to two different superconducting states. The new intermediate superconducting phase characterized by the quasi-long-range order exclusively in one of these bands will vanish when the disorder strength exceeds the critical value $f_c\approx 100$. Meanwhile in the strong disorder regime, our numerical data on the Edwards-Anderson order parameter support a zero-temperature transition from the normal state to the glassy phase.

We derive the normal and anomalous proper polarization functions and the screened Coulomb interactions in a two-dimensional superfluid electron-hole bilayer, including all first-order corrections beyond the Random Phase Approximation (RPA) at zero temperature. This requires a modification of the perturbation method as first noted by Nozières and Schrieffer. We discuss the physical origin and magnitude of the first-order corrections in a superfluid system with long-range Coulomb interactions. The screened electron-electron, hole-hole, and electron-hole interactions in the superfluid state are evaluated as functions of the carrier density. We find that at low density, the strong cancellations between the normal and anomalous components that make screening of the interactions negligible, apply not only within RPA but also with the first-order corrections included. As the density is increased, the normal-anomalous cancellation weakens and screening becomes increasingly significant. We find that the first-order corrections amplify the normal-anomalous difference but only at large momenta exchanged in the two-particle scattering, so their effect on the interactions remains modest. We conclude that the superfluid state RPA is an excellent approximation for the screening and for the effective electron-hole pairing in this superfluid system over the range of densities up to the maximum of the superfluid gap.

Electron-plasmon coupling, in which carriers are dressed by collective electronic oscillations, represents an important many-body interaction in solids. Here, using angle-resolved photoemission spectroscopy, we report the tunable electron-plasmon coupling in the layered transition metal dichalcogenide system ${\mathrm{ZrSe}}_{2-x}{\mathrm{Te}}_x$. The coupling manifests itself as nondispersive satellite features below the conduction-band minimum at the $\overline{M}$ point, with the energy scale significantly exceeding the longitudinal optical phonon energy in this case. We demonstrate that the plasmon energy can be tuned through two synergistic routes. First, the chemical substitution of Te modifies the band structure and dielectric constant, leading to a twofold increase in energy. In contrast, in situ alkali-metal deposition varies the carrier concentration, affecting both the effective mass and plasmon energy. By comparing the plasmon energy with the carrier concentration and effective mass, our results provide compelling evidence for the electron-plasmon coupling in this system. This work establishes ${\mathrm{ZrSe}}_{2-x}{\mathrm{Te}}_x$ as a versatile platform for engineering collective electronic excitations and offers intriguing means for manipulating many-body interactions in layered quantum materials.

We report on a muon spin rotation study of the wide band-gap material ${\mathrm{La}}_2{\mathrm{O}}_3$. The focus is on the implantation process and the configurations formed at the end of the implantation trajectory. The data are analyzed in the recently developed “doorway model.” First-principles density-functional calculations were performed to identify the different muonium configurations in the lattice and their hyperfine interaction properties. A detailed description of the reaction mechanism at the end of the muon trajectory is given.

Motivated by recent progress in employing two key classes of two-dimensional materials—topological insulators and transition-metal dichalcogenides (TMDCs)—as spin sources for generating spin-orbit torque (SOT), we investigate current-induced spin polarization and the resulting SOT in bilayers composed of a TMDC (${\mathrm{WSe}}_2$ or ${\mathrm{MoSe}}_2$) and ferromagnetic chromium iodide (${\mathrm{CrI}}_3$), beyond the linear response regime. Using the steady-state Boltzmann equation, we find that intraband transitions yield a strong fieldlike torque on the ${\mathrm{CrI}}_3$ layer, while interband transitions give rise to a comparatively weaker dampinglike torque in the ${\mathrm{WSe}}_2/{\mathrm{CrI}}_3$ system. Remarkably, the dampinglike component is enhanced by up to three orders of magnitude in $n$-doped ${\mathrm{MoSe}}_2$, reaching a strength comparable to the fieldlike torque, which itself is an order of magnitude larger than that in the ${\mathrm{WSe}}_2$-based bilayer. Both torque components exhibit strong asymmetry between $n$- and $p$-type doping in ${\mathrm{WSe}}_2$ and ${\mathrm{MoSe}}_2$ systems. Furthermore, we demonstrate that the twist angle plays a crucial role: depending on the TMDC and chemical potential, twisting can reverse the sign of the SOT and significantly modulate its magnitude. Finally, we show that a transverse gate electric field enables substantial tunability of the SOT, by nearly one order of magnitude, and induces a sign reversal at a twist angle of $10.{16}^{\circ }$.

Twistoptics has recently emerged as a branch of nano-optics that explores light propagation in stacks of thin anisotropic layers rotated relative to one another. The concept is particularly relevant for polaritons—hybrid light-matter quasiparticles—in van der Waals (vdW) materials, where strong in-plane anisotropy and deep subwavelength confinement make the polaritonic dispersion highly sensitive to interlayer twist angles. This sensitivity enables exotic phenomena such as canalization, i.e., diffraction-free propagation, with potential applications ranging from thermal management to super-resolution imaging. Despite rapid progress, a general analytical framework to describe polariton propagation in twisted planar heterostructures has been missing. Here we present an analytical model for planar stacks comprising an arbitrary number of finite-thickness anisotropic (biaxial) layers and infinitesimally thin anisotropic conductive sheets. The formalism and its high-momentum and thin-film approximations predict key polaritonic observables, such as wavelength, propagation length, and electromagnetic field distributions. We also provide open-access numerical scripts implementing the model to support their practical use. Together, these results provide a general theoretical foundation for twistoptics and should facilitate the discovery and accelerate the implementation of twist-engineered polaritonic phenomena across the electromagnetic spectrum.

At strong on-site repulsion $U$, the fermionic Hubbard model realizes an extremely correlated electron system. In this regime, it is natural to derive the low-energy physics with the help of noncanonical operators acting on a projected Hilbert space without double occupancies. Using a strong-coupling functional renormalization group technique, we study the physics of such extreme correlations in the strict $U=\infty$ limit, where only kinematic interactions due to the Hilbert space projection remain. For nearest-neighbor hopping on a square lattice, we find that the electronic spectrum is significantly renormalized, with bandwidth and quasiparticle residue strongly decreasing with increasing electron density. On the other hand, damping and particle-hole asymmetry increase, while a polaronic continuum forms in the hole sector, below the single-particle band. Fermi liquid phenomenology applies only at low densities, where the system remains paramagnetic. At higher densities, we find a bad metal with strong magnetic correlations, indicating that the ground state is the Nagaoka ferromagnet at high densities and a stripe antiferromagnet at intermediate densities. Both in the paramagnetic and the ferromagnetic regimes, we observe a violation of Luttinger's theorem.

${\mathrm{CsCr}}_3{\mathrm{Sb}}_5$ is a newly identified strongly correlated kagome superconductor, characterized by non-Fermi-liquid behavior at elevated temperatures and intertwined charge and spin density wave order below $T_{\text{DW}}\approx 54\mathrm{K}$. Under external pressure, this order is suppressed and a superconducting phase emerges. This phase diagram, which closely resembles that of high-$T_c$ superconductors, together with a kagome flat band near the Fermi level and possible altermagnetic order, has motivated extensive theoretical and experimental investigations. To better understand how pressure influences the ordered states, we present a systematic study of the evolution of the electronic properties under applied pressure. Performing density functional theory combined with dynamical mean field theory calculations, we uncover a complex interplay between the redistribution of spectral weight in the flat bands and the strength of electronic correlations under pressure. Our results further strengthen the interpretation that pressure effectively weakens electronic correlations through enhanced orbital hybridization. This, in turn, strongly suggests that superconductivity emerges as a direct consequence of the suppression of the system's ordered phase.

We study the dynamic equilibrium between trions and excitons in monolayers of transition metal dichalcogenides in the presence of resident charge carriers and continuous photogeneration of excitons. We show that heating of the system via Drude absorption of low-frequency radiation leads to bistability of the steady-state equilibrium. The first is a low-temperature state, in which almost all resident charge carriers are bound into trions. The second state occurs at high temperatures, where most trions are dissociated; in this regime, the heating is more efficient due to the higher Drude conductivity of unbound charge carriers compared to trions. Switching between these two states occurs on a timescale of tens to hundreds of picoseconds and is accompanied by a jump in various observables such as temperature, current, and the intensity of exciton or trion luminescence.

We present a symmetry-adapted extension of quantum selected configuration interaction and sample-based quantum diagonalization that rigorously embeds space-group symmetry into the many-body subspace sampled by quantum hardware. The method is benchmarked on the two-leg ladder Hubbard model using both molecular orbital and momentum bases. Energy convergence is shown to be improved in the momentum basis compared to the molecular orbital basis for both the spin-quintet ground state and the spin-singlet excited state. We clarify the relationship between the compactness of the many-body wave function and the sparsity of the representation matrices of symmetry operations. Furthermore, an enhancement in the superconducting correlation function due to the Coulomb interaction is observed in the finite-size system. Our method highlights the importance of symmetry-adapted postprocessing in random-sampling quantum simulation of correlated systems.

We present a semiempirical total-energy tight binding (TETB) parametrization for bilayer graphene that is transferable to arbitrary interlayer separation and local disregistry, and therefore to arbitrary interlayer twist angle. We use this model to investigate how tight binding forces influence the relaxed structures of twisted bilayer graphene (tBLG). The TETB model uses existing Slater-Koster style distance-dependent tight binding parameters to represent the electronic contribution to the energy. In order to capture the remaining contributions to the total energy, we define residual potentials—classical interatomic potentials that, when added to the tight binding band energy of a system, allow us to recreate a potential energy surface of arbitrary interlayer separation and local disregistry. We fit the residual potentials to diffusion quantum Monte Carlo (QMC) and density functional theory (DFT) total-energy data for a range of disregistries, interlayer separations, and biaxial strains. The resulting parametrization enables large-scale calculations (tens of thousands of atoms) with QMC-accurate total energies and built-in electronic structure dependence. As a demonstration, the models are used to relax tBLG with twist angles as low as $\theta =0.{88}^{\circ }$, with unit cells containing as many as 16 876 atoms. At this scale, the relaxations are orders of magnitude faster than recent DFT results. We find that structures relaxed with the TETB model differ from structures relaxed using a reactive bond order (REBO) potential plus Kolmogorov-Crespi classical potential, despite both models being fit to the same total-energy data. These structural differences lead to differences in band structures and even magic angles. We find that the TETB model has a negative out-of-plane Poisson ratio, comparable to results obtained by DFT, whereas the classical model has a Poisson ratio close to zero. Given the differences in out-of-plane Poisson ratio, we show that in-plane biaxial strain has little effect on the magic angle in TETB-relaxed structures but significantly affects the magic angle in structures relaxed with the classical model. We conclude that the difference in magic angle behavior is due to the inclusion of tight binding forces in the relaxation method.

We investigate the low-temperature magnetic properties of the organic Mott insulator $\kappa \text{−}{\left(\mathrm{BEST}\right)}_2{\mathrm{Cu}}_2{\left(\mathrm{CN}\right)}_3$ using ${}^{13}\mathrm{C}$ nuclear magnetic resonance (NMR) and electron paramagnetic resonance (EPR), where BEST denotes bis(ethylenediseleno)tetrathiafulvalene. The NMR Knight shift and EPR integrated intensity together reveal a pronounced drop in spin susceptibility below $T^{*}\approx$ 10 K, accompanied by a broad hump in the nuclear spin-lattice relaxation rate and the growth of a Lorentzian EPR component. These features closely mirror the “6 K anomaly” previously reported for the isostructural $\kappa \text{−}{\left(\mathrm{ET}\right)}_2{\mathrm{Cu}}_2{\left(\mathrm{CN}\right)}_3$, where ET is bis(ethylenedithio)tetrathiafulvalene. Although the two salts share nearly identical electronic correlations and qualitatively exhibit the same magnetic behavior, the combined NMR and EPR results indicate that substituting the heavier BEST for ET enhances the transition temperature to the spin-singlet state. We propose that the increase in molecular mass may induce lattice softening, thereby stabilizing the lattice distortion required for valence-bond-solid order. Our results suggest that lattice dynamics play an important role in the valence-bond-solid transition in $\kappa \text{−}\mathrm{type}$ organics.

Quasinormal modes (QNMs) and coherent control of light-matter interactions (through synchronized multiple coherent incident waves) are profound and pervasive concepts in and beyond photonics, making accessible photonic manipulations with extreme precision and efficiency. Though each has been playing essential roles along its own vein, these two sweeping concepts remain largely segregated with little interaction, blocking vast opportunities of cross-fertilizations to explore. Here we unify both concepts into a fully vectorial framework of coherent control for light interacting with open photonic systems. From the QNM perspective, scattered waves are superimposed radiations from all QNMs excited, and thus coherent controls can be mapped into another problem of QNM excitation manipulations. Within our vectorial framework, coherent control is formulated as interference among distinct QNM excitation channels associated with mutually coherent waves incident from different directions. The amplitudes, phases, polarizations, and propagation directions of these waves can be exploited simultaneously, providing additional independent degrees of freedom for tailoring QNM excitations. This multichannel formulation offers three main advances: (i) Multiple incident waves enlarge the accessible control space for manipulating QNM excitation amplitudes and phases; (ii) appropriately designed excitation channels enable the selective excitation or suppression of individual QNMs, as well as the simultaneous control of multiple QNMs; and (iii) independent control of the relative amplitudes and phases of different QNM radiation components enables the directional scattering polarization to span the entire Poincaré sphere. Given the ubiquity and profundity of QNMs and coherent control in almost all branches of wave physics, the vectorial framework and its underlying principles established will inspire further fundamental explorations and practical applications beyond photonics, opening new opportunities for various forms of wave-matter interactions.

We investigate the temperature dependence of the pseudospin phase transition at filling factor $\nu =2$ in the quantum Hall regime. Experiments were performed on a high-quality, wide AlAs quantum well, where the pseudospin degree of freedom arises naturally from the near degeneracy and equivalence of the two conduction-band valleys. The anisotropic $g$-factor inherent to AlAs enabled independent measurement of the spin resonance response for each valley, thereby allowing quantification of the pseudospin polarization. The observed critical behavior of the pseudospin polarization yielded a critical temperature ${\text{T}}_c=0.9\mathrm{K}$ for the phase transition. The value of ${\text{T}}_c$ was further verified by the temperature dependence of the resistance spike that emerges at the phase transition.

We introduce a non-Abelian geometric quantum energy pump realized by a transitionless geometric quantum drive—a time-dependent Hamiltonian supplemented by a counterdiabatic term generated by a prescribed trajectory on a smooth control manifold—that coherently transports states within a degenerate subspace. When the coordinates of the trajectory are independently addressable by external drives, the net energy transferred between drives is set by the non-Abelian Berry curvature tensor. The trajectory-averaged pumping power is separately controlled by the initial state and by the Hamiltonian topology through the Euler class. We outline an implementation with artificial atoms, which are realizable on various platforms including trapped atoms/ions, superconducting circuits, and semiconductor quantum dots. The resulting energy pump can serve as a quantum transducer or charger, and as a metrological tool for measuring phase coherences in quantum states.

Higher-order topological superconductors host Majorana zero modes localized at corners or hinges, providing a promising route toward scalable and controllable Majorana networks without vortices or magnetic flux. Here we propose a symmetry-enforced higher-order topological superconductivity based on antiferromagnetic topological insulators, specifically realized in $\mathrm{Mn}X{\mathrm{Pb}}_2\text{-Pb}$ ($X$ = Se, Te) heterostructure. We show that the intrinsic boundary dichotomy—gapless Dirac states protected by an effective time-reversal symmetry on antiferromagnetic edges and magnetic gaps on ferromagnetic edges—naturally generates Majorana corner modes as mass domain walls. Superconducting proximity converts the antiferromagnetic edges into one-dimensional topological superconductors, and the intersections between superconducting and magnetic edges bind Majorana zero modes as mass domain walls. Combining first-principles calculations with a calibrated effective boundary theory, we demonstrate robust corner localization and purely electrical control of Majorana fusion and braiding in a triangular geometry. Our results establish $\mathrm{Mn}X{\mathrm{Pb}}_2$ as an experimentally promising platform for electrically programmable Majorana networks in two dimensions.

We propose a material platform comprising transition-metal dichalcogenide (TMDC) heterostructures to realize two-dimensional (2D) helical superconductivity with an intrinsic gap. By van der Waals stacking a 2D superconductor ($1T^{\prime }\text{−}{\mathrm{WS}}_2$ with inversion symmetry) on top of a 2D semiconductor $2H{\text{−}WS}_2$, the resulting TMDC bilayer exhibits helical superconductivity. Under an external in-plane magnetic field, the system can host finite-momentum Cooper pairing, evidenced by the divergence in the particle-particle susceptibility of a $k·p$ Hamiltonian fitted to the ab initio theory band structure. By varying the strength of the in-plane magnetic field, we demonstrate that the edge can undergo a phase transition to a one-dimensional gapless phase with narrow Fermi segments corresponding to zero-energy Bogoliubov quasiparticles. The controllable one-dimensional gapless phase serves as a clear experimental fingerprint of 2D helical superconductivity. The proposed 2D TMDC heterostructure is promising for intrinsic nonreciprocal superconducting transport and the development of Majorana-based quantum devices.

In interacting quantum many-body systems, relaxation toward equilibrium reflects a competition between internal chaotic dynamics and environmental dissipation. While conventional Markovian baths typically produce exponential decay, non-Markovian dissipation can give rise to more intricate behavior, including algebraic relaxation. We study an open Sachdev-Ye-Kitaev (SYK) model coupled to a pseudogapped fermionic bath, using the Keldysh formalism to compute steady-state correlations in the large-$N$ limit. Our results uncover a rich dynamical phase diagram, with regimes of bath-driven power-law relaxation, chaos-driven exponential decay, and an intermediate prerelaxation phase where exponential decay crosses over to algebraic decay. These findings demonstrate that non-Markovian environments can qualitatively reshape relaxation mechanisms in strongly correlated quantum many-body systems.