We study an extended Kitaev model on the triangular lattice in a limit where the symmetric off-diagonal bond-dependent and Heisenberg interactions together map onto an XXZ model, in addition to the Kitaev interaction. Within the previously identified ${\mathbb{Z}}_2$ vortex regime, we uncover a ferrichiral skyrmion phase characterized by a sublattice-resolved scalar chirality: two of the three sublattices carry unit skyrmion charge, while the third remains nonchiral. Using classical Monte Carlo simulations, we show that this ferrichiral skyrmion phase emerges at zero temperature and in the absence of both an external magnetic field and Dzyaloshinskii-Moriya interactions, in sharp contrast to conventional skyrmion-hosting systems. The phase is stable over a wide parameter window and persists to relatively high temperatures. Our results reveal an unconventional route to skyrmion physics driven purely by frustrated exchange interactions and highlight the emergence of rich topological structures. Since both XXZ anisotropy and Kitaev interactions originate from the same spin-orbit-coupling mechanism, materials traditionally classified as XXZ magnets are expected to host finite Kitaev interactions as well. The potential for ferrichirality in these systems therefore warrants further investigation.

We investigate an $S=3/2$ quantum spin model on a two-dimensional honeycomb lattice that continuously interpolates between two paradigmatic quantum disordered states with distinct entanglement structures: the Kitaev quantum spin liquid and the Affleck-Kennedy-Lieb-Tasaki (AKLT) valence bond solid. Combining classical, semiclassical, and exact diagonalization approaches, we map out the ground-state phase diagram and elucidate the role of quantum fluctuations across the entire parameter range. While classical and semiclassical frameworks predict noncoplanar orders competing with a collinear Néel state, we find these phases to be fragile: once full quantum fluctuations are included, they melt into a quantum-entangled state characterized by suppressed spin correlations and enhanced entanglement entropy. Our findings highlight how competition between qualitatively different quantum disordered phases provides a fertile playground for unconventional phases emerging from their interplay and quantum fluctuations.

We study how an out-of-plane magnetic field $B\left(\mathbf{r}\right)$ and a Berry curvature $\mathrm{\Omega }\left(\mathbf{k}\right)$ modify the exchange interactions in a two-dimensional Wigner crystal (WC) using a semiclassical large-$r_s$ expansion. When only a magnetic field is present, ring-exchange processes arise from multiparticle tunneling through complex trajectories which constitute complex instanton solutions of the coordinate-space path integral. To leading order in $B$, each exchange constant $J_a$ acquires an Aharonov-Bohm phase along the zero-field tunneling trajectory. When a Berry curvature is present, the multiparticle tunneling must be considered in a complexified phase space $(\mathbf{r},\mathbf{k})$. To leading order in $\mathrm{\Omega }, J_a$ acquires a Berry phase along a purely imaginary momentum-space trajectory. When $B$ and $\mathrm{\Omega }$ are both present, in addition to having both Aharonov-Bohm and Berry phases, the exchange magnitude $|J_a|$ is also exponentially modified due to an effective-mass renormalization. These effects could be relevant for the WC and proximate phases recently observed in rhombohedral multilayer graphene.

We present a systematic study of the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction in ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$ films under both dark and illuminated conditions. In the dark, the intrinsic magnetism of ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$ is shown to yield a stronger anisotropic RKKY spin model compared to nonmagnetic topological insulators, providing a clear signature for differentiating these systems. Furthermore, key band properties—such as energy gap, band degeneracy/splitting, and topological deformations of the Fermi surface—imprint distinct signatures on the RKKY interaction, enabling clear discrimination between axion insulators (AI) and quantum anomalous Hall (QAH) insulators in even- and odd-septuple-layer (SL) films. This discrimination manifests in multiple ways: through the Fermi-energy dependence or spatial oscillations of the interaction for impurities on the same surface, or via the presence versus absence of spin-frustrated terms for those on different surfaces. Under off-resonant circularly polarized light, additional phase-transition-related fingerprints also emerge to distinguish these two phases, such as sign reversals of spin-frustrated terms in even-SL films versus chirality-selective double-dip structures of collinear RKKY components in odd-SL films. Overall, this work establishes RKKY interactions as a sensitive magnetic probe for distinguishing between AI phase (even-SL) and QAH phase (odd-SL), thereby complementing conventional electrical measurements while providing new insights into the influence of intrinsic magnetism on the surface-state band structure.

Mean-field one-dimensional topological superconductors host edge Majorana zero modes (MZMs) that encode a topologically protected ground-state degeneracy and enable robust braiding operations within this subspace. Mean-field states lack definite particle number and thus cannot represent isolated systems. Nevertheless, projecting them onto fixed particle number can yield good approximations to an isolated superconductor ground state. In earlier work [Sajith et al., Phys. Rev. B 109, 184509 (2024)] we showed that the projected Kitaev wave function of a single wire preserves some important mean-field features, such as the zero-energy spectral peaks near the wire edges. However, uniqueness of the fixed-number ground state does not allow for any nontrivial operations in the ground-state subspace. To overcome this limitation, here we consider the case of multiple wires with a conserved total charge, using the same approach. In the limit of vanishing interwire coupling, this system has a macroscopic ground-state degeneracy. We show how this degeneracy is resolved by coherent single-particle tunneling, and identify special many-body states which play the same role as the MZM parity states in the mean field. We demonstrate how braiding operations can be implemented and discuss both intrinsic and extrinsic limits on their fidelity.

We theoretically investigate the spin structure and spin dynamics of excitons in bulk lead halide perovskite semiconductors with cubic, tetragonal, and orthorhombic crystal symmetry. The exciton spin structure and its modification by an external magnetic field are modeled for different regimes defined by the relative magnitude of the electron–hole exchange interaction (splitting between dark and bright states) and the Zeeman spin splitting. The effects of crystal symmetry and magnetic field orientation with respect to the crystal axes are considered for lead halide perovskite crystals with band gaps in the range of 1.4 to 3.5 eV, having different ratios of electron and hole $g$-factors. For cubic symmetry, in a longitudinal magnetic field, our theory predicts quantum beats between the bright exciton states under linearly polarized excitation and detection, while the dark exciton remains optically inactive. In a transverse magnetic field, all exciton spin states become optically active and can be excited by circularly polarized light. Reduction of the crystal symmetry leads to a zero-field offset of the exciton Larmor precession frequencies, modifying the Zeeman splitting energy dependence on magnetic field. This theoretical framework allows for the extraction of the strength of the exchange interaction and the crystal symmetry. Experimentally, we measure the exciton spin coherence via time-resolved photoluminescence at a temperature of 1.6 K in longitudinal and transverse magnetic fields in orthorhombic ${\mathrm{MAPbI}}_3$ crystals. Polarization beats at the frequency of the bright exciton are observed in both configurations. Comparison with theory indicates that the excitons are in the strong exchange interaction regime, and the reduction of symmetry does not lead to a significant splitting of the exciton spin levels.

Flat bands in moiré lattices of transition-metal dichalcogenides have emerged as a powerful platform for exploring strongly correlated electron physics. However, the moiré flat bands in these systems caused by band folding can originate from different valleys, which complicates the physics of the flat bands. Here, we combine a band-unfolding technique with symmetry analysis to visualize how the $\mathrm{\Gamma }$ valley gives rise to flat bands for the $R$- and $H$-stacking moiré superlattices of the homobilayers. We demonstrate that the $\mathrm{\Gamma }$ valley—at the center of the Brillouin zone—interacts with its folded bands under the effect of moiré potential, then forms flat bands as the twist angle decreases. Furthermore, the topmost two $\mathrm{\Gamma }$ flat bands in $R$-stacking moiré systems maintain a robust Dirac node at the K point protected by $D_3$ symmetry. In contrast, the $\mathrm{\Gamma }$ flat bands open a gap at K in the $H$-stacking moiré superlattices due to having a $C_3$ symmetry. As a result, the change from the $R$-stacking order to the $H$-stacking order in the moiré systems causes the $\mathrm{\Gamma }$ flat bands to transform from a honeycomb lattice to a triangular lattice. The K-valley flat bands for both types of moiré superlattices exhibit similar dispersion features.

The Hubbard model on the kagome lattice is a widely used interacting model for describing the electronic properties of various transition-metal-based kagome materials. We find altermagnetism driven by Coulomb interaction in the kagome Hubbard model at Dirac filling with no spin-orbit coupling nor explicit spatial symmetry breaking present. We show how this insulating altermagnet is relevant to other lattices with larger unit cells such as the Lieb-kagome lattice. The altermagnet found displays a characteristic magnon splitting, which can be detected in inelastic neutron scattering experiments on interacting kagome materials.

In isolated quantum many-body systems periodically driven in time, the asymptotic dynamics at late times can exhibit distinct behavior such as thermalization or dynamical freezing. Understanding the properties of and the convergence towards infinite-time (nonequilibrium) steady states, however, remains a challenging endeavor. We propose a physically motivated Krylov space perspective on Floquet thermalization which offers a natural framework to study rates of convergence towards steady states and, simultaneously, an efficient numerical algorithm to evaluate infinite-time averages of observables within the diagonal ensemble. The effectiveness of our algorithm is demonstrated by applying it to the periodically driven mixed-field Ising model, reaching system sizes of up to 30 spins. Our method successfully resolves the transition between the ergodic and dynamically frozen phases and provides insight into the nature of the Floquet eigenstates across the phase diagram. Furthermore, we show that the long-time behavior is encoded within the localization properties of the Ritz vectors under the Floquet evolution, providing an accurate diagnostic of ergodicity.

We develop a Monte Carlo sampling algorithm to numerically evaluate the Lehmann representation for the finite temperature single-particle Green's function in the repulsive Lieb-Liniger model. This allows us to determine the spectral function in the full range of temperatures and interactions, as well as in generalized Gibbs ensembles. We test our results against known results for dynamics at infinite interaction strength and static correlators, and find excellent agreement.

Rabi oscillations characterize light-matter hybridization in the waveguide quantum electrodynamics (WQED) framework, with their associated decay rates reflecting excitation damping, yet their behavior remains unresolved when collective emitters are coupled to a collective waveguide mode. This scenario reveals a conceptually novel collective-light-collective-matter interaction, realizable when a timed-Dicke state of subwavelength emitters couples to a slow, delocalized surface-plasmon mode, forming a hybridized plasmon-polariton (HPP). The HPP acquires its directionality from the timed-Dicke state via momentum matching. It also exhibits plasmonic characteristics, with excitation frequencies following the surface-plasmon dispersion relation. We obtain a Rabi oscillation and a long-time decay that describe the HPP and use them to reveal weak- and strong-coupling regimes through the emergence of normal-mode splitting. By performing a finite-time Lyapunov-exponent analysis, we show that the HPP also exhibits instantaneous decay and numerically identify three distinct decay regimes: early-time rapid, transient-time oscillatory, and long-time classical. Finally, by analyzing the emission spectrum, we observe an anticrossing of the peak doublets—a feature also seen in cavity quantum electrodynamics setups—which originates from quantum vacuum effects and the resulting non-Markovian HPP evolution in our WQED.

We consider the excitation of Higgs modes via the modulation of the Bardeen-Cooper-Schrieffer coupling within the Migdal-Eliashberg-Keldysh theory of time-dependent superconductivity. Despite the presence of phonons, which break integrability, we observe Higgs amplitude oscillations reminiscent of the integrable case. The dynamics of quasiparticles follows from the effective Bogolyubov–de Gennes (BdG) equations, which represent a Floquet problem for the Bogolyubov quasiparticles. We find that when the Floquet-Bogolyubov bands overlap, the homogeneous solution formally leads to a negative superfluid density, which is no longer proportional to the amplitude of the order parameter. This result indicates an instability, which we explore using spatially resolved BdG equations. Spontaneous appearance of spatial inhomogeneities in the order parameter is observed and they first occur when the superfluid density becomes unphysical. We conclude that the homogeneous solution to time-dependent superconductivity is generally unstable and breaks up into a complicated spatial landscape via an avalanche of topological excitations.

Using a self-consistent Hartree-Fock theory, we show that the recently observed ferromagnetism in twisted bilayer ${\mathrm{WSe}}_2$ [Nat. Commun. 16, 1959 (2025)] can be understood as a Stoner-like instability of interaction-renormalized moiré bands. We quantitatively reproduce the observed Lifshitz transition as function of hole filling and applied electric field that marks the boundary between layer-hybridized and layer-polarized regimes. The former supports a ferromagnetic valley-polarized ground state below half-filling, developing a topological charge gap at half-filling for smaller twist angles. At larger twist angles, the system hosts a gapped triangular Néel antiferromagnet. On the other hand, the layer-polarized regime supports a stripe antiferromagnet below half-filling and a wing-shaped multiferroic ground state above half-filling. We map the evolution of these states as a function of filling factor, electric field, twist angle, and interaction strength. Our results demonstrate that long-range exchange in a symmetry-unbroken parent state with strongly renormalized moiré bands provides a broadly applicable framework to understand itinerant magnetism in moiré TMDs.

Vortex waves carrying quantized orbital angular momentum (OAM) have been extensively studied in optical and acoustic systems, yet their realization in fully vectorial elastic media is generally hindered by intrinsic longitudinal-transverse coupling. Here, we introduce a cut-and-glue topological lattice defect (TLD) in a three-dimensional elastic Weyl phononic crystal and realize a defect-bound mode propagating along the defect line with a vortex phase structure. The TLD preserves axial translational symmetry, giving rise to a one-dimensional defect band dispersing with ${\mathit{k}}_{\mathit{z}}$ that topologically connects a pair of Weyl points with opposite chirality in momentum space. The resulting defect-bound mode is strongly localized at the defect core, carries a well-defined OAM with winding number $\ell =1$, and maintains longitudinal-dominant polarization across the entire defect band. These findings establish a bulk-defect topological mechanism for robust OAM-guided transport in fully vectorial three-dimensional elastic media.

Bound states in the continuum (BICs) have emerged as a powerful platform for enhancing wave-matter interactions, owing to their ultrahigh quality ($Q$) factors arising from extreme energy localization. While symmetry breaking in BIC-based metasurfaces can yield strong chiral responses, most existing studies have focused on single BIC systems, leaving their coupling and interactions largely unexplored. In this work, we propose a coupling-induced chiral quasi-BIC (QBIC) in an acoustic system. By introducing a coupling channel between a pair of BICs, perfect vortex patterns are observed in the far-field radiation at both ends, where the chiral responses are independently tunable for each port and maintain robustness against parameter perturbations. In a scattering network, the designed chiral QBIC exhibits maximum chirality-selective transmission for acoustic vortices, which is validated by an experimental prototype. Additionally, an exceptional line emerges in the parameter space of the scattering matrix as a non-Hermitian feature, revealing the origin of robust chirality and enabling tunability of the $Q$ factor. Our work provides an efficient way for chirality manipulation using BICs and may enrich the understanding of wave physics.

Dirac semimetals, with their protected Dirac points, present an ideal platform for realizing intrinsic topological superconductivity. In this work, we investigate superconductivity in a two-dimensional, square-lattice nonsymmorphic Dirac semimetal. In the normal state near half-filling, the Fermi surface consists of two distinct pockets, each enclosing a Dirac point at a time-reversal invariant momentum $[\mathbf{X}=(\pi ,0)$ and $\mathbf{Y}=(0,\pi )]$. Considering an on-site repulsive and nearest-neighbor attractive interaction, we use self-consistent mean-field theory to determine the ground-state pairing symmetry. We find that an even-parity, spin-singlet $d_{x^2-y^2}$-wave pairing is favored as it gives rise to a fully gapped superconducting state. Since the pairing amplitude has opposite signs on the two Dirac Fermi pockets, the superconducting state is identified as a second-order topological superconductor. The hallmark of this topological phase is the emergence of Majorana zero modes at the system's boundaries. Notably, the positions of these Majorana modes are highly controllable and can be manipulated simply by tailoring the boundary sublattice terminations. Our results highlight the promise of nonsymmorphic Dirac semimetals for realizing and manipulating Majorana modes.

Bound electron–hole pairs in semiconductors known as excitons are the subject of intense research because of their potential for optoelectronic devices and applications, especially in the realm of two-dimensional materials. While the properties of free excitons in these systems are well understood, a general description of the interactions between these quasiparticles is complicated owing to their composite nature, which leads to important exchange processes that can take place between the identical fermions of different excitons. In this work, we employ a variational approach to study interactions between Wannier excitons and obtain an effective interaction potential between two ground-state excitons in a system of spin-degenerate electrons and holes. This potential is in general nonlocal in position space and depends on the combined spin configurations of the electrons and holes. When particularized to the case of hydrogen-like excitons with a heavy hole, this potential becomes local and exactly reproduces the Heitler–London result for two interacting hydrogen atoms. Thus, our result can be interpreted as a generalization of the Heitler–London potential to the case of arbitrary masses. We also show how including corrections arising from excited states into the theory results in a van der Waals potential at large distances, which is expected given the induced dipole–dipole nature of the interactions. Our approach is also applicable to more complicated systems with nonhydrogenic exciton series, such as layered semiconductors with Rytova–Keldysh interactions. Additionally, we use a path-integral formalism to develop a many-body theory for a dilute gas of excitons, resulting in an excitonic action that formally includes many-body interactions between excitons. While in this approach the field representing the excitons is exactly bosonic, we clarify how the internal exchange processes arise in the field-theoretical treatment, and show that the diagrams corresponding to the interactions between excitons align with our variational calculation when evaluated on shell. Our methods and results lay the groundwork for a generalized theory of exciton–exciton interactions and their application to the study of biexciton spectra and correlated excitonic matter.

Deconfined quantum critical points are intriguing transition points not predicted by the Landau-Ginzburg-Wilson symmetry-breaking paradigm which are usually identified by the appearance of a continuous phase transition between locally ordered phases. Here, we reveal the presence of deconfined quantum critical points with unexplored properties. Contrary to previously known examples, we show that the phenomenon of spin-charge separation peculiar to interacting low-dimensional fermions can allow for the appearance of partially gapped deconfined quantum critical points. We first infer this point by performing a field theory analysis of generic one-dimensional fermionic systems in the low energy limit. Subsequently, we derive a microscopic model where phase transitions between different locally ordered phases can take place. Here, by performing a numerical analysis we explicitly derive, among others, the gaps, local order parameters and correlation functions behavior, supporting the presence of partially gapped deconfined quantum critical points. Our results thus provide interesting insights on the widely investigated topic of quantum phase transitions.

Transition metal dichalcogenides, a family of two-dimensional compounds, are of interest for a range of technological applications. ${\mathrm{MoS}}_2$, the most researched member of this family, is hexagonal, from which monolayers may be isolated. Under ambient conditions and during growth/processing, contamination by impurities can occur, of which carbon is significant due to its presence in the common growth techniques. We have performed extensive computational investigations of carbon point defects, examining substitutional and interstitial locations. Previously unreported thermodynamically stable configurations: Fourfold coordinated monocarbon and dicarbon substitutions of Mo, and a complex of carbon substitution of sulfur bound to interstitial sulfur have been identified. We find no evidence to support recent assertions that carbon defects are responsible for electrical doping of ${\mathrm{MoS}}_2$, finding all energetically favorable forms have only deep charge transition levels and would act as carrier traps. To aid in the unambiguous identification of carbon defects, we present electronic and vibrational data for comparison with spectroscopy.

Oxygen vacancies play a critical role in stabilizing the ferroelectric phase of hafnia-zirconia thin films, yet their interplay with zirconium doping is less well understood. In this work, we employ Density Functional Theory to uncover how local cation environments caused by zirconium doping, and charge states ($q=-2$ to $+2$), shape the oxygen vacancy landscape in ferroelectric hafnia-zirconia (${\mathrm{Hf}}_{1-x}{\mathrm{Zr}}_x{\mathrm{O}}_2$; $x=0.25,0.50,0.75$). We reveal the crucial role of the local cation environment in facilitating the formation of oxygen vacancies in ferroelectric hafnia-zirconia compared to hafnia, extending the analysis to nine unique environments. The environment comprised of four zirconium cations was found to have the lowest formation energy out of the set of local cation environments for neutral oxygen vacancy formation. The charge transition to positively charged vacancies ($q=+2$) dramatically increases the number of available sites, suggesting that electric field–induced charge transitions can facilitate vacancy diffusion. We find that the concentration of zirconium doping surprisingly did not impact the formation energies of oxygen vacancies, instead acting as a lever controlling the population of available sites for oxygen vacancy formation. Bonding analysis reveals that zirconium content weakens the cation-oxygen bond by introducing occupied antibonding states, further stabilizing defect formation. These findings reveal how doping and charge state modulate the defect landscape, offering a blueprint for tuning oxygen vacancy behavior and, by extension, the performance of next-generation ferroelectric devices.