We study the $T=0$ phase diagrams of models of bilayers of $S=1/2$ square lattices antiferromagnets with SU(2) Heisenberg symmetry that have 2, 4, and 6 spin exchanges. We study two families of bilayer models with distinct internal symmetries and, hence, different phase diagram topologies. A traditional bilayer model in which the interlayer interaction is Heisenberg so that the two layers can exchange spin (and energy) with each other, making it possible to achieve a simple dimerized valence bond liquidlike state. The resulting phase diagram is rich with Néel, valence bond solid and simple dimer phases, and both first-order and continuous transitions, which we demonstrate are consistent with the conventional Landau theory of order parameters. In the second family of models in which the layers can exchange only energy but no spin (reminiscent of the Ashkin-Teller coupling), the simple dimer state cannot occur. The phase diagrams reveal a number of phase transitions that are accessed. We find that the phase transition between Néel and valence bond solid (VBS) is first order in both the spin-spin and energy-energy coupled models, although they have strikingly distinct finite-size scaling behavior and that the transition from VBS to dimer in the spin-spin coupling model deviates from the expected scenario of an XY model with dangerously irrelevant fourfold anisotropy.

The interplay between charge stripes and pairing has long been a subject of scrutiny in a broad class of unconventional superconductors, as in some cases it is unclear whether this interplay benefits the ensuing superfluidity. Experiments that explore the out-of-equilibrium dynamics of these systems aim to tip the balance toward one phase or the other by selectively coupling to relevant modes. Leveraging the fact that competition between stripes and pairing is not exclusive to fermionic systems, we explore the photoirradiation dynamics of interacting hardcore bosons in which density-wave phase-reversal melting leads to enhanced phase-coherent transport response, as quantified by the dynamic amplification of both the zero-momentum occupancy and the condensate fraction, as well as finite out-of-equilibrium charge stiffness and superfluid weight, for a given system size. Our results, obtained using unbiased methods for an interacting system on a ladder geometry, demonstrate how one can engineer time-dependent perturbations to release suppressed orders, potentially providing insight into the underlying mechanism in related experiments.

We propose and analyze a microwave spin-photon interface based on a circular double quantum dot, inspired by recent experimental observations of anisotropic $g$-factors and orbital ring states in InAs nanowires. We develop an effective theoretical model, capturing the interplay of spin-orbit coupling and the magnetic flux, and show how ring states form at crossings of odd and even geometric parity orbital states. Similar to bonding and antibonding states of conventional double quantum dots, the ring states can be changed into single dot states by detuning the dots, which enables a high degree of control over the system's properties. Applying a tilted magnetic field induces spin-charge hybridization which enables spin-photon coupling. For low disorder, the photons couple states of simultaneously (almost) opposite spin and angular momentum. With increasing disorder, the spin-photon coupling becomes analogous to the flopping mode mechanism of conventional double quantum dots with intrinsic spin-orbit coupling where the spin is hybridized with the bonding and antibonding orbital states. We show that the system exhibits a third-order charge-noise sweet spot at a specific magnetic field angle, which lowers the system's sensitivity to dephasing while retaining a substantial spin-photon coupling strength. Moreover, the photon coupling mechanism can be switched off either electrically, by detuning to the single-dot regime, or magnetically, by rotating the field to disable the spin-charge hybridization.

We investigate the nonlinear transverse planar transport phenomena (viz., nonlinear Hall, thermal Hall, and Nernst coefficients) induced by chiral anomaly in three-dimensional spin-orbit coupled metallic systems. Unlike Weyl semimetals, these systems do not possess multiple Weyl nodes located at isolated points in the momentum space but instead host a pair of Fermi surfaces characterized by opposite Berry curvature fluxes enclosing the same band-degeneracy point. Using semiclassical Boltzmann transport formalism within the relaxation time approximation, we derive the second-order transverse planar transport coefficients induced by electrical and thermal gradients in the presence of an in-plane magnetic field. Our analysis reveals distinctive angular dependencies of the nonlinear transport coefficients, along with characteristic scaling behavior with the magnetic field strength. Furthermore, we demonstrate that the anomaly induced transport coefficients exhibit an exponential temperature dependence. This unconventional behavior leads to the violations of the Wiedemann-Franz law and Mott relation, highlighting unique thermoelectric signatures that can be probed experimentally in three-dimensional spin-orbit coupled metallic systems.

The recently discovered family of kagome metals has attracted significant interest as a result of reports of charge-bond order, orbital magnetism, and superconductivity. Some of these phases may exhibit time-reversal symmetry breaking. More recently, experiments have reported the emergence of nematic order that lowers the rotational symmetry of the system from sixfold to twofold. Here, we investigate the mechanism behind a nematic loop-current bond order (NLCBO) that breaks both rotational and time-reversal symmetries. Examining an effective patch model that captures one $p$-type and one $m$-type van Hove singularity at each $M$ point, we find that frustration of the complex order-parameter phases leads to NLCBO. We further present conditions for overcoming other competing phases, including isotropic charge-bond and loop-current orders. Applying our findings to a previously studied model for ${A\mathrm{V}}_3{\mathrm{Sb}}_5$ $(A=\mathrm{K},\mathrm{Rb},\mathrm{Cs})$, we find that NLCBO emerges within a small region of phase space within mean-field theory. Our theory provides a microscopic description that goes beyond symmetry-allowed free-energy analyses and is broadly applicable to other kagome metals featuring van Hove singularities near the Fermi level.

Quantum phase transitions (QPTs) in the spin-boson model with/without the rotating-wave approximation (RWA) are systematically investigated through variational calculations using a sub-Ohmic bath with high spectral density. Four cases involving different system-environment interactions are examined, where transition points and critical exponents are accurately determined across varying tunneling strengths. Contrary to prior work, a rich phase diagram is revealed in the tunneling-coupling plane even at the low spectral exponent $s<1/2$, with a novel U(1)-symmetric phase being identified. As coupling increases, a multistage QPT sequence arises for the tunneling $0<\mathrm{\Delta }<{\mathrm{\Delta }}^{*}=0.074\left(1\right)$, whereas a single transition occurs beyond this range. Furthermore, an odd-parity phase is found to emerge even under the positive tunneling, exhibiting distinct characteristics relative to the prototype model.

Quantum materials with bands of narrow bandwidth near the Fermi level represent a promising platform for exploring a diverse range of fascinating physical phenomena, as the high density of states within the small energy window often enables the emergence of many-body physics. On one hand, flat bands can arise from strong Coulomb interactions that localize atomic orbitals. On the other hand, quantum destructive interference can quench the electronic kinetic energy. Although both have a narrow bandwidth, the two types of flat bands should exhibit very distinct spectral properties arising from their distinctive origins. So far, the two types of flat bands have only been realized in very different material settings and chemical environments, preventing a direct comparison. Here we report the observation of the two types of flat bands within the same material system—an above-room-temperature van der Waals ferromagnet, ${\mathrm{Fe}}_{5-x}{\mathrm{GeTe}}_2$, distinguishable by a switchable iron site order. The contrasting nature of the flat bands is also identified by the remarkably distinctive temperature evolution of the spectral features, indicating that one arises from electron correlations in the Fe(1) site-disordered phase, while the other geometrical frustration in the Fe(1) site-ordered phase. Our results therefore provide a direct juxtaposition of the distinct formation mechanism of flat bands in quantum materials and an avenue for understanding the distinctive roles flat bands play in the presence of magnetism, topology, and lattice geometrical frustration, utilizing sublattice ordering as a key control parameter.

We compute the electronic and emission properties of Coulomb–correlated multiparticle states ($X^0, X^{\pm }, \mathit{XX}$) in weakly confining GaAs/AlGaAs quantum dots using an eight-band $\mathbf{k}·\mathbf{p}$ model coupled to continuum elasticity and configuration interaction (CI). We evaluate polarization-resolved oscillator strengths and radiative rates both in the dipole approximation and in a quasielectrostatic beyond-dipole (BDA) longitudinal formulation implemented via a Poisson reformulation exactly equivalent to the dyadic Green-tensor kernel. For the dots studied, BDA yields lifetimes in quantitative agreement with experiments, e.g., ${\tau }^X=0.279\mathrm{ns}$ vs $0.267\mathrm{ns}$ (exp.) and ${\tau }^{XX}=0.101\mathrm{ns}$ vs $0.115\mathrm{ns}$ (exp.). The framework also reproduces electric-field tuning of the multiparticle electronic structure and emission—including the indistinguishability inferred from $P={\tau }^X/({\tau }^X+{\tau }^{XX})$—and we assess sensitivity to CI-basis size and to electron–electron and hole–hole exchange.

Size-dependent anisotropy in lattice parameters governs the bright-exciton fine-structure splitting (BEFSS) in ${\mathrm{CsPbI}}_3$ nanocrystals. By using the continuum elastic theory, we demonstrate that an additional lattice anisotropy in a pseudocubic nanocrystal is determined by the elastic coupling between bulk and surfaces. For the ${\mathrm{CsPbI}}_3$ nanocrystal in orthorhombic phase, it reveals that the surface stress anisotropy of the (001) surface plays a more important role than that of the (110) surface in inducing the size-dependent lattice anisotropy. According to the first-principles calculations, the intrinsic surface stress anisotropy is relatively small for either (110) or (001) surface. As organic ligands are always involved in the synthesis of ${\mathrm{CsPbI}}_3$ nanocrystals, they may introduce stochastic surface stress anisotropy and thus result in divergency of BEFSS. Therefore, we further study the surface with typical substitutional short-chain ligands, namely methylammonium and formate ions. It turns out that the surface stress anisotropy is large on the (110) surface but small on the (001) surface due to the cooperative geometric effect of ligand and surface structure. Consequently, the BEFSS variations become negligible at various coverages with the short-chain ligands. These findings enable precise control of the BEFSS through surface stress engineering by carefully selecting the suitable ligands, improving the application of ${\mathrm{CsPbI}}_3$ nanocrystals in quantum optics and information technologies.

Dirac points close to the Fermi level can cause geometrical quantities, such as the Berry curvature and quantum metric, to diverge, thereby significantly influencing the transverse electronic responses of the system. Theoretical studies have demonstrated that Dirac points can emerge in the band structure of a carbon nanoscroll under the influence of a magnetic flux. Here, we demonstrate that their number can be tuned by adjusting the applied flux and the number of turns in the scroll. To characterize the topological properties of these Dirac points, we analyze the winding number through our diagrammatic technique, correlating it to the dominant accumulated phases along different hopping paths. Our approach provides a general framework that can be extended to investigate other nanomaterial systems with chiral symmetry.

Glass phases can be stabilized by quenched disorders, as in most spin-glass materials, or self-generated through kinetic freezing in disorder-free systems. A canonical example of the latter is structural glasses, which have been extensively studied for many decades. Yet, how the rugged energy landscape of a glass phase is spontaneously generated in disorder-free systems remains one of the key questions in glass physics. Here we present a general electronic mechanism for the emergence of glassy phase using the example of itinerant electrons coupled to XY spins on a lattice. This model can also be viewed as the mean-field theory of a superconducting system with attractive density-density interactions. Intriguingly, the electron gauge symmetry in the strong pairing limit gives rise to a macroscopic degeneracy of XY spins. In the presence of electron hopping that breaks the gauge symmetry, the lifting of the extensive degeneracy leads to a glass phase with disordered pairings. Our findings highlight a scenario in which a glassy state originates from the breaking of quantum gauge symmetry without quenched disorders.

The Wiedemann-Franz (WF) law, relating the electronic thermal conductivity (${\kappa }_{\mathrm{e}}$) to the electrical conductivity, is vital in numerous applications such as in the design of thermoelectric materials and in the experimental determination of the lattice thermal conductivity (${\kappa }_{\mathrm{L}}$). While the WF law is generally robust, violations are frequently observed, typically manifesting in a reduced Lorenz number ($L$) relative to the Sommerfeld value ($L_0$) due to inelastic scattering. Here, we report a pronounced departure from the WF law in the topological semimetal CoSi, where the electronic Lorenz number ($L_{\mathrm{e}}$) instead rises up to $\sim 40\%$ above $L_0$. We demonstrate that this anomaly arises from strong bipolar diffusive transport, enabled by topological band-induced electron-hole compensation, which allows electrons and holes to flow cooperatively and additively enhance the heat current. Concurrently, we unveil that the lattice contribution to thermal conductivity is anomalously large and becomes the dominant component below room temperature. As a result, if ${\kappa }_{\mathrm{L}}$ is assumed negligible—as conventional in metals—the resulting $L$ from the total thermal conductivity (${\kappa }_{\mathrm{tot}}={\kappa }_{\mathrm{L}}+{\kappa }_{\mathrm{e}}$) deviates from $L_0$ by more than a factor of 3. Our work provides deeper insight into the unconventional thermal transport physics in topological semimetals.

Higher-order topological insulators have attracted significant attention in the classical wave manipulation field because of their ability to host lower-dimensional topological protected states. The introduction of the valley degree of freedom confers the typical valley-selective excitation characteristic on higher-order topological corner states. However, in valley phononic crystals, the effect of corner structure types on the excitation characteristics of corner states has not been widely discussed or investigated. In this work, we realize multiple distinct corner-dependent excitation characteristics of higher-order topological corner states in the same bulk band gap based on two-dimensional valley phononic crystals. We propose a valley phononic crystal based on a plate trimer, achieving corner states with tunability and valley-switchable excitation characteristics. Further, we design a multicorner superstructure containing various intersection angles. Within this system, we achieve the two typical excitation characteristics supported by traditional corner structures with valley selectivity and valley switching capability. Interestingly, we find that certain specific corner structures simultaneously support the excitation of both corner states with distinct valleys, with no need to rotate the scatterers. And our study confirms that the corner states enable reconfigurability and exhibit robustness against defects and disorders. This study enhances the spatial multiplexing capability and structural design flexibility of higher-order topological corner states, laying the foundation for the development of programmable topological functional devices in phononic systems.

Charge density waves (CDWs), spontaneous modulations of charge and lattice, are one of the fundamental orders in condensed matter systems. Since the modulation period of CDW strongly modifies the electronic structure and influences other emergent orders, unraveling the mechanism responsible for the periodicity is one of the major challenges in controlling quantum materials. While current phenomenological models based on the instability of the electronic structure have successfully described some of their characteristic features, the full complexity of CDW formation points to the energy landscape with much richer structures calling for additional or alternative underlying mechanisms. Here, we investigate the canonical CDW material $1\mathit{H}\text{−}{\mathrm{TaSe}}_2$, which undergoes multiple CDW phase transitions. By constructing a highly accurate interatomic potential trained on first-principles data, we achieved sub-meV-per-atom precision and reproduced the full experimental phase diagram, capturing discontinuous changes in modulation period and symmetry-breaking transitions. Based on these results, we construct an interacting eigenmode model that quantitatively accounts for the whole details using only a few low-energy eigenmodes and short-range anharmonic force constants. Our calculations clearly demonstrate that lattice anharmonicity is the primary driving mechanism for the complex evolution of CDW periodicity in transition-metal dichalcogenides. Our interacting eigenmode model further yields a simple condition governing whether CDWs in triangular and square lattices emerge uniaxially or as two-dimensionally.

The thermoelectric properties of the Zintl compound RbSnSb are systematically investigated based on first-principles calculations. The delocalized nature of Rb atoms, together with the rattling vibrations of Sb atoms, induces pronounced lattice anharmonicity, driving the lattice thermal conductivity well below 1 W/mK. Due to strong optical-acoustic hybridization, acoustic modes are suppressed and optical phonons emerge as the dominant heat carriers, contributing more than 40% to ${\kappa }_L$ and elucidating the microscopic origin of the ultralow thermal conductivity. In addition, the multivalley electronic structure enhances the power factor, while spin-orbit coupling reconstructs the band structure and mitigates the adverse effect of piezoelectric scattering in the noncentrosymmetric lattice. Benefiting from these synergistic features, RbSnSb achieves a remarkable figure of merit reaching 4.89 at 900 K, underscoring its promise as a high-performance thermoelectric material.

Beyond conventional electric and magnetic monopoles, electric and magnetic toroidal monopoles, which are rank-0 multipoles distinguished by opposite parities under spatial inversion and time reversal, can exist in nature. The recent observation of electric-field-induced directional dichroism in antiferromagnetic olivine ${\mathrm{Co}}_2{\mathrm{SiO}}_4$ has provided the first concrete example of a magnetic toroidal monopole; however, its microscopic origin remains elusive. Here, we propose a minimal spin model that incorporates magnetoelectric coupling via the $d\text{−}p$ hybridization mechanism and analyze it within the mean-field approximation. The model qualitatively reproduces the experimentally observed temperature dependence of the dielectric constant and its pronounced sensitivity to the direction of the applied electric field. Furthermore, it elucidates the temperature evolution of the magnetic toroidal monopole and the strong electric-field-direction dependence of the magnetic toroidal moment. Our calculations also predict a second-order nonlinear magnetoelectric response, consistent with the symmetry classification of ${\mathrm{Co}}_2{\mathrm{SiO}}_4$ as an altermagnet. Additionally, we demonstrate that the same framework is applicable to other antiferromagnetic olivines with analogous magnetic order, indicating the robustness and generality of the toroidal-type magnetoelectric response in this material family.

The Kitaev model belongs to an unconventional class of two-dimensional spin systems characterized by anisotropic, bond-dependent interactions that give rise to quantum spin liquid (QSL) states. These exotic phases, marked by the absence of magnetic ordering even at zero temperature, support fractionalized excitations and emergent gauge fields. A particularly compelling feature of the Kitaev model is its exact solvability, which reveals low-energy excitations in the form of itinerant Majorana fermions—quasiparticles that obey non-Abelian statistics—and are of central interest in topological quantum computation because of their inherent robustness against local perturbations and decoherence. Despite extensive theoretical advancements, the experimental identification of QSLs remains challenging, as conventional magnetic probes fail to detect their defining properties. In this work, we present a theoretical investigation of spin-current injection from a superconducting metal into a Kitaev quantum spin liquid. By employing a spintronic framework, we derive the dynamics of the injected spin current and demonstrate how its signatures can be traced back to the underlying Majorana excitations in the spin liquid phase. Superconductivity plays a pivotal role in this context, not only as a source of coherent quasiparticles but also as a platform with potential for interfacing with topological quantum devices. The interplay between superconductivity and the Kitaev QSL enables a novel mechanism for probing the spin transport mediated by Majorana fermions. Our analysis contrasts the Kitaev-superconductor interface with conventional ferromagnetic junctions, where spin transport is carried by magnons, and highlights distinctive features in the spin-current response. These findings suggest approaches for the detection of QSLs and contribute to the broader effort of integrating topological quantum materials into scalable quantum technologies.

Using first-principles density functional theory and determinant quantum Monte Carlo methods, we show that Janus graphene nanoribbons with topological defect arrays ($m=2$) exhibit robust intrinsic ferromagnetism across widths $W=2\text{–}6$, with bandgaps exceeding $200\mathrm{meV}$ and stable ferromagnetic ground states. Notably, uniaxial tensile strain significantly enhances their ferromagnetic properties: at 25% strain, the Curie temperature increases to $222\mathrm{K}$—a fivefold improvement over unstrained systems and the highest reported for graphene-based nanoribbons. Strain also induces a reversible transition to a bipolar magnetic semiconductor, with spin-flipped valence and conduction band edges beyond 10% strain. This dual functionality—strain-enhanced ferromagnetism and strain-induced spin flip—stems from strain-modulated $p_z$ orbital hybridization and strong direct exchange interaction. Among these, $W=5$ Janus graphene nanoribbons emerge as potential candidates for room-temperature spintronic devices and strain-programmable quantum transport systems.

We theoretically demonstrate high harmonic generation (HHG) of the ac charge current, in which the emitted current contains integer multiples of the fundamental frequency. By solving the Landau-Lifshitz-Gilbert equation under a GHz ac magnetic field and evaluating the pumped charge current using Brouwer's formula, we find that standing spin waves generate ac charge currents containing exclusively even-order harmonics. Remarkably, the harmonic order exceeds 800 where electron scattering due to the spin waves is resonant, reaching to the THz regime. Our numerical results are explained by a microscopic theory based on spin gauge fields, revealing a mechanism driven solely by interaction between conduction electrons spin and spin waves.

Disorder in frustrated quantum systems can critically influence their magnetic ground states and drive exotic correlated behavior. In the $S=\frac{1}{2}$ system ktenasite, ${\mathrm{Cu}}_{\text{2.7}}{\mathrm{Zn}}_{\text{2.3}}{\left({\mathrm{SO}}_{\text{4}}\right)}_{\text{2}}{\left(\mathrm{OH}\right)}_{\text{6}}·6{\mathrm{H}}_{\text{2}}\mathrm{O}$, we show that structural disorder drives an unexpected dimensional crossover and stabilizes a rare coexistence of distinct magnetic states. Neutron diffraction reveals significant Cu/Zn mixing at the Cu2 site, which tunes the ${\mathrm{Cu}}^{2+}$ sublattice from a two-dimensional scalene-distorted triangular lattice into a one-dimensional spin-chain network. Magnetic susceptibility, neutron diffraction, ac susceptibility, and specific heat measurements collectively indicate magnetic duality: a coexistence of incommensurate long-range magnetic order below $T_{\text{N}}=4$ K and a cluster spin-glass state with $T_{\text{f}}=3.28$ K at $\nu =10$ Hz. Our findings highlight ktenasite as a rare platform where structural disorder tunes the effective dimensionality and stabilizes coexisting ordered and glassy magnetic phases, offering a unique opportunity to explore the interplay of frustration, disorder, and dimensional crossover in quantum magnets.