We systematically derive the dissipationless quantum kinetic equation for a multiband free fermionic system with $U$(1) symmetry. Using the Moyal product formalism, we fully band diagonalize the dynamics. Expanding to the second order in gradients, which is beyond the semiclassical limit, we give a complete analysis of the band-resolved thermodynamics and transport properties, especially those arising from the quantum geometric tensor. We apply our framework to a Bloch band theory under electric fields near equilibrium and find the linear and nonlinear transport coefficients. We also obtain the dynamical density-density response functions in the metallic case, including quantum metric corrections. Our results and approach can be applied very generally to multiband problems even in situations with spatially varying Hamiltonians and distributions.

Many studies on ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$ doped with various dopants have been conducted to explore the relationship between charge density waves (CDWs) and superconductivity. Among them, only Cr-doped ${\mathrm{CsV}}_3{\mathrm{Sb}}_5\left[\mathrm{Cs}{\left({\mathrm{V}}_{1-x}\mathrm{C}{\mathrm{r}}_x\right)}_3\mathrm{S}{\mathrm{b}}_5\right]$ exhibits a simultaneous weakening of the CDW and superconductivity as the doping concentration increases. In this study, we investigated two Cr-doped ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$ samples with doping levels of $x$ = 0.024 and 0.057 using optical spectroscopy with a focus on the CDW phase. By combining the experimental results with first-principles calculations, we found that the low-energy interband transitions were tuned by the saddle point at the $M$ point and were significantly enhanced by Cr doping. Furthermore, from a two-Drude-Lorentz model analysis, selective spectral weight transfer, previously observed in the pristine ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$ compound, was confirmed to exist in the Cr-doped samples and systematically changed with Cr doping. We observed that the spectral weight of the narrow Drude component decreased as the Cr doping increased, suggesting a reduction in superconductivity. We extracted the CDW gaps of the two Cr-doped samples and found that the CDW order was suppressed more rapidly with Cr doping than expected based solely on the transition temperature. We expect that our results will provide insights into understanding the simultaneous weakening of both the CDW and superconductivity in Cr-doped ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$.

Rhombohedral trilayer graphene (RTG) hosts exotic electronic, magnetic, and superconductivity properties, offering a versatile platform to study correlated and topological phases. We systematically investigate topological phase transitions in RTG under optical fields and staggered electric potentials. Using an effective two-band model, we capture multiple Lifshitz transitions and high-Chern-number states induced by these fields, while an extended Hubbard model is employed to investigate magnetic phase transitions in RTG. Our analysis reveals intriguing quarter-metal (QM) and three types of half-metal (HM) states under external fields, and we further examine the symmetry properties and temperature dependence of the resulting phase diagrams. These findings highlight the potential for spintronic and valleytronic applications of RTG.

We investigate the coupling of two spatially separated qubits via topologically protected edge states in a two-dimensional Hofstadter lattice. In this hybrid platform, the qubits are coupled to distinct edge sites of the lattice, enabling long-range interactions mediated by topological edge modes. We solve the full system Hamiltonian and analyze the resulting eigenstate structure to uncover the conditions under which coherent qubit interactions emerge. Our analysis reveals that the effective coupling is highly sensitive to the qubit placement, energy detuning, and the topological character of the edge spectrum. We obtain an analytical solution that goes beyond the perturbative regime, capturing the full interplay between the qubits and edge modes. These results provide a foundation for exploring information transport and many-body effects in engineered quantum systems where interactions are mediated by topological edge modes.

We study the honeycomb lattice with a single magnetic impurity modeled by adding imaginary next-nearest-neighbor hopping $ih$ on a single hexagon. This Haldane defect gives a topological mass term to the gapless Dirac cones and generates chirality. For a small density of defects, Neehus et al. [Phys. Rev. Lett. 135, 126604 (2025)] found that the system's chirality reverses at a critical $h_c\approx 0.95$ associated with an unexpected tricritical point of Dirac fermions at zero defect density. We investigate this zero-density limit by analyzing a single defect and computing two experimentally relevant measures of chirality: (1) orbital magnetization via local Chern marker, a bulk probe of all occupied states; and (2) electronic currents of low-energy states. Both probes show a chirality reversal at a critical $h_c\approx 0.9\text{–}1.0$. Motivated by this consistency, we propose a defect-scale toy model whose low-energy states reverse their chirality at $h_c^{\prime }\approx 0.87$. Remarkably, the same pair of zero-energy bound states also generates the critical point $h_c$ in the full impurity projected T-matrix. Our results show how the chirality reversal produced by an impurity can be observed either in local probes or in the global topology, and suggest a possible role of the microscopic defect structure at the critical point.

We compute the disorder averaged dc conductance in the non-equilibrium steady state that sets in between a longer-range Kitaev chain and a lead connected to an external reservoir, as a function of the system parameters and of the disorder strength. From our results, we map out the phase diagram of the disordered chain for different types of disorder and discuss the corresponding effects of the interplay between topology and disorder in the system. To do so, we set up a combined analytical and numerical approach, which is potentially amenable of straightforward generalizations to other disordered topological systems.

The quality of the silicon-oxide interface plays a crucial role in fabricating reproducible silicon spin qubits. In this work we characterize interface quality by performing mobility measurements on silicon Hall bars. We find a peak electron mobility of nearly $40000{\text{cm}}^2/\text{Vs}$ in a device with a $21\text{nm}$ oxide layer, and a peak hole mobility of about $2000{\text{cm}}^2/\text{Vs}$ in a device with $8\text{nm}$ oxide, the latter being the highest recorded mobility for a p-type silicon MOSFET. Despite the high device quality, we note an order-of-magnitude difference in mobility between electrons and holes. By studying additional n-type and p-type devices with identical oxides, and fitting to transport theory, we show that this mobility discrepancy is due to valence band nonparabolicity. The nonparabolicity endows holes with a density-dependent transverse effective mass ranging from $0.6m_0$ to $0.7m_0$, significantly larger than the usually quoted band-edge mass of $0.22m_0$. Finally, we perform magnetotransport measurements to extract electron momentum and quantum scattering lifetimes.

Owing to the coupled ferroic order parameters, two-dimensional ferroelastic-ferroelectric multiferroics have emerged as promising candidates for nonvolatile memories, multifield-responsive sensors, and intelligent thermal management devices. However, realizing such functionalities in intrinsically nonferroic systems like monolayer hexagonal boron nitride ($h$-BN) remains a critical challenge. Here, we demonstrate that hydrogenation could induces robust multiferroicity and switchable thermal transport in monolayer $h$-BN. First-principles calculations reveal that hydrogenation triggers an $s{p}^2$-to-$s{p}^3$ rehybridization, breaking the pristine $C_3$ rotational symmetry, generating spontaneous polarizations of 160.79 and 90.53 $\mu \mathrm{C}/{\mathrm{cm}}^2$ in $\alpha \text{−}{\mathrm{H}}_2\mathrm{BN}$ and $\delta \text{−}{\mathrm{H}}_2\mathrm{BN}$, far exceeding conventional ${\mathrm{BaTiO}}_3$ ($\sim 26$ $\mu \mathrm{C}/{\mathrm{cm}}^2$ ). Meanwhile, the ferroelastic-ferroelectric switching barriers are 0.15 and 0.12 eV/atom, guaranteeing ultralow-energy bit operations. By combining machine learning interatomic potentials with the phonon Boltzmann transport equation, we further show that $\alpha \text{−}{\mathrm{H}}_2\mathrm{BN}$ and $\delta \text{−}{\mathrm{H}}_2\mathrm{BN}$ exhibit anisotropic lattice thermal conductivities (considering both three-phonon and four-phonon scattering) of 133 (327) and $140\left(84\right)\mathrm{W}{\mathrm{m}}^{-1}{\mathrm{K}}^{-1}$ along the $xx$ ($yy$) direction. The thermal conductivity can be reversibly switched via ferroelastic-ferroelectric coupling, achieving ON/OFF ratios of 2.46 and 1.67. These results clarify the combination of giant ferroelectricity, low switching energy, and real-time tunable thermal conductivity endows $\alpha \text{−}{\mathrm{H}}_2\mathrm{BN}$ and $\delta \text{−}{\mathrm{H}}_2\mathrm{BN}$ as compelling candidates for monolithic devices that unify nonvolatile data storage and adaptive thermal management.

Time crystals are a peculiar state of matter. Their emergence hinges on ergodicity breaking, which typically originates from many-body localization or Floquet prethermalization. Here we propose a novel scheme for devising robust dissipative time crystals where the ergodicity is broken through symmetry-induced fragmentation. Building upon a $U\left(1\right)$-symmetry-induced Liouville-space fragmentation, we first propose a generic Liouvillian with long-time oscillations typical of time crystals. We then show that, even when the $U\left(1\right)$ symmetry is broken, a prethermal time-crystal behavior survives, with distinct oscillation frequencies at different times of the steady-state approaching dynamics. Intriguingly, the stagewise prethermal dynamics derive from Fermi statistics and the Liouvillian skin effect of our model—as the excitations above the boundary-localized dark states can be mapped to the irreducible representations of the permutation group, the branching rules of the permutation group ensure the robustness of the prethermal time crystal. Our work paves the way for devising time crystals through Hilbert-space fragmentation. It also sheds light on the dynamic effects of non-Hermitian physics in many-body quantum open systems.

We provide a theory of spin and acoustic wave coupled nonlinear dynamics in continuum systems. Combining the Landau-Lifshitz-Gilbert equations with the magnetoelastic Hamiltonian, we derive classical equations of motion for the magnetization and acoustic wave amplitudes, which include magnonic nonlinearity—both three- and four-magnon processes—as well as linear and nonlinear magnetoelastic interactions. We focus on two-dimensional magnetic films sustaining surface acoustic waves, a geometry where our model successfully reproduces our recent experimental observation of phonon-to-magnon down conversion under acoustic drive. We provide analytical expressions for all the rates in our equations, which make them particularly suitable for quantization. We then quantize our model, deriving Heisenberg-Langevin equations of motion for magnon and phonon operators, and show how to compute quantum expectation values in the mean-field approximation. Our work paves the way toward acoustic control of magnons in the quantum regime.

Inspired by the observation of extreme field-boosted superconductivity in uranium ditelluride, we present a scenario in which superconductivity can be induced by a strong Zeeman field, rather than destroyed by it, as is the case in Pauli-limited superconductivity. The resulting superconducting state has an upper critical field far greater than expected from its superconducting transition temperature, and, with spin-orbit coupling, it is sensitive to the field orientation. We demonstrate the interplay between superconductivity and metamagnetism in a simple effective theory.

Current research continuously discovers new superionic phases and provokes extensive discussion, but remains confronted with several key questions. For instance, do other hybrid states exist during the transition from solid to superionic to liquid? How do high temperature and pressure synergistically influence these states? Here, we perform a systematic investigation of the structural evolution and physical properties of ${\mathrm{Li}}_x\mathrm{Ga}$ $(x=1–8)$ compounds between 0 and 100 GPa using crystal structure prediction, first-principles calculations, and deep potential molecular dynamics simulations. Our findings reveal that when the Li atom concentration exceeds twice that of Ga, Li sublattices undergo melting, thereby inducing the emergence of a superionic state. During this process, the hybrid state arising from the premelting phenomenon is identified. While the pressure did not change the phase sequence of solid-hybrid-superionic-hybrid-liquid, it significantly affected the stable regions of each state. Furthermore, we calculated the superconductivity of ${\mathrm{Li}}_x\mathrm{Ga}$ $(x=1–2)$ from 0 to 50 GPa, exploring the intrinsic mechanisms underlying their pressure dependence. Finally, we constructed a pressure-temperature phase diagram for ${\mathrm{Li}}_x\mathrm{Ga}$ $(x=2, 4)$ within 0–5 GPa and 0–1200 K. This provides a theoretical insight into the correlation of physical properties in the other Li-based compounds over a broad range of temperatures and pressures.

Scanning photoemission microscopy (SPEM) and angle-resolved photoemission spectroscopy (ARPES) are used to investigate the electronic structure of ${\mathrm{Cu}}_{0.33}{\mathrm{TiSe}}_2$ across the charge density wave (CDW) transition. The electronic structure is found to be largely homogeneous in the normal state above the CDW transition. The ARPES measurements using a submicron beam size reveal a large electronlike pocket due to the Ti 3d band at the $M$ point while the Se $4p$ band at the $\mathrm{\Gamma }$ point is absent due to electron doping by the intercalated Cu. We have observed $\sqrt{3}\times \sqrt{3}$ Fermi surface reconstruction below the CDW transition, in which Ti $3d$ bands are folded into a reconstructed BZ rotated by ${30}^{\circ }$, making the $\mathrm{\Gamma }$ and $K$ points equivalent. The results demonstrate the presence of a $\sqrt{3}\times \sqrt{3}$ CDW phase in the electronic structure of heavily Cu-intercalated ${\mathrm{Cu}}_{0.33}{\mathrm{TiSe}}_2$. The findings reveal a platform for investigating the two-dimensional electronic phase in 1T-${\mathrm{TiSe}}_2$ chalcogenides, which exhibit a unique interplay between CDW, superconductivity, and excitonic/lattice instabilities.

We derive and study a model of square planar, $d^8$ spin crossover materials that treats elastic, magnetic, and spin-orbit interactions on an equal footing. For 1D chains density matrix renormalization group calculations show that the competition between these interactions leads to six different phases. For weak spin-orbit coupling (SOC) and large antiferromagnetic interactions we find a symmetry-protected topological (SPT) Haldane phase. Moderate SOC introduces dynamical low-spin (LS) impurities that rapidly destroy the SPT state and drive the system into a topologically trivial high-spin (THS) phase. For strong SOC the $S^z=\pm 1$ HS states are high-energy excitations. Thus, the system can be understood as a transverse field Ising model with the SOC playing the role of the transverse field. Consistent with this, we find a quantum phase transition between the THS phase and a quantum disordered (QD) phase. However, if the magnetic coupling is nonzero or the HS and LS states of a single molecule are nondegenerate, the $Z_2$ (Ising) symmetry is broken and the phase transition becomes a crossover. Thus, the QD phase and the THS phases are adiabatically connected, as, equivalently, are the large-$D$ phase of the spin-1 Heisenberg model and the QD phase of the transverse field Ising model. We also find a ferroelastic LS phase, an antiferroelastic phase, with alternating HS and LS complexes, and a dimer phase, which results from the competition between antiferromagnetic and antiferroelastic interactions.

${\mathrm{BaMg}}_2{\mathrm{Bi}}_2$ is a Dirac semimetal characterized by a simple Dirac cone crossing the Fermi level at the center of the Brillouin zone, protected by $C_3$ rotational symmetry. Together with its Sr-based analog ${\mathrm{SrMg}}_2{\mathrm{Bi}}_2$, it has been proposed as a promising candidate for a chemically driven topological switch: while ${\mathrm{SrMg}}_2{\mathrm{Bi}}_2$ is an insulator, ${\mathrm{BaMg}}_2{\mathrm{Bi}}_2$ exhibits nontrivial topological features. A detailed understanding of its electronic structure is essential to elucidate its electronic and transport properties. Previous photoemission studies confirmed the Dirac nature of ${\mathrm{BaMg}}_2{\mathrm{Bi}}_2$, but were limited to high photon energies, which hindered direct comparison with density functional theory calculations (DFT), due to reduced resolution and higher-frequency matrix-element modulation in that regime. In this work, we combine high-resolution angle-resolved photoemission spectroscopy and DFT calculations to get full insight into the valence band states, providing a comprehensive picture of the low-energy electronic structure. Our measurements reveal the presence of previously unobserved surface states. We found that they are topologically trivial, but they unlock a more comprehensive understanding of the material's behavior, reconciling previous discrepancies between experiment and theory.

The quasi-one-dimensional material $\alpha \text{−}{\mathrm{Bi}}_4{\mathrm{Br}}_4$ was recently predicted as a higher-order topological insulator with helical hinge states strongly dependent on the (001) surface termination. Here, we theoretically investigate how step defects on the (001) surface of $\alpha \text{−}{\mathrm{Bi}}_4{\mathrm{Br}}_4$ nanowires reconfigure helical hinge states. Our results show that step defects, depending on the (001) termination type and the in-plane location of the step, can either enable lateral and vertical repositioning of helical hinge states or generate additional helical hinge channels. Moreover, we propose a three-dimensional helical beam splitter utilizing step defects with gate tunability. Transport simulations show electrical control of hinge current splitting, highlighting step engineering's potential for tunable topological quantum devices. Especially, the three-dimensional geometry enables more control capabilities, unattainable in conventional planar beam splitters. Our findings establish atomic step engineering as a versatile platform for manipulating helical hinge states in $\alpha \text{−}{\mathrm{Bi}}_4{\mathrm{Br}}_4$.

Honeycomb bismuthene structures on Ag(111) were investigated using low-energy electron diffraction (LEED) and density functional theory. LEED I(V) analysis revealed that 0.5 monolayer (ML) of Bi forms an ultraflat honeycomb lattice with negligible buckling at ∼120 K, which transforms into other structures upon warming to room temperature. A similar flat bismuthene structure also forms in Mn/Bi/Ag(111), which remains stable even at room temperature. Mn deposition on (p × $\sqrt{3}$)-rect Bi/Ag(111) induces Bi surface segregation, as confirmed by x-ray photoelectron spectroscopy, resulting in a p(2 × 2) honeycomb bismuthene. The detailed structural investigation provides fundamental basis for characterizing the two-dimensional topological properties of bismuthene grown on Ag(111).