Temperature-dependent reflectivity studies on the nonmagnetic kagome metal ${\mathrm{RbV}}_3{\mathrm{Sb}}_5$ in a broad energy range ($50–20000{\mathrm{cm}}^{-1}$, equivalent to 6 meV–2.5 eV) down to 10 K are reported. Below $T_{\mathrm{CDW}}=102$ K, the optical spectra demonstrate a prominent spectral-weight transfer from low to higher energies as the fingerprint of the charge-density-wave (CDW) formation with the opening of a partial gap. A detailed analysis reveals two energy scales of respectively $\sim 800 {\mathrm{cm}}^{-1}$ (100 meV) and $360{\mathrm{cm}}^{-1}$ (45 meV), the latter visible below 50 K only. Additionally, two modes at respectively $160{\mathrm{cm}}^{-1}$ (20 meV) and $430{\mathrm{cm}}^{-1}$ (53 meV) can be traced both above and below $T_{\mathrm{CDW}}$. They show strong anomalies already above $T_{\mathrm{CDW}}$ with a further renormalization across the transition, suggesting the importance of the electron-phonon coupling in ${\mathrm{RbV}}_3{\mathrm{Sb}}_5$ in both normal and CDW states. While the $160{\mathrm{cm}}^{-1}$ mode can be attributed to the $E_{1u}$ phonon, the $430{\mathrm{cm}}^{-1}$ mode could not be reproduced in our phonon calculations. The antiresonance nature of this mode suggests a nontrivial electron-phonon coupling in ${\mathrm{RbV}}_3{\mathrm{Sb}}_5$. A distinct localization peak observed at all temperatures signals damped electron dynamics, whereas the reduced Drude spectral weight manifests moderate deviations from the band picture in ${\mathrm{RbV}}_3{\mathrm{Sb}}_5$.

We compute both analytically and numerically the quantum metric tensor and its scalar curvature for the Dicke model. In the analytical setting we consider the thermodynamic limit and carry out the computations by means of the truncated Holstein-Primakoff approximation. We also study the exactly solvable case ${\omega }_0=0$ and find that the corresponding non-Abelian QMT effectively reduces to just one metric tensor with zero determinant. In the numerical case we use an efficient basis to diagonalize the Hamiltonian for four different system's sizes. For the components of the quantum metric tensor and their derivatives, we find a remarkable agreement between the numerical and analytical results, with the metric's peaks signaling the precursors of the quantum phase transition. In the case of the scalar curvature, there are some differences between the numerical and analytical results that can be traced back to the behavior of the combination of the metric components' derivatives. Notably, the scalar curvature in the thermodynamic limit is continuous across the quantum phase transition and, in that zone, it approximately matches the numerical results.

In this study, we develop a classical interatomic potential for the ${\mathrm{Ba}}_{1-x}{\mathrm{Ca}}_x{\mathrm{ZrO}}_3$ alloy based on the bond-valence theory. The bond-valence model enables rapid and large-scale molecular dynamics simulations of alloys with variable compositions. Molecular dynamics simulations based on this force field can reproduce the experimentally observed composition-dependent dielectric responses and lattice constants very well, indicating the validity and robustness of this force field. Based on molecular dynamics simulations, we demonstrate that ${\mathrm{Ba}}_{1-x}{\mathrm{Ca}}_x{\mathrm{ZrO}}_3$ can adopt a ferroelectric phase under a tensile strain, while the ${\mathrm{Ba}}_{1-x}{\mathrm{Ca}}_x{\mathrm{ZrO}}_3$ solid solution under zero strain is paraelectric. In addition, we find that strain can invoke a phase transition from antiferroelectric to ferroelectric in the ${\mathrm{BaZrO}}_3/{\mathrm{CaZrO}}_3$ superlattice. These results successfully explain previous experimental results and first-principles calculations. This interatomic potential provides a powerful tool for simulating the physical properties of perovskite alloys at nanoscale and thus has great significance in understanding the underlying physical mechanisms and designing functional materials.

Feynman path integrals provide an elegant, classically inspired representation for the quantum propagator and the quantum dynamics, through summing over a huge manifold of all possible paths. From computational and simulational perspectives, the ergodic tracking of the whole path manifold is a hard problem. Machine learning can help, in an efficient manner, to identify the relevant subspace and the intrinsic structure residing at a small fraction of the vast path manifold. In this work, we propose the Feynman path generator for quantum mechanical systems, which efficiently generates Feynman paths with fixed endpoints, from a (low-dimensional) latent space and by targeting a desired density of paths in the Euclidean space-time. With such path generators, the Euclidean propagator as well as the ground-state wave function can be estimated efficiently for a generic potential energy. Our work provides an alternative approach for calculating the quantum propagator and the ground-state wave function, paves the way toward generative modeling of quantum mechanical Feynman paths, and offers a different perspective to understand the quantum-classical correspondence through deep learning.

Recent experimental advances in Floquet engineering and controlling dissipation in open systems have brought about unprecedented flexibility in tailoring novel phenomena without any static and Hermitian analogues. It can be epitomized by the various Floquet and non-Hermitian topological phases. Topological classifications of either static/Floquet Hermitian or static non-Hermitian systems based on the underlying symmetries have been well established in the past several years. However, a coherent understanding and classification of Floquet non-Hermitian (FNH) topological phases have not been achieved yet. Here we systematically classify FNH topological bands for 54-fold generalized Bernard-LeClair (GBL) symmetry classes and arbitrary spatial dimensions using $K$ theory. The classification distinguishes two different scenarios of the Floquet operator's spectrum gaps [dubbed as Floquet operator (FO) angle-gapped and FO angle-gapless]. The results culminate into two periodic tables, each containing 54-fold GBL symmetry classes. Our scheme reveals FNH topological phases without any static/Floquet Hermitian and static non-Hermitian counterparts. And our results naturally produce the periodic tables of Floquet Hermitian topological insulators and Floquet unitaries. The framework can also be applied to characterize the topological phases of bosonic systems. We provide concrete examples of one- and two-dimensional fermionic/bosonic systems. And we elucidate the meaning of the topological invariants and their physical consequences. Our paper lays the foundation for a comprehensive exploration of FNH topological bands. And it opens a broad avenue toward uncovering unique phenomena and functionalities emerging from the synthesis of periodic driving, non-Hermiticity, and band topology.

Quasiperiodic lattice systems offer diverse transport properties. In this paper, we investigate environment-induced effects on the transport properties for quasiperiodic systems, namely the one-dimensional Aubry-André-Harper (AAH) lattice chain and its generalized version (GAAH), by considering the Büttiker probe approach. We first consider a voltage-probe situation and study the electrical conductance properties in the linear-response regime. At zero temperature, we observe an enhancement in conductance for small probe coupling strength $\gamma$ with a power-law scaling ${\gamma }^4$ at all the no-transport regimes, located both inside and outside of the band of the original system. For large probe coupling strengths, the conductance at all Fermi energies is the same and decays as a power law with scaling $1/{\gamma }^4$. This particular scaling survives even in the finite-temperature limit. Interestingly, this scaling result is different from the one recently predicted following the local Lindblad master equation approach. The transport eventually becomes diffusive for all energy ranges and in all regimes of the original model for a sufficiently strong coupling with the probes. We further extend our study and consider voltage-temperature probes to analyze the thermoelectric performance of the chain in terms of the figure of merit. We also demonstrate the validity of two recently obtained bounds on thermoelectric efficiency which are tighter than the seminal Carnot bound, in the presence of voltage-temperature probes.

Quantum transport is developed in the Wigner function representation for a Bloch electron quasiparticle interacting with a disordered binary alloy in the presence of a homogeneous electric field of arbitrary time dependence and amplitude. The electron quasiparticle is described by a single-band effective Hamiltonian, and the homogeneous electric field is treated in the vector potential gauge. The methodology for the quantum transport analysis proceeds by first transforming the Liouville equation to one in which the interaction Hamiltonian (the binary alloy Hamiltonian) appears quadratically. The basis states employed in evaluating the requisite matrix elements are the instantaneous eigenstates of the electron quasiparticle Hamiltonian in the presence of the electric field. The Wigner quantum transport equations are derived, and the binary alloy collision term is suitably ensemble averaged over the disordered binary alloy matrix elements. In addition, the general drift and diffusion terms are exactly obtained, resulting in the complete Wigner-Boltzmann equation for the binary alloy system. In approximating the collision term for the two separate cases of parabolic energy dispersion and the long-wavelength limit, it is found that the reduced Wigner-Boltzmann equation includes the manifestation of the intracollisional field effect and other quantum generalities. As a contrast to the actual random alloy treatment, attention is given to the canonical problem of quantum transport for a virtual crystal (VC). As an alternative to the random binary alloy scattering problem, the VC Hamiltonian adopted for this treatment is derived by ensemble averaging the random binary alloy Hamiltonian. The resulting Wigner transport equation for the VC case is descriptive of Bloch dynamics in a graded semiconductor alloy.

Noise-induced synchronization is a phenomenon where several oscillators with different initial conditions show synchronized motion, even in the absence of a coupling between them, when common stochastic input signals are injected. The phenomenon has attracted attention from nonlinear science, as well as applied physics, because it enables environmental noise to be used to synchronize many oscillators and is a necessary condition for brain-inspired computing. Here we develop a theoretical analysis of noise-induced synchronization in spin-torque oscillators (STOs). The analytical form for the Lyapunov exponent for the present model we derive indicates that there are two contributions from input signal to noise-induced synchronization in STOs. The first is that the input signal directly aligns the phases of the magnetizations, and the second is that the input signal changes the oscillating amplitude, and the amplitude-phase coupling results in synchronization. The validity of the analytical results was qualitatively confirmed by numerical simulation. We also show the existence of on-off intermittency at finite temperature, whose statistical properties are similar to those of other oscillator systems.

$\tilde{S}=1/2$ triangular-lattice $f$-electron materials define a dynamic research area in condensed matter magnetism. In various Yb $4f^{13}$ triangular-lattice compounds, for example, spin-liquid ground states seem to be realized. Using ab initio quantum chemical methods, we here investigate how correlation effects involving the $4f$ electrons affect the on-site $f\text{−}f$ excitation spectrum in $\mathrm{Na}\mathrm{Yb}{\mathrm{Se}}_2$. The system is well suited for such a study since unambiguous inelastic neutron scattering data are available for the ${\mathrm{Yb}}^{3+}f\text{−}f$ transitions. The excitation energies obtained by configuration-interaction calculations with single and double substitutions agree within 3–4 meV with experimental values, which provides a not so expensive alternative to fitting experimental data at the model-Hamiltonian level in order to analyze $f$-center multiplet structures.

Predictions of localized Majorana modes, and ideas for manipulating these degrees of freedom, are the two key ingredients in proposals for physical platforms for Majorana quantum computation. Several proposals envisage a scalable network of such Majorana modes coupled bilinearly to each other by quantum-mechanical mixing amplitudes. Here, we develop a theoretical framework for characterizing collective topologically protected zero-energy Majorana fermion excitations of such networks of localized Majorana modes. A key ingredient in our work is the Gallai-Edmonds decomposition of a general graph, which we use to obtain an alternate “local” proof of a “global” result of Lovász and Anderson on the dimension of the topologically protected null space of real skew-symmetric (or pure-imaginary Hermitian) adjacency matrices of general graphs. Our approach to Lovász and Anderson's result constructs a maximally localized basis for the said null space from the Gallai-Edmonds decomposition of the graph. Applied to the graph of the Majorana network in question, this gives a method for characterizing basis-independent properties of these collective topologically protected Majorana fermion excitations, and relating these properties to the correlation function of monomers in the ensemble of maximum matchings (maximally packed dimer covers) of the corresponding network graph. Our approach can also be used to identify signatures of zero-energy excitations in systems modeled by a free-fermion Hamiltonian with a hopping matrix of this type; an interesting example is provided by vacancy-induced Curie tails in generalizations (on nonbipartite lattices) of Kitaev's honeycomb model.

The intrinsic spin Hall effect and topological properties of ${\mathrm{ThAl}}_2$ and ${\mathrm{ThGa}}_2$ have been investigated based on first-principles electronic structure calculations. The band structures calculated without the spin-orbital coupling (SOC) show that both ${\mathrm{ThAl}}_2$ and ${\mathrm{ThGa}}_2$ host multiple nodal lines near the Fermi level. Once the SOC is taken into account, these nodal lines are gapped and huge spin Berry curvatures can be generated. As a result, giant spin Hall conductances of ${\mathrm{ThAl}}_2$ and ${\mathrm{ThGa}}_2$ are induced around the Fermi level. Moreover, large spin Hall angles can be achieved due to the giant spin Hall conductance and the small electric conductivity. Our work indicates that ${\mathrm{ThAl}}_2$ and ${\mathrm{ThGa}}_2$ not only provide an ideal platform for exploring the interplay between spin Hall effect and topological property, but also have promising applications in spin-orbitronic devices.

Fractonic phases of matter are novel quantum ground states supporting subdimensional emergent excitations with mobility restrictions. Due to a subextensive ground state degeneracy that is dependent on the geometry of the underlying lattice, fractonic phases are considered as models for quantum memory or quantum glass. While there exist a number of exactly solvable models with interactions between multiple particles/spins, the realization of such models in real materials is extremely challenging. In this work, we provide a realistic quantum model of quadratic spin interactions on the breathing pyrochlore lattice of existing materials. We show that the emergent “cluster charge” excitations arise as vacuum fluctuations residing on the boundary of membrane objects, and move in a subdimensional space. Using the membrane operators, we demonstrate the existence of a subextensive ground state degeneracy explicitly depending on the lattice geometry, which is a useful resource for novel quantum memory.

Motivated by manipulating the magnetic order of bilayer $\mathrm{Cr}{\mathrm{I}}_3$, we carry out microscopic calculations to find the magnetic order and various magnetic domains of the system in the presence of an electric field. Making use of density functional simulations, a spin model Hamiltonian is introduced consisting of isotropic exchange couplings, Dzyaloshinskii-Moriya (DM) interaction, and on-site magnetic anisotropy. The spin dynamics of two well-known states of bilayer $\mathrm{Cr}{\mathrm{I}}_3$, low-temperature (LT) and high-temperature (HT) phases, are obtained by solving the Landau-Lifshitz-Gilbert equation. We show that the magnetic texture is stacking-dependent in bilayer $\mathrm{Cr}{\mathrm{I}}_3$ and stable magnetic domains can appear in the HT stack, which are tunable by external electric and magnetic fields. Therefore, we suggest that the HT phase represents a promising candidate for data storage in the modern generation of spintronic devices working on magnetic domain engineering.

The quantum diffusion in a one-dimensional lattice in a tunneling regime under the influence of spatially inhomogeneous Gaussian noise is studied theoretically. It is shown that the thermopower appears at an arbitrary value of the tunneling coupling. In the strong tunneling regime at an extremely high temperature, when the drift component of the charge transport can be neglected, the thermoelectric effect takes place only in the presence of a constant external electric field. The Seebeck and Peltier coefficients for the closed-circuit mode are obtained for a one-dimensional quantum diffusion medium in the tunneling regime. In the strong tunneling regime, both the thermopower and the Peltier effect are essentially nonlinear phenomena.

Magnetic focusing of charge carriers in two-dimensional systems provides a solid state version of a mass spectrometer. In the presence of a spin-orbit interaction, the first focusing peak splits into two spin dependent peaks, allowing focusing to be used to measure spin polarization and the strength of the spin-orbit interaction. In hole systems, the $k^3$ dependence of the Rashba spin-orbit term allows the spatial separation of spins to be changed in situ using a voltage applied to an overall top gate. Here, we demonstrate that this can be used to control the splitting of the magnetic focusing peaks. Additionally, we compare the focusing peak splitting to that predicted by Shubnikov–de Haas oscillations and $k·p$ band-structure calculations. We find that the focusing peak splitting is consistently larger than expected, suggesting further work is needed on understanding spin dependent magnetic focusing.

We investigate the relative thermodynamic stability of the $3\times 3$, $5\times 5$, $7\times 7$, $9\times 9$, and infinitely large structures related to the dimer-adatom-stacking fault family of $\mathrm{Si}\left(111\right)$ surface reconstructions by means of first-principles calculations. Upon accounting for the vibrational contribution to the surface free energy, we find that the $5\times 5$ structure is more stable than the $7\times 7$ at low temperatures. While a phase transition is anticipated to occur at around room temperature, the $7\times 7\to 5\times 5$ transformation upon cooling is hindered by the limited mobility of Si atoms. The results not only flag a crucial role of vibrational entropy in the formation of the $7\times 7$ structure at elevated temperatures, but also point to its metastable nature below room temperature.

We propose a transport probe scheme to detect the nonadiabatic transition caused by moving Majorana zero modes. The scheme is analyzed theoretically by means of a time-dependent single-electron-wave-function approach to quantum transport. Based on the Kitaev model, we simulate the time-dependent currents and examine the feasibility of using the currents to infer the nonadiabatic transition. In particular, we develop a method to determine the Landau-Zener tunneling ratio in the context of transport, and compare it with the result computed from the same Majorana moving in the isolated quantum wire. Desirable agreements are demonstrated for the whole proposed scheme.

We study the role of non-Markovian effects in the emission spectrum of a quantum emitter resonantly coupled to a surface plasmon in a metal-dielectric structure as the system transitions to strong coupling regime. By using a recent quantum approach to interacting plasmons that incorporates the effects of host material's optical dispersion and losses in the coupling parameters, we obtain analytically the emission spectrum for a plasmonic system of arbitrary shape with characteristic size below the diffraction limit. In the weak coupling regime, the dispersion-induced non-Markovian effects are weak and do not significantly affect the spectral shape of the emission peak. In contrast, in the strong coupling regime, the non-Markovian effects lead to dramatic changes in the emission spectra by causing inversion of spectral asymmetry, as compared with classical and quantum models based on the Markov approximation, which results in a strong enhancement of the lower frequency polaritonic band, consistent with the experiment.

We study the equilibrium arrangements of polarization vortices in (${\mathrm{PbTiO}}_3{)}_n/({\mathrm{SrTiO}}_3{)}_n$ superlattices by means of second-principles simulations. We find that, at low temperatures, polarization vortices organize in a regular arrangement in which clockwise and counterclockwise vortices alternate positions, leading to a crystal-like structure with well-defined handedness. This chiral crystal melts at a critical temperature $T_{\mathrm{M}}$ into a chiral liquid where long-range order is lost but handedness is preserved. At even higher temperatures $T_{\mathrm{C}}$, a second phase transition occurs at which the chiral liquid of polarization vortices loses its handedness. Both phase transitions can be readily identified by the adequate choices of order parameters.