Metastability is a quintessential feature of first-order quantum phase transitions, which is lost either by dynamical instability or by nucleating bubbles of a true vacuum through quantum tunneling. By considering a drive across the first-order quantum phase transition in the quantum Ising chain in the presence of both transverse and longitudinal fields, we reveal multiple regions in the parameter space where the initial metastable state loses its metastability in successive stages. The mechanism responsible is found to be semidegenerate resonant tunnelings to states with specific bubble sizes. We show that such dynamics of quantized bubble nucleations can be understood in terms of Landau-Zener transitions, which provide quantitative predictions of nucleation probabilities for different bubble sizes.

The quantum-limit Hall effect at $\nu =nh/eB\sim O\left(1\right)$ that hosts a variety of exotic quantum phenomena requires demanding strong magnetic field $B$ and low carrier density $n$. We propose to realize the quantum-limit Hall effect even in the presence of large carrier density residues $n_e$ and $n_h$ in a relatively weak and variable magnetic field $B$ in topological semimetals, where a single Fermi-surface contour allows both electron-type and hole-type carriers and approaches charge neutrality when $n_e\approx n_h$. The underlying filling factor $\nu =|n_e-n_h|h/eB$ also offers an example that violates Onsager's relation for quantum oscillations.

The coexistence of Mott localized $f$ states with wide conduction and valence bands in $f$-electron semiconductors results, quite generically, in a complex optical response with the nature of the absorption edge difficult to resolve both experimentally and theoretically. Here, we combine a dynamical mean-field theory approach to localized $4f$ shells with an improved description of band gaps by a semilocal exchange-correlation potential to calculate the optical properties of the light rare-earth fluorosulfides $Ln\mathrm{SF}$ ($Ln=\mathrm{Pr}$, Nd, Sm, Gd) from first principles. In agreement with experiment, we find the absorption edge in SmSF to stem from $\mathrm{S}\text{−}3p$ to $\mathrm{Sm}\text{−}4f$ transitions, while the Gd compound behaves as an ordinary $p\text{−}d$ gap semiconductor. In the unexplored PrSF and NdSF systems we predict a rather unique occurrence of strongly hybridized $4f\text{−}5d$ states at the bottom of the conduction band. The nature of the absorption edge results in a characteristic anisotropy of the optical conductivity in each system, which may be used as a fingerprint of the relative energetic positions of different states.

The influence of disorder on the critical electron states in a quasiperiodic lattice is a subject of intense research. In this work, we report the occurrence of Anderson localization in an icosahedral (i) polygrain quasicrystal Al-Pd-Re due to site disorder using hard x-ray photoemission spectroscopy, resistivity, and density functional theory (DFT). Photoemission spectroscopy shows that the density of states is enhanced at the Fermi level in polygrain $i$-Al-Pd-Re compared to single-grain $i$-Al-Pd-Re. In contrast, the conductivity of the former is an order of magnitude reduced compared to the latter, indicating that these electron states are localized. DFT shows that these states originate primarily from Re $5d$-Pd $4d$ hybridization and are enhanced in polygrain $i$-Al-Pd-Re due to compositional difference, but are broadened because of disorder that brings about Anderson localization. This is established by the Mott variable range hopping behavior of conductivity, and the estimated localization length is 23 Å.

Here we present a study of the phonon dynamics in the honeycomb Kitaev spin model at finite temperatures. We show that the fractionalized spin excitations of the Kitaev spin liquid, the itinerant Majorana fermions, and static $Z_2$ fluxes have distinct effects on the phonon dynamics, which makes the phonon dynamics a promising tool for exploring the Kitaev spin liquid candidate materials. In particular, we focus on the signature of the fractionalized excitations in the thermodynamic behavior of the sound attenuation and the phonon Hall viscosity: the former describes the phonon decay into the fractionalized excitations, and the latter is the leading order time-reversal symmetry breaking effect on the acoustic phonon. We find that the angular dependence of the attenuation coefficient and its magnitude is modified by the thermal excitation of the $Z_2$ fluxes. The strength of this effect strongly depends on the relative magnitude of the sound velocity and the Fermi velocity characterizing the low-energy Majorana fermions. We also show that the Hall viscosity is strongly suppressed by the increase of the density of the $Z_2$ fluxes at finite temperatures. All these observations reflect the effects of the emergent disorder on the Majorana fermions introduced by the $Z_2$ fluxes. Our analysis is based on the complementary analytical calculations in the low-temperature zero-flux sector, and numerical calculations in the inhomogeneous flux sectors at intermediate and high temperatures with stratified Monte Carlo (strMC) method.

Using atomic simulations, we characterize the scattering time, the nature, and the weight of thermal excitations in the example system ${\mathrm{Li}}_2\mathrm{S}$ from its amorphous solid state to its superionic and liquid states. For the amorphous solid state at 300 K, the vibration scattering time was found to range from a few femtoseconds to several picoseconds. As a result, both propagons and diffusons are the main heat carriers and contribute largely to the total thermal conductivity. The enhancement of scattering among vibrations and between vibrations and the free ions' flow due to the increase of temperature leads to a large reduction of the scattering times and of the acoustic vibrational thermal conductivity, i.e., 0.8 W/mK at 300 K to 0.56 W/mK in the superionic ${\mathrm{Li}}_2\mathrm{S}$ at 700 K. In this latter state, the thermal conductivity contributed by convection increases to half of the total, as a result of the cross correlation between the virial heat flux and the free ions' one in liquid ${\mathrm{Li}}_2\mathrm{S}$ at 1100 K. The vibrational scattering time can be large ($\sim 1.5$ ps) yet, and the vibrational conductivity is reduced to a still significant fraction highlighting the unexpected role of acoustic transverse and longitudinal vibrations. Our study provides a fundamental understanding of the thermal excitations at play in amorphous materials from solid to liquid, including the intermediate superionic phase.

We present a new geometric approach to Floquet many-body systems described by inhomogeneous conformal field theory in $1+1$ dimensions. It is based on an exact correspondence with dynamical systems on the circle that we establish and use to prove existence of (non)heating phases characterized by the (absence) presence of fixed or higher-periodic points of coordinate transformations encoding the time evolution: Heating corresponds to energy and excitations concentrating exponentially fast at unstable such points while nonheating to pseudoperiodic motion. We show that the heating rate (serving as the order parameter for transitions between these two) can have cusps, even within the overall heating phase, and that there is a rich structure of phase diagrams with different heating phases distinguishable through kinks in the entanglement entropy, reminiscent of Lifshitz phase transitions. Our geometric approach generalizes previous results for a subfamily of similar systems that used only the $\mathfrak{sl}\left(2\right)$ algebra to general smooth deformations that require the full infinite-dimensional Virasoro algebra, and we argue that it has wider applicability, even beyond conformal field theory.

Unlike van der Waals materials, perovskite oxide may undergo structural reconstruction when reduced to two dimensions. Therefore, it is still an open question whether some of the ferroelectric mechanisms and magnetoelectric coupling effects found in perovskite bulks can be maintained at the two-dimensional (2D) limit. Here we demonstrate that the ferroelectricity induced by octahedral rotation and the magnetoelectric coupling mechanism dependent on Dzyaloshinskii-Moriya (DM) interaction exist in a proposed perovskite bilayer. Although the octahedral rotation distortion of the ${\mathrm{Ca}}_3{\mathrm{Mn}}_2{\mathrm{O}}_7$ bilayer is reconstructed with respect to its layered bulk phase, tensile strain can induce the same octahedral rotation type as its bulk phase, resulting in the emergence of ferroelectric phase. We show that the modulation of the DM vector by octahedral rotation distortion provides a coupling between polarization and the induced magnetization. In ferroelectric switching, the polarization is reversed by a ${180}^{\circ }$ rotation of the tilt axis within the $ab$ plane, which also results in a reversal of magnetization. This work is expected to provide a guidance for realizing electric-field control of magnetization direction in 2D perovskite oxides.

We study the effects of vector Bose gases, which are described by massive vector Bose fields, on fermionic superfluids in the low-energy region. It is demonstrated that the vector Bose gases can give rise to the Meissner effect, the flux quantization, and the Josephson effect, which are similar to those of the electromagnetic field in fermionic superfluids. However, unlike the electromagnetic field where the Meissner effect is unrelated to mass and the quantized flux is a constant, the Meissner effect of the vector Bose gases can be related to mass via a classical kinetic energy, and the value of the quantized flux is tunable. Our analyses also show that, although the origin model violates gauge invariance, the vector Bose gases in fact are gauge-invariant Maxwell-Chern-Simons systems under a Gauss constraint. It is also proposed that these effects can be observed by using the spin-1 cold atomic Bose gases in experiments.

We have measured low-temperature variation of resistance and nonlinear current-voltage behavior in antidot graphene in the vicinity of the charge neutrality point. The data are found to be consistent with the manifestations of a variable-range hopping electronic density of states (DOS) with a small hard gap of $\sim 1$ meV around the Fermi level, in conjunction with a parallel tunneling conduction channel that exists at the center of the gap. The hard gap is confirmed by the appearance of a low-conductive plateau at low-bias electric field, whereas the parallel tunneling conduction channel, with temperature-independent conductance, is manifest through the nonlinear electric field variation. Unified good agreement between the temperature and electric field dependencies of conductance, for both channels, is obtained with the predictions of a proposed DOS model. An increase in the gap size with applied magnetic field is observed.

Three-dimensional Weyl semimetals have pairs of topologically protected Weyl nodes whose projections onto the surface Brillouin zone are the end points of zero-energy surface states called Fermi arcs. At the endpoints of the Fermi arcs, surface states extend into and are hybridized with the bulk. Here, we consider a two-dimensional junction of two identical Weyl semimetals whose surfaces are twisted with respect to each other and tunnel coupled. Confining ourselves to commensurate angles (such that a larger unit cell preserves a reduced translation symmetry at the interface) enables us to analyze arbitrary strengths of the tunnel coupling. We study the evolution of the Fermi arcs at the interface, in detail, as a function of the twisting angle and the strength of the tunnel coupling. We show unambiguously that in certain parameter regimes, all surface states decay exponentially into the bulk, and the Fermi arcs become Fermi loops without endpoints. We study the evolution of the Fermi surfaces of these surface states as the tunnel-coupling strengths vary. We show that changes in the connectivity of the Fermi arcs/loops have interesting signatures in the optical conductivity in the presence of a magnetic field perpendicular to the surface.

Phonon interactions are inevitable in cavity quantum electrodynamical systems based on solid-state emitters or fluorescent molecules, where vibrations of the lattice or chemical bonds couple to the electronic degrees of freedom. Due to the non-Markovian response of the vibrational environment, it remains a significant theoretical challenge to describe such effects in a computationally efficient manner. This is particularly pronounced when the emitter-cavity coupling is comparable with or larger than the typical phonon energy range, and polariton formation coincides with vibrational dressing of the optical transitions. In this paper, we consider four non-Markovian perturbative master equation approaches to describe such dynamics over a broad range of light-matter coupling strengths and compare them with numerically exact reference calculations using a tensor network. The master equations are derived using different basis transformations, and a perturbative expansion in the transformed basis is subsequently introduced and analyzed. We find that two approaches are particularly successful and robust. The first of these is suggested and developed in this paper and is based on a vibrational dressing of the exciton-cavity polaritons. This enables the description of distinct phonon-polariton sidebands that appear when the polariton splitting exceeds the typical phonon frequency scale in the environment. The second approach is based on a variationally optimized polaronic vibrational dressing of the electronic state. Both of these approaches demonstrate good qualitative and quantitative agreement with reference calculations of the emission spectrum and are numerically robust, even at elevated temperatures, where the thermal phonon population is significant.

In this paper we study the third-order nonlinear optical response due to transitions between excitonic levels in two-dimensional transition metal dichalcogenides. To accomplish this we use methods not applied to the description of excitons in two-dimensional materials so far and combined with a variational approach to describe the $1s$ excitonic state. The aforementioned transitions allow us to probe dark states which are not revealed in absorption experiments. We present general formulas capable of describing any third-order process. The specific case of two-photon absorption in ${\mathrm{WSe}}_2$ is studied. The case of the circular well is also studied as a benchmark of the theory.

We present a hierarchical equations of motion approach, which allows a numerically exact simulation of nonequilibrium transport in general open quantum systems involving multiple macroscopic bosonic and fermionic environments. The performance of the method is demonstrated for a model of a nanosystem, which involves interacting electronic and vibrational degrees of freedom and is coupled to fermionic and bosonic baths. The results show the intricate interplay of electronic and vibrational degrees of freedom in this nonequilibrium transport scenario for both voltage and thermally driven transport processes. Furthermore, the use of importance criteria to improve the efficiency of the method is discussed.

As control of the Rashba spin splitting size is highly desirable for spintronic devices, intensive studies have been performed to vary the splitting size by, for example, applying an electric field or designing novel heterostructures. However, direct observation of Rashba spin splitting size change via spectroscopic measurements has not been done so far. Here, we report results of angle-resolved photoemission studies on ferroelectric $\alpha$-GeTe(111). We observe that the Rashba splitting size of $\alpha$-GeTe(111) surface states is reduced upon dosing with potassium (K) which has a very low electron affinity. Based on density functional theory calculations, we find that the electric potential energy at the topmost atomic layer and the surface potential energy barrier is reduced upon K dosing. This change in the surface potential induces both delocalization of the surface states and reduction of the splitting size. We expect to increase the splitting size by dosing other elements or molecules with high electron affinity.

Magnetic dipole (MD) spontaneous emission plays a vital role in the field of optical magnetism, which has been observed in trivalent lanthanide ions. In this case, the luminescence properties of the MD are largely affected by the crystal field symmetry at the embedding trivalent lanthanide ion site, but the correlation between the doping properties and the MD emission has not been fully understood. Here, we systematically investigate the doping properties of Eu in $\mathrm{Sr}M{\mathrm{O}}_3$ ($M=\mathrm{Ti},\mathrm{Zr},\mathrm{Hf}$) using first-principles calculations, in order to maximize the MD emission efficiency of the Eu-doped perovskites. By analyzing the formation energies under the accessible growth conditions, we determine the ideal conditions for Eu-doped perovskites that could maximize the MD emission. We also theoretically demonstrate the spin flipping emission mechanism of ${\mathrm{Eu}}^{3+}$. Our study thus provides a guideline for the design of highly efficient MD emission materials.

We clarify theoretically that the topological ring-cavity (TRC) modes propagating along the interface between two honeycomb-type photonic crystals distinct in topology can be exploited for achieving stable single-mode lasing, with the maximal intensity larger than a whispering-gallery-mode counterpart by order of magnitude. Especially, we show that the TRC modes located at the bulk band-gap center benefit maximally from the gain profile since they are most concentrated and uniform along the interface, and that, inheriting from the Dirac-like dispersion of topological interface states, they are separated in frequency from each other and from other photonic modes, both favoring intrinsically single-mode lasing. A TRC mode running in a specific direction with desired orbital angular momentum can be stimulated selectively by injecting a circularly polarized beam. With the scheme proposed in the present work one can fabricate a topological laser of tens of micrometers in linear size by means of advanced semiconductor nanotechnologies, which generates chiral laser beams ideal for novel photonic functions.

Understanding nonequilibrium electron dynamics in a tunable-barrier dynamic quantum dot is vital to achieve high-accuracy single-electron pumping, which has applications to metrological standards and quantum information devices. However, the dynamic mechanism determining the pumping process has not been fully understood. In this paper, we study and clarify the physical mechanisms of single-electron pumping by analyzing and calculating master equations with a realistic model in detail. Our focus is mainly on the regime which lies at the crossover between the two extreme physical models of single-electron pumping. The mechanism crossover strongly depends on a capacitive coupling between a gate for tuning the barrier and the quantum dot. We find that there is an optimal value of the coupling with the best pumping accuracy and the mechanism crossover depends on gate voltage in some range of the coupling values. Our results offer a guideline for evaluation of pumping characteristics and for optimization of the device structure, which is an important step toward understanding high-speed nonequilibrium electron dynamics and realizing reproducible high-accuracy single-electron pumps.

The quantum Hall effect (QHE) is usually observed in two-dimensional systems. Recently, the QHE has been proposed to exist in three-dimensional (3D) Weyl semimetals when the Fermi energy is precisely at the Weyl nodes at zero temperature, which is hardly achievable experimentally. However, the QHE has been experimentally realized in the 3D topological semimetal ${\text{Cd}}_3{\text{As}}_2$ at finite temperatures. It has been found that a Weyl semimetal may translate to a chiral excitonic insulator due to the interplay between the interaction and band topology. In a Weyl semimetal slab, we show that the Fermi arc surface states can exist not only in the Weyl semimetal but also in the chiral excitonic insulator. As long as the Fermi energy crosses the Fermi arc states, the QHE can be generated under the combined action of bulk states and Fermi arc surface states. Therefore, the integer quantization of Hall conductivity can be observed for a finite energy range, or equivalently at finite temperatures.