Recent breakthroughs in quantum-dot circuit-quantum-electrodynamics systems are promising both from fundamental perspectives and from the point of view of quantum photonic devices. However, understanding such setups as potential thermoelectric devices has been missing. In this paper, via the Keldysh nonequilibrium Green's function approach, we show that cavity-coupled double quantum-dots can serve as excellent quantum thermoelectric diodes and transistors. Using an enhanced perturbation approach based on the exact polaron transformation, we find dependencies of thermoelectric transport properties on the electron-photon interaction beyond the predictions from the conventional second-order perturbation theory. In particular, strong light-matter interaction leads to pronounced rectification effects for both charge and heat, as well as thermal transistor effects in the linear transport regime, which opens up a cutting-edge frontier for quantum thermoelectric devices.

We study the optical response of nanoribbons made from the $\alpha -T_3$ lattice under a weak magnetic field in the terahertz to far-infrared regime. It is found that the magnetic field can open a gap in the band structure and induce a new absorption peak with much reduced frequency in metallic armchair ribbons and a class of zigzag ribbons with particular boundaries. This tunable magneto-optical modulation effect is attributed to the interband transitions between the flat band and the propagating bands. By contrast, this magnetic modulation of gap opening and optical conductance is much weaker in metallic armchair graphene ribbons (the case of $\alpha =0$) in which the flat band is absent. The enhancement in the $\alpha -T_3$ model is analytically investigated and explained within the perturbation theory for metallic armchair ribbons. The magnetic field induced valley degeneracy lifting and valley splitting of the absorption peak are also discussed in the case of zigzag ribbons. These findings pave the way for magneto-optics devices based on the $\alpha -T_3$ model materials.

We analyze the cooling of a mechanical resonator coupled to an ensemble of interacting two-level systems via an open quantum systems approach. Using an exact analytical result, we find optimal cooling occurs when the phonon mode is critically coupled $\left(\gamma \sim g\right)$ to the two-level system ensemble. Typical systems operate in suboptimal cooling regimes due to the intrinsic parameter mismatch $\left(\gamma \gg g\right)$ between the dissipative decay rate $\gamma$ and the coupling factor $g$. To overcome this obstacle, we show that carefully engineering the coupling parameters through the strain profile of the mechanical resonator allows phonon cooling to proceed through the dark (subradiant) entangled states of an interacting ensemble, thereby resulting in optimal phonon cooling. Our results provide an avenue for ground-state cooling and should be accessible for experimental demonstrations.

We experimentally and theoretically characterize the angular-dependent microwave response of three-macrospin-vertex structures that can serve as a node in various spin ice lattices. The macrospins consist of patterned permalloy thin films with an elliptical cross section together with an in-plane aspect ratio allowing an Ising-like behavior together with bulk modes as low-frequency excitations in the field range of interest. Various branches of the frequency-magnetic field curves display atypical behaviors and discontinuities, together with softening due to macrospin reversals. The overall behavior observed accurately corresponds to a superposition of the spectra of the individual macrospins. The measured ferromagnetic resonance spectra are in good agreement with theoretical modeling. In particular, they reveal a close correlation between the field direction (relative to the individual macrospins), and the corresponding frequency-magnetic field curve, i.e., between the geometry and the magnetic response.

We consider a type-II superconducting thin film in contact with a Néel skyrmion. The skyrmion induces spontaneous currents in the superconducting layer, which under the right condition generates a superconducting vortex in the absence of an external magnetic field. We compute the magnetic field and current distributions in the superconducting layer in the presence of Néel skyrmion.

We study multiterminal Majorana and conventional superconducting islands in the vicinity of the charge degeneracy point using bosonization and the numerical renormalization group. Both models map to the multichannel charge Kondo problem, but for noninteracting normal leads they flow to different non-Fermi-liquid fixed points at low temperatures. We compare and contrast both cases by numerically obtaining the full crossover to the low-temperature regime, and we predict distinctive transport signatures. The differences between the two types of islands result from a crucial distinction between charge-$2e$ and charge-$e$ transfer in the conventional and topological case, respectively. In the conventional case, our results establish $s$-wave islands as a platform to study the intermediate multichannel Kondo fixed point. In the topological setup, the crossover temperature to non-Fermi-liquid behavior is relatively high as it is proportional to the level broadening, and the transport results are not sensitive to channel coupling anisotropy, moving away from the charge degeneracy point or including a small Majorana hybridization, which makes our proposal experimentally feasible.

Like ferroelectric or ferroelastic phase transitions, antiferroelectric transitions can be predicted by symmetry. This property is illustrated by a theoretical analysis of the complex phase transitions, occurring in the perovskite compounds $\mathrm{PbZr}{\mathrm{O}}_3$, $\mathrm{NaNb}{\mathrm{O}}_3$, and $\mathrm{PbHf}{\mathrm{O}}_3$, which involve the coupling of several order parameters. The symmetry-breaking order parameters associated with the transitions in the three compounds are determined. The antiferroelectric transitions are shown to verify specific symmetry criteria, which consist of the emergence at the local scale of polar site symmetries coinciding with symmorphic subgroups of the antiferroelectric space group. The theoretical temperature–electric-field phase diagram of $\mathrm{PbZr}{\mathrm{O}}_3$ involving ferrielectric and ferroelectric phases is worked out.

Within the framework of density functional theory (DFT), we carry out a random structure search for an ${\mathrm{MnCO}}_3$ system under high pressures. The results identify a number of candidate structures under pressure ranged from 50 to 110 GPa, and we found several stable structures including the $P\overline{1}$ symmetry structure, which was suggested by previous experiments; the structural properties are highly consistent with measured structural data. Furthermore, our results also indicate that the $P\overline{1}$ symmetry structure becomes more energetically stable than the $R\overline{3}c$ symmetry structure above 55 GPa, which corresponds to the measured structural discontinuity in the experiment. Our results indicate that there are some candidate structures to be energetically comparable to the observed $P\overline{1}$ symmetry structure, which may explain the inconsistency between the previous experimental results. Above 65 GPa, a phase transition into a high-pressure phase with $P{2}_1/c$ symmetry is also predicted. As the pressure increases, the coordinated number of Mn atoms is increased, from sixfold-coordinated octahedral geometry (${\mathrm{MnO}}_6$, below 53.5 GPa) to sevenfold- and eightfold-coordinated polyhedron mixture phases (${\mathrm{MnO}}_7$ and ${\mathrm{MnO}}_8$, 53.5–65 GPa), and finally to the eightfold-coordinated polyhedron (${\mathrm{MnO}}_8$, above 65 GPa). For C–O bondings, the trigonal planar carbonate ${\mathrm{CO}}_3^{2-}$ ion transits into the fourfold coordinated ${\mathrm{CO}}_4^{4-}$ ion with distorted tetrahedron geometry and forms a one-dimensional chain structure above 65 GPa, which has also been predicted and observed in analogous systems.

In this paper we introduce a method to resolve transient excitations in time-frequency space from molecular dynamics simulations. Our technique is based on continuous wavelet transform of velocity time series coupled to a threshold-dependent filtering procedure to isolate excitation events from background noise in a given spectral region. By following in time the center of mass of the reference frequency interval, the data can be easily exploited to investigate the statistics of the burst excitation dynamics, by computing, for instance, the distribution of the burst lifetimes, excitation times, amplitudes and energies. As an illustration of our method, we investigate transient excitations in the gap of NaI crystals at thermal equilibrium at different temperatures. Our results reveal complex ensembles of transient nonlinear bursts in the gap, whose lifetime and excitation rate increase with temperature. The method described in this paper is a powerful tool to investigate transient excitations in many-body systems at thermal equilibrium. Our procedure gives access to both the equilibrium and the kinetics of transient excitation processes, allowing one in principle to reconstruct the full picture of the dynamical process under examination.

We present a derivation of the recently discovered duality between the free massless $(2+1)$-dimensional Dirac fermion and ${\mathrm{QED}}_3$. Our derivation is based on a regularized lattice model of the Dirac fermion and is similar to the more familiar derivation of the boson-vortex duality. It also highlights the important role played by the parity anomaly, which is somewhat less obvious in other discussions of this duality in the literature.

Using the first-principles density functional approach, we comparatively studied the electronic and magnetic properties of two series of ordered double perovskite, $\mathrm{B}{\mathrm{a}}_{2}B\mathrm{Os}{\mathrm{O}}_6(B=\mathrm{Sc},\text{Y},\text{and In})$ and $A_2\mathrm{ScOs}{\mathrm{O}}_6(A=\text{Ba},\text{Sr},\text{and Ca})$. The electronic structure calculations indicate that the Os ion bears the nominal $t_{2g}^3$ state with oxidation state $5+$ and is the only magnetically active one in all compounds. In nice agreement with the experimental observations, all of them stabilized in the type I antiferromagnetic alignment, which is mediated by the moderate nearest-neighbor antiferromagnetic interactions through the $\mathrm{Os}\text{−}\mathrm{O}\text{−}B\text{−}\mathrm{O}\text{−}\mathrm{Os}90$° route and accompanied by mild next-nearest-neighbor antiferromagnetic coupling via the $\mathrm{Os}\text{−}\mathrm{O}\text{−}B\text{−}\mathrm{O}\text{−}\mathrm{Os}180$° route, in spite of the geometric frustration. The mechanism of the antiferromagnetic interaction has been proposed based on the density of states analysis. In addition, the computed magnetic moment on Os ions, $\sim 2{\mu }_B$, is remarkably smaller than the ideal spin-only value. In conjunction with the modest spin-orbit coupling effect, strong covalency between the $\mathrm{Os}\text{−}5d$ and $\mathrm{O}\text{−}2p$ states reduces the magnetic moments. More importantly, our results demonstrated that the magnitude of the magnetic couplings as well as magnetic transition temperature $T_N$ are directly proportional to the reciprocal of the absolute value of energy mismatch between the $B\text{−}nd$ and occupied $\mathrm{Os}\text{−}{t}_{2g}$ band; in addition, they are also a function of the Sc-O-Os bond angles, i.e., the buckling of the octahedral connectivity. Moreover, comparison of these two series helps to identify the energy mismatch as the efficient factor to adjust the magnetic interactions.

Magnetic chains on superconducting systems have emerged as a platform for realization of Majorana bound states (MBSs) in condensed-matter systems with possible applications to topological quantum computation. In this work, we study the MBSs formed in magnetic chains on two-dimensional honeycomb materials with induced superconductivity. We establish chemical potential vs Zeeman splitting phase diagrams showing that the topological regions (where MBSs appear) are strongly dependent on the spiral angle along the magnetic chain. In some of those regions, the topological phase is robust even for large values of the local Zeeman field, thus producing topological regions which are, in a sense, “unbounded” in the large-field limit. Moreover, we show that the energy oscillations with magnetic field strength due to MBS splitting can show very different behaviors depending on the parameters. In some regimes, we find oscillations with increasing amplitudes and decreasing periods, while in the other regimes the complete opposite behavior is found. We also find that the topological phase can become dependent on the chain length, particularly in topological regions with a very high or no upper bound. In these systems, we see a very smooth evolution from MBSs localized at chain end points to in-gap Andreev bound states spread over the full chain.

Bound states in the continuum (BIC) have been studied mainly in optics. Recently, electronic BIC have been proposed. They appear as points in the momentum space and are protected topologically by the Chern number. In this study we propose a type of BIC protected by the winding number, which is one of the topological invariants. These BIC appear as lines in the momentum space, and are realized in a multilayer model consisting of honeycomb-lattice layers. We show band structure and spatial localization of the BIC in this model. The wave numbers at which the BIC appear can be explained in terms of topology in the momentum space.

We systematically study the coupling of longitudinal modes (shells) in a carbon nanotube quantum dot. Inelastic cotunneling spectroscopy is used to probe the excitation spectrum in parallel, perpendicular, and rotating magnetic fields. The data is compared to a theoretical model, including coupling between shells based on atomically sharp disorder in the nanotube. The calculated excitation spectra show good correspondence with experimental data.

We present a real-space theoretical formalism and its numerical implementation for investigating nonequilibrium quantum Coulomb drag between parallel two-terminal transport structures in quasi-one-dimension. In addition to the Coulomb interaction and the finite external potential bias, our formalism takes into account the effects of impurity disorder. The theory is formulated in the nonequilibrium Green's function formalism, with the long-range Coulomb interaction treated at the many-body GW approximation level and the disorder-average carried out at the coherent potential approximation (CPA) level. The coupled GW and CPA equations are solved self-consistently so that the fundamental conservation laws are ensured. The effects related to the electron-hole symmetry in the Coulomb drag physics have been generalized to the nonlinear transport regime. A set of symmetry-induced relations linking physical quantities with the particle distribution were established on generic footing and, remarkably, they are found robust against uniformly distributed impurities. As an application, the theoretical formalism is employed to analyze the Coulomb drag transport physics in quasi-one-dimensional systems. The dependencies of the drag current on external bias, chemical potential, temperature, and the system sizes are predicted.

The discovery of high-mobility electron gas at the two-dimensional (2D) $\mathrm{LaAl}{\mathrm{O}}_3/\mathrm{SrTi}{\mathrm{O}}_3$ heterointerface [A. Ohtomo and H. Hwang, Nature (London) 427, 423 (2004)] has attracted great research interest due to the ballistic transport of spatially confined electrons. When such kind of valence discontinuity extends to the interface of a well-chosen in-planar 2D heterostructure, a one-dimensional (1D) confined interface can be generated. Herein, through first-principles modeling, we demonstrate that Dirac-cone-like bands emerge at the 1D zigzag interface of a $\mathrm{ZnO}/\mathrm{Mo}{\mathrm{S}}_2$ lateral heterostructure (LHS), creating a highly mobile 1D transport channel with a high Fermi velocity of $4.5\times {10}^5\mathrm{m}/\mathrm{s}$. The metallic state at the 1D heterointerface is attributed to the polar discontinuity that introduces excess charge carriers. Nontrivial polarization (excess carriers appear at boundaries) results in a large built-in electric field within the ZnO and $\mathrm{Mo}{\mathrm{S}}_2$ monolayers. Remarkably, the ultrafast 1D conducting channel is independent of the arrangements of width $(m$ and $n)$ in the ${\left(\mathrm{ZnO}\right)}_m{\left(\mathrm{Mo}{\mathrm{S}}_2\right)}_n$ LHS, which are further illustrated by tight-binding-based orbital and polarization analysis.

We explore the problem of localization in topological and nontopological nearly-flat subbands derived from the lowest Landau level, in the presence of quenched disorder and short-range interactions. We consider two models: a suitably engineered periodic potential and randomly distributed pointlike impurities. We perform numerical exact diagonalization on a torus geometry and use the mean level spacing ratio $\langle r\rangle$ as a diagnostic of ergodicity. For topological subbands, we find there is no ergodicity breaking in both the one- and two-dimensional thermodynamic limits. For nontopological subbands, in contrast, we find evidence of an ergodicity breaking transition at finite disorder strength in the one-dimensional thermodynamic limit. Intriguingly, indications of similar behavior in the two-dimensional thermodynamic limit are found as well. This constitutes a novel, continuum setting for the study of the many-body localization transition in one and two dimensions.