We study energy transport in disordered $XXZ$ spin-1/2 chains driven to nonequilibrium configurations by thermal reservoirs of different temperatures at the boundaries, using large-scale matrix product simulations. In particular we discuss the transition between diffusive and subdiffusive transport in sectors of zero and finite magnetization at high temperature. At large anisotropies we find that diffusive energy transport prevails over a large range of disorder strengths, which is in contrast to spin transport that is subdiffusive in the same regime for weak disorder. However, at finite magnetization both energy and spin currents decay as a function of the system size with the same exponent. We conclude that diffusion of energy is much more pervasive than that of magnetization in these disordered spin-1/2 systems, and occurs across a significant range of the interaction-disorder parameter phase space. We support the existence of this asymmetry, reminiscent of that in the clean limit, by an analytical estimation of diffusion constants for weak disorder.

We present an experimental demonstration as well as a theoretical model of an integrated circuit designed for the manipulation of a microwave field down to the single-photon level. The device is made of a superconducting resonator coupled to a transmission line via a second frequency-tunable resonator. The tunable resonator can be used as a tunable coupler between the fixed resonator and the transmission line. Moreover, the manipulation of the microwave field between the two resonators is possible. In particular, we demonstrate the swapping of the field from one resonator to the other by pulsing the frequency detuning between the two resonators. The behavior of the system, which determines how the device can be operated, is analyzed as a function of one key parameter of the system, the damping ratio of the coupled resonators. We show a good agreement between experiments and simulations, realized by solving a set of coupled differential equations.

We identify a new “order parameter” for the disorder-driven many-body localization transition by leveraging machine learning. Contrary to previous studies, our method is almost entirely unsupervised. A game theoretic process between neural networks defines an adversarial setup with conflicting objectives to identify what characteristic features to base efficient predictions on. This reduces the numerical effort for mapping out the phase diagram by a factor of $100\times$ and allows us to pin down the transition, as the point at which the physics changes qualitatively, in an objective and cleaner way than is possible with the existing diverse array of quantities. Our approach of automated discovery is applicable specifically to poorly understood phase transitions and is a starting point for a research program leveraging the potential of machine learning–assisted research in physics.

In the first-principles bulk-layer model the superlattice structure and polarization are determined by first-principles computation of the bulk responses of the constituents to the electrical and mechanical boundary conditions in an insulating superlattice. In this work the model is extended to predict functional properties, specifically dielectric permittivity and piezoelectric response. A detailed comparison between the bulk-layer model and full first-principles calculations for three sets of perovskite oxide superlattices, ${\mathrm{PbTiO}}_3/{\mathrm{BaTiO}}_3$, ${\mathrm{BaTiO}}_3/{\mathrm{SrTiO}}_3$, and ${\mathrm{PbTiO}}_3/{\mathrm{SrTiO}}_3$, is presented. The bulk-layer model is shown to give an excellent first approximation to these important functional properties and to allow for the identification and investigation of additional physics, including interface reconstruction and finite-size effects. Technical issues in the generation of the necessary data for constituent compounds are addressed. These results form the foundation for a powerful data-driven method to facilitate discovery and design of superlattice systems with enhanced and tunable polarization, dielectric permittivity, and piezoelectric response.

Level statistics of systems that undergo many-body localization transition are studied. An analysis of the gap ratio statistics from the perspective of inter- and intrasample randomness allows us to pin point differences between transitions in random and quasirandom disorder, showing the effects due to Griffiths rare events for the former case. It is argued that the transition in the case of random disorder exhibits universal features that are identified by constructing an appropriate model of intermediate spectral statistics which is a generalization of the family of short-range plasma models. The considered weighted short-range plasma model yields a very good agreement both for level spacing distribution including its exponential tail and the number variance up to tens of level spacings outperforming previously proposed models. In particular, our model grasps the critical level statistics which arise at disorder strength for which the intersample fluctuations are the strongest. Going beyond the paradigmatic examples of many-body localization in spin systems, we show that the considered model also grasps the level statistics of disordered Bose- and Fermi-Hubbard models. The remaining deviations for long-range spectral correlations are discussed and attributed mainly to the intricacies of level unfolding.

We consider a magnetic insulator in contact with a normal metal. We derive a self-consistent Keldysh effective action for the magnon gas that contains the effects of magnon-magnon interactions and contact with the metal to lowest order. Self-consistent expressions for the dispersion relation, temperature, and chemical potential for magnons are derived. Based on this effective action, we study instabilities of the magnon gas that arise due to spin current flowing across the interface between the normal metal and the magnetic insulator. We find that the stability phase diagram is modified by an interference between magnon-magnon interactions and interfacial magnon-electron coupling. These effects persist at low temperatures and for thin magnetic insulators.

We represent an approach to calculate micromagnetic model parameters such as the tensor of exchange stiffness, Dzyaloshinskii-Moriya interaction (DMI), as well as spin-wave stiffness. The scheme is based on the fully relativistic Korringa-Kohn-Rostoker Green function (KKR-GF) technique and can be seen as a relativistic extension of the work of Lichtenstein et al. [J. Magn. Magn. Mater. 67, 65 (1987)]. The expression for $D^{z\alpha }$ elements of DMI differ from the expressions for $D^{x\alpha }$ and $D^{y\alpha }$ elements as the former are derived via second-order perturbation term of the energy caused by spin-spiral while the latter are associated with the first-order term. Corresponding numerical results are compared with those obtained using other schemes reported in the literature.

We observed quantum oscillations in thermoelectric and magnetic torque signals in noncentrosymmetric superconductor ${\mathrm{PbTaSe}}_2$. One oscillatory frequency stems from the orbits formed by magnetic breakdown, while others are from two-dimensional-like Fermi surfaces near the topological nodal rings. Our comprehensive understanding of the Fermi surface topology of ${\mathrm{PbTaSe}}_2$, including nailing down the Fermi level and detecting the Berry phases near the nodal rings, is crucial for searching plausible topological superconductivity in its bulk and surface states.

Cubic antiperovskite ${\mathrm{Ca}}_3\mathrm{PbO}$ is a candidate for three-dimensional (3D) Dirac fermion systems that have recently emerged as a class of topological materials exhibiting linear energy dispersion in the bulk. We report magnetotransport and tunnel diode oscillation (TDO) measurements on Bi-doped ${\mathrm{Ca}}_3\mathrm{PbO}$ in magnetic fields up to 55 T and temperatures between 2 and 78 K. By observing the Shubnikov–de Haas (SdH) oscillations, we resolve the bulk 3D Fermi surface (FS) with distinctive features of Dirac fermions including linear magnetoresistance, light effective mass, and a nontrivial phase shift. TDO measurements under high fields reveal the existence of two primary SdH frequencies, one being twice the other. Together with the low effective mass, the oscillations of these two frequencies account for the emergence of the Landau level splitting that persists up to 43 K. The field-angular-dependence of the oscillation frequencies with three branches confirms that the FS is composed of three pairs of hole pockets with uniaxial anisotropy on the Γ-X path, as predicted for bulk ${\mathrm{Ca}}_3\mathrm{PbO}$ crystal by density functional theory calculation.

In analogy with equilibrium phase transitions, we address the problem of the instability to symmetry-breaking perturbations of systems undergoing a laser transition. The symmetry in question is the $\text{U}\left(1\right)$ invariance with respect to a phase factor, and the perturbation is a coherent field $E$, coupled to the exciton. At the rate-equation level we analyze first the case of a cavity containing a single, two-level emitter, and then a chain of such cavities interacting by photon-hopping processes. In both cases, spontaneous symmetry breaking takes place when the system is in the lasing phase. For the laser transition, the analog of the thermodynamic limit is the scaling limit of vanishing cavity loss and light-matter coupling, $\kappa \to 0$, $g\to 0$, so that $g^2/\kappa$ remains finite. We show that in the lasing regime, anomalous averages persist in the $E\to 0$ limit, provided that the scaling limit is performed first. Lasing diagnosis based on robust anomalous averages is compared numerically with the familiar coherence criterion $g^{\left(2\right)}\left(0\right)=1$, and the advantages of the former are discussed.

A new type of quantum dot (QD) structure that enables us to control a spatial localization of electrons by changing the strain distribution in the vicinity of QDs is proposed. The structures represent a combination of large ($\sim 200\mathrm{nm}$) GeSi disklike quantum dots (nanodisks) and groups of smaller ($\sim 30\mathrm{nm}$) laterally ordered QDs grown over a nanodisk template. Electron localization has been studied by the electron spin resonance (ESR) method. Analysis of experimental results has been supported by calculations of electron binding energies and carrier distribution probabilities taking into account strain effects and real geometry of nano-objects. Results show that the strain field produced by the nanodisk can be used for tuning the energy levels of electrons in different $\mathrm{\Delta }$ valleys and makes possible successful realization of simultaneous localization of two electrons with different $g$ factors in the vicinity of the same QD.

Graphene-based hyperbolic metamaterials have been predicted to transport evanescent fields with extraordinarily large vacuum wave vectors. It is particularly at much higher wave vector values that the commonly employed descriptional models involving structure homogenization and assumptions of an approximatively local graphene conductivity start breaking down. Here, we combine a nonlocal quantum conductivity model of graphene with an exact mathematical treatment of the periodic structure to develop a toolset for determining the hyperbolic behavior of these graphene-based hyperbolic metamaterials. The quantum conductivity model of graphene facilitates us to predict the plasmonic amplification in graphene sheets of the considered structures. This allows us to reverse the problem of Ohmic and temperature losses, making this simple yet powerful arrangement practically applicable. We analyze the electric field distribution inside the finite structures, concluding that Bloch boundary solutions can be used to predict their behavior. With the transfer-matrix method, we show that at finite temperature and collision loss, we can compensate for losses, restoring imaging qualities of the finite structure via an introduction of chemical imbalance.

We study the semiclassical theory of wave packet dynamics in crystalline solids extended to include the effects of a nonuniform electric field. In particular, we derive a correction to the semiclassical equations of motion (EOMs) for the dynamics of the wave packet center that depends on the gradient of the electric field and on the quantum metric (also called the Fubini-Study, Bures, or Bloch metric) on the Brillouin zone. We show that the physical origin of this term is a contribution to the total energy of the wave packet that depends on its electric quadrupole moment and on the electric-field gradient. We also derive an equation relating the electric quadrupole moment of a sharply peaked wave packet to the quantum metric evaluated at the wave packet center in reciprocal space. Finally, we explore the physical consequences of this correction to the semiclassical EOMs. We show that in a metal with broken time-reversal and inversion symmetry, an electric-field gradient can generate a longitudinal current which is linear in the electric-field gradient, and which depends on the quantum metric at the Fermi surface. We then give two examples of concrete lattice models in which this effect occurs. Our results show that nonuniform electric fields can be used to probe the quantum geometry of the electronic bands in metals and open the door to further studies of the effects of nonuniform electric fields in solids.

Understanding charge transport properties of a molecular junction plays a crucial role in the possible application of such devices. In this work we present a theoretical study on the transport properties of molecular transistors in the sequential tunneling regime. Special attention is paid to the origins of the asymmetries in the charge transport properties as presented by the commonly measured conductance stability diagrams (two-dimensional plots of the differential conductance against gate and bias voltages). It is found that both the external factors, including the bias coupling constant and the coupling strength between the molecule and the electrodes, and the intrinsic molecular properties such as the frequency differences between different charging states, the anharmonicity of the potential energy surface, and the mode-mixing effect can lead to asymmetric conductance stability diagrams. Especially, we show that the uneven bias coupling between the molecule and the two electrodes can result in strong current rectifications. These results successfully reproduce the change in rectification directions of molecular diodes as observed in a recent experiment [L. Yuan et al., Nat. Commun. 6, 6324 (2015)]. The differences in the asymmetric stability diagram caused by the studied factors are also discussed, which helps us to correctly interpret the experimental results.

Element doping and pressure compression may change material properties for improved performance in applications. We report pressure-induced metallization in the semiconductor $\mathrm{C}{\mathrm{d}}_{0.90}\mathrm{Z}{\mathrm{n}}_{0.1}\mathrm{Te}$. Transport measurements showed an overall resistivity drop of 11 orders of magnitude under compression up to 12 GPa, which is indicative of a metallization transition. X-ray diffraction measurements revealed that the sample underwent a structural transition from a cubic-$F4\overline{3}m$ phase (zinc blende) to a cubic-$Fm\overline{3}m$ phase (rock salt) at about 5.5 GPa, followed by another transition to an orthorhombic $Cmcm$ structure at 13 GPa. A huge volume collapse of about 18% was observed during the first phase transition, suggesting a first-order phase transition. The disappearance or weakening of Raman modes, temperature-dependent resistivity, and $abinitio$ calculation results depict the metallic nature of both the rock-salt and $Cmcm$ phases. The band structure changes and increased carrier density (especially at the first structural transition) are likely a consequence of the structural transition.

We investigated the ultrafast Dirac-electron dynamics on the surface of a magnetically doped topological-insulator (TI) system, ${\mathrm{V}}_{0.04}{\left({\mathrm{Bi}}_x{\mathrm{Sb}}_{1-x}\right)}_2{\mathrm{Te}}_3$. Even when the Fermi level is located around the Dirac point of the topological surface states (TSSs), the recovery was accomplished shortly, within 3 ps; namely, a bottleneck-type slowing of the recovery was not observed. Besides, the recovery was independent of energy. The unique shortness and independence can be ascribed to the $s-d$ interaction between the TSSs and the impurity states of V formed near the Fermi level. Magnetic doping thus provides a new route to control the nonequilibrium dynamics on a TI surface.

We used single-crystal neutron diffraction to investigate the magnetic structures of the multiferroic ${\mathrm{GdMn}}_2{\mathrm{O}}_5$. The system undergoes a first-order incommensurate to commensurate magnetic transition below 33 K, accompanied by the appearance of electric polarization $P$. Upon cooling, $P$ increases smoothly while the magnetic order shows an abrupt enhancement in intensity below 20 K owing to the large increase of the rare-earth Gd moment. The contrasting temperature evolution of the magnetic order and polarization indicates the polarization is mainly driven by the exchange striction between the magnetic Mn ions. The incommensurate phase in the intermediate temperature range has a cycloidal modulation along the $c$ axis, with spin configuration projected in the $ab$ plane similar to the commensurate phase. The lack of observable bulk polarization suggests the exchange striction is not the dominant mechanism for the polarization in the incommensurate phase.

Noncollinear triangular antiferromagnets with a coplanar spin arrangement and vanishing net magnetic moment can exhibit a large anomalous Hall effect owing to their nonvanishing momentum space Berry curvature. Here we show the existence of a large field-induced topological Hall effect in the noncoplanar triangular antiferromagnetic geometry of ${\mathrm{Mn}}_3\mathrm{Sn}$. A detailed magnetic and Hall effect measurements demonstrate the presence of three distinct Hall contributions: a high-temperature anomalous Hall effect, a low-temperature topological Hall effect, and an intermediate-temperature region with coexistence of both the effects. The origin of the observed topological Hall effect is attributed to the scalar spin chirality induced real-space Berry curvature that appears when the system goes from a trivial noncoplanar triangular spin alignment to a topologically protected nontrivial spin texture like skyrmions by application of magnetic field at low temperatures.