We generalize the hierarchical equations of motion method to study electron transport through a quantum dot or molecule coupled to one-dimensional interacting leads that can be described as Luttinger liquids. Such leads can be realized, for example, by quantum wires or fractional quantum Hall edge states. In comparison to noninteracting metallic leads, Luttinger liquid leads involve many-body correlations and the single-particle tunneling density of states shows a power-law singularity at the chemical potential. Using the generalized hierarchical equations of motion method, we assess the importance of the singularity and the next-to-leading order many-body correlations. To this end, we compare numerically converged results with second- and first-order results of the hybridization expansion that is inherent to our method. As a test case, we study transport through a single-level quantum dot or molecule that can be described by an Anderson impurity model. Cotunneling effects turn out to be most pronounced for attractive interactions in the leads or repulsive ones if an excitonic coupling between the dot and the leads is realized. We also find that an interaction-induced negative differential conductance near the Coulomb blockade thresholds is slightly suppressed as compared to a first-order and/or rate equation result. Moreover, we find that the two-particle ($n$-particle) correlations enter as a second-order ($n$-order) effect and are, thus, not very pronounced at the high temperatures and parameters that we consider.

We study the mass transport dynamics of an adsorbed layer near a discontinuous incommensurate striped-honeycomb phase transition via numerical simulations of a coarse-grained model focusing on the motion of domain walls rather than individual atoms. Following an initial step profile created in the incommensurate striped phase, an intermediate hexagonal incommensurate phase nucleates and grows, leading to a bifurcation into two sharp profiles propagating in opposite directions as opposed to broad profiles induced by atomic diffusive motion. Our results are in agreement with recent numerical simulations of a microscopic model as well as experimental observations for the Pb/Si(111) adsorbate system.

Using the tight-binding model, we investigate the influence of vacancy disorder on electrical transport in graphene Hall bars in the presence of quantizing magnetic fields. Disorder, induced by a random distribution of monovacancies, breaks the graphene sublattice symmetry and creates states localized on the vacancies. These states are observable in the bend resistance, as well as in the total DOS. Their energy is proportional to the square root of the magnetic field, while their localization length is proportional to the cyclotron radius. At the energies of these localized states, the electron current flows around the monovacancies and, as we show, it can follow unexpected paths depending on the particular arrangement of vacancies. We study how these localized states change with the vacancy concentration, and what are the effects of including the next-nearest-neighbor hopping term. Our results are also compared with the situation when double vacancies are present in the system. Double vacancies also induce localized states, but their energy and magnetic field dependencies are different. Their localization energy scales linearly with the magnetic field, and their localization length appears not to depend on the field strength.

The single-impurity Anderson model is studied within the i-DFT framework, a recently proposed extension of density functional theory (DFT) for the description of electron transport in the steady state. i-DFT is designed to give both the steady current and density at the impurity, and it requires the knowledge of the exchange-correlation (xc) bias and on-site potential (gate). In this work we construct an approximation for both quantities which is accurate in a wide range of temperatures, gates, and biases, thus providing a simple and unifying framework to calculate the differential conductance at negligible computational cost in different regimes. Our results mark a substantial advance for DFT and may inform the construction of functionals applicable to other correlated systems.

We utilize nonlinear absorption to design all-optical protocols that improve both charge-state initialization and spin readout for the nitrogen-vacancy (NV) center in diamond. Nonmonotonic variations in the equilibrium charge state as a function of visible and near-infrared optical power are attributed to competing multiphoton absorption processes. In certain regimes, multicolor illumination enhances the steady-state population of the NV's negative charge state above 90%. At higher NIR intensities, selective ionization of the singlet manifold facilitates a protocol for spin-to-charge conversion that dramatically enhances the spin readout fidelity. We demonstrate a sixfold increase in the signal-to-noise ratio for single-shot spin measurements and demonstrate a pathway towards single-shot electron spin readout at room temperature.

We employ equation-of-motion techniques to study the nonequilibrium dynamics in a lattice model of weakly interacting spinless fermions. Our model provides a simple setting for analyzing the effects of weak integrability-breaking perturbations on the time evolution after a quantum quench. We establish the accuracy of the method by comparing results at short and intermediate times to time-dependent density matrix renormalization group computations. For sufficiently weak integrability-breaking interactions we always observe prethermalization plateaus, where local observables relax to nonthermal values at intermediate time scales. At later times a crossover towards thermal behavior sets in. We determine the associated time scale, which depends on the initial state, the band structure of the noninteracting theory, and the strength of the integrability-breaking perturbation. Our method allows us to analyze in some detail the spreading of correlations and in particular the structure of the associated light cones in our model. We find that the interior and exterior of the light cone are separated by an intermediate region, the temporal width of which appears to scale with a universal power law $t^{1/3}$.

The phonovoltaic cell harvests optical phonons like a photovoltaic harvests photons, that is, a nonequilibrium (hot) population of optical phonons (at temperature $T_{p,\mathrm{O}}$) more energetic than the band gap produces electron-hole pairs in a $p\text{−}n$ junction, which separates these pairs to produce power. A phonovoltaic material requires an optical phonon mode more energetic than its band gap and much more energetic than the thermal energy $\left(E_{p,\mathrm{O}}>\mathrm{\Delta }{E}_{e,g}\gg k_{\mathrm{B}}T\right)$, which relaxes by generating electrons and power (at rate ${\dot{\gamma }}_{e\text{−}p}$) rather than acoustic phonons and heat (at rate ${\dot{\gamma }}_{p\text{−}p}$). Graphene (h-C) is the most promising material candidate: when its band gap is tuned to its optical phonon energy without greatly reducing the electron-phonon $\left(e\text{−}p\right)$ coupling, it reaches a substantial figure of merit $[Z_{\mathrm{pV}}=\mathrm{\Delta }{E}_{e,g}{\dot{\gamma }}_{e\text{−}p}/E_{p,\mathrm{O}}({\dot{\gamma }}_{e\text{−}p}+{\dot{\gamma }}_{p\text{−}p})\approx 0.8]$. A simple tight-binding (TB) model presented here predicts that lifting the sublattice symmetry of graphene in order to open a band gap proscribes the $e\text{−}p$ interaction at the band edge, such that ${\dot{\gamma }}_{e\text{−}p}\to 0$ as $\mathrm{\Delta }{E}_{e,g}\to E_{p,\mathrm{O}}$. However, ab initio (DFT-LDA) simulations of layered h-C/BN and substitutional h-C:BN show that the $e\text{−}p$ coupling remains substantial in these asymmetric crystals. Indeed, h-C:BN achieves a high figure of merit $\left(Z_{\mathrm{pV}}\approx 0.6\right)$. At 300 K and for a Carnot limit of $0.5(T_{p,\mathrm{O}}=600\text{K})$, a h-C:BN phonovoltaic can reach an efficiency of ${\eta }_{\mathrm{pV}}\approx 0.2$, double the thermoelectric efficiency $\left(ZT\approx 1\right)$ under similar conditions.

Pressure and shear strain-induced phase transformations (PTs) in a nanograined bicrystal at the evolving dislocations pile-up have been studied utilizing a phase field approach (PFA). The complete system of PFA equations for coupled martensitic PT, dislocation evolution, and mechanics at large strains is presented and solved using the finite element method (FEM). The nucleation pressure for the high-pressure phase (HPP) under hydrostatic conditions near a single dislocation was determined to be 15.9 GPa. Under shear, a dislocation pile-up that appears in the left grain creates strong stress concentration near its tip and significantly increases the local thermodynamic driving force for PT, which causes nucleation of HPP even at zero pressure. At pressures of 1.59 and 5 GPa and shear, a major part of a grain transforms to HPP. When dislocations are considered in the transforming grain as well, they relax stresses and lead to a slightly smaller stationary HPP region than without dislocations. However, they strongly suppress nucleation of HPP and require larger shear. Unexpectedly, the stationary HPP morphology is governed by the simplest thermodynamic equilibrium conditions, which do not contain contributions from plasticity and surface energy. These equilibrium conditions are fulfilled either for the majority of points of phase interfaces or (approximately) in terms of stresses averaged over the HPP region or for the entire grain, despite the strong heterogeneity of stress fields. The major part of the driving force for PT in the stationary state is due to deviatoric stresses rather than pressure. While the least number of dislocations in a pile-up to nucleate HPP linearly decreases with increasing applied pressure, the least corresponding shear strain depends on pressure nonmonotonously. Surprisingly, the ratio of kinetic coefficients for PT and dislocations affect the stationary solution and the nanostructure. Consequently, there are multiple stationary solutions under the same applied load and PT, and deformation processes are path dependent. With an increase in the size of the sample by a factor of two, no effect was found on the average pressure and shear stress and HPP nanostructure, despite the different number of dislocations in a pile-up. The obtained results represent a nanoscale basis for understanding and description of PTs under compression and shear in a rotational diamond anvil cell and high-pressure torsion.

We study a class of periodically driven $d$-dimensional integrable models and show that after $n$ drive cycles with frequency $\omega$, pure states with non-area-law entanglement entropy $S_n\left(l\right)\sim l^{\alpha (n,\omega )}$ are generated, where $l$ is the linear dimension of the subsystem, and $d-1\le \alpha (n,\omega )\le d$. The exponent $\alpha (n,\omega )$ eventually approaches $d$ (volume law) for large enough $l$ when $n\to \infty$. We identify and analyze the crossover phenomenon from an area $(S\sim l^{d-1}$ for $d\ge 1$) to a volume $\left(S\sim l^d\right)$ law and provide a criterion for their occurrence which constitutes a generalization of Hastings's theorem to driven integrable systems in one dimension. We also find that $S_n$ generically decays to $S_{\infty }$ as ${(\omega /n)}^{(d+2)/2}$ for fast and ${(\omega /n)}^{d/2}$ for slow periodic drives; these two dynamical phases are separated by a topological transition in the eigenspectrum of the Floquet Hamiltonian. This dynamical transition manifests itself in the temporal behavior of all local correlation functions and does not require a critical point crossing during the drive. We find that these dynamical phases show a rich re-entrant behavior as a function of $\omega$ for $d=1$ models and also discuss the dynamical transition for $d>1$ models. Finally, we study entanglement properties of the steady state and show that singular features (cusps and kinks in $d=1$) appear in $S_{\infty }$ as a function of $\omega$ whenever there is a crossing of the Floquet bands. We discuss experiments which can test our theory.

We have performed the detailed investigations of the magnetization of the $S=\frac{1}{2}$ triangular-lattice antiferromagnets ${\mathrm{Ba}}_3{\mathrm{CoSb}}_2{\mathrm{O}}_9$ and ${\mathrm{CsCuCl}}_3$ with a ${120}^{\circ }$ spin structure in the $ab$ plane. In ${\mathrm{Ba}}_3{\mathrm{CoSb}}_2{\mathrm{O}}_9$, the magnetic susceptibility ($\chi$) exhibits a broad maximum above the Néel temperature ($T_{\mathrm{N}}$) as is expected in the low-dimensional antiferromagnet (AFM). In ${\mathrm{CsCuCl}}_3, \chi$ exhibits a continuous increase down to $T_{\mathrm{N}}$ as if it is the three-dimensional AFM. This is induced by the strong ferromagnetic (FM) interaction along the $c$ axis. The magnetic phase diagrams are also very different. Although the transition field from the umbrella to the 2-1-coplanar phase ($H_{u-c}$) for $H\parallel c$ is almost independent of temperature in ${\mathrm{Ba}}_3{\mathrm{CoSb}}_2{\mathrm{O}}_9$, it shows a considerable decrease with increasing temperature in ${\mathrm{CsCuCl}}_3$. The temperature independent $H_{u-c}$ in ${\mathrm{Ba}}_3{\mathrm{CoSb}}_2{\mathrm{O}}_9$ originates from the magnetic anisotropy from the van Vleck contribution, which does not depend so much on the temperature. The temperature dependent $H_{u-c}$ in ${\mathrm{CsCuCl}}_3$ originates from the magnetic anisotropy from the Dzyaloshinskii-Moriya (DM) interaction, which decreases with increasing temperature. For $H\parallel ab$, the clear transition from the Y-coplanar to the up-up-down ($uud$) phase was observed in ${\mathrm{Ba}}_3{\mathrm{CoSb}}_2{\mathrm{O}}_9$ but not in ${\mathrm{CsCuCl}}_3$. While the reentrant behavior of $T_{\mathrm{N}}$ originating from the thermal and quantum spin fluctuations is observed in both compounds, it is pronounced in ${\mathrm{Ba}}_3{\mathrm{CoSb}}_2{\mathrm{O}}_9$ but small in ${\mathrm{CsCuCl}}_3$. These differences originate from the existence or nonexistence of the DM interaction. The DM interaction in ${\mathrm{CsCuCl}}_3$ suppresses those fluctuations in the $ab$ plane, leading to the less pronounced reentrant behavior of $T_{\mathrm{N}}$ and the broad crossover in place of the phase transition. We analyzed the anisotropic magnetization of ${\mathrm{Ba}}_3{\mathrm{CoSb}}_2{\mathrm{O}}_9$ in the paramagnetic region by the mean field calculation. The spin-orbit (SO) coupling, the uniaxial crystalline electric field, and the isotropic exchange interaction were taken into account. We could estimate the anisotropy ratio of the exchange interaction $J/J_z=1.19$ and $g_{\perp }/g_{\parallel }=1.13$ with $g_{\perp }=4.45$ and $g_{\parallel }=3.95$ in the ground state. We emphasized that although the isotropic exchange interaction was used, the above anisotropies at low temperatures are induced simultaneously through the SO coupling and the uniaxial crystalline electric field and they are closely associated with each other.

We analyze Cooper pairing instabilities in strongly driven electron-phonon systems. The light-induced nonequilibrium state of phonons results in a simultaneous increase of the superconducting coupling constant and the electron scattering. We demonstrate that the competition between these effects leads to an enhanced superconducting transition temperature in a broad range of parameters. Our results may explain the observed transient enhancement of superconductivity in several classes of materials upon irradiation with high intensity pulses of terahertz light, and may pave new ways for engineering high-temperature light-induced superconducting states.

We propose a scheme which can generate a highly directional single photon with almost 100% efficiency. Therefore we can get a useful single photon when it is required. An initial excited atom is placed inside a special Fabry-Pérot cavity whose walls consist of left-handed and zero-index metamaterials. The left-handed slabs work as a closed shell to avoid dissipation of the emitted photon, while the outer zero-index metamaterial slabs act as a special shutter which is transparent only for normal incidence, so that the photon emitted by the atom can only escape out of the cavity unidirectionally. Furthermore, we design the cavity with currently available metamaterials made of two-dimensional dielectric photonic crystals, and simulate the radiative field of an electric dipole to confirm our prediction. Differently from the previous proposal of single-photon sources which demanded complicated structure design and subtle mode analysis, our scheme is simple and robust for atomic position. This work has promising applications for quantum communication and optical quantum computing.

Lattice vibrations in ${\mathrm{CH}}_3{\mathrm{NH}}_3{\mathrm{PbI}}_3$ are strongly interacting, with double-well instabilities present at the Brillouin zone boundary. Analysis within a first-principles lattice-dynamics framework reveals anharmonic potentials with short phonon quasiparticle lifetimes and mean free paths. The phonon behavior is distinct from the inorganic semiconductors GaAs and CdTe where three-phonon interaction strengths are three orders of magnitude smaller. The implications for the applications of hybrid halide perovskites arising from thermal conductivity, band-gap deformation, and charge-carrier scattering through electron-phonon coupling, are presented.

An aspect of a magnetic vortex whose dynamics strongly affects the magnetic structures of the environment is experimentally shown. We exploit a nanodot of an exchange-coupled bilayer with a soft magnetic $\mathrm{N}{\mathrm{i}}_{81}\mathrm{F}{\mathrm{e}}_{19}$ [permalloy (Py)] having a magnetic vortex and a perpendicularly magnetized $L{1}_0\text{−}\mathrm{FePt}$ exhibiting a large switching field $\left(H_{\mathrm{sw}}\right)$. The vortex dynamics with azimuthal spin waves makes the excess energy accumulate in the Py, which triggers reversed-domain nucleation in $L{1}_0\text{−}\mathrm{FePt}$ at a low magnetic field. Our experimental and numerical results shed light on the essence of reversed-domain nucleation, and provide a route for efficient $H_{\mathrm{sw}}$ reduction.

${\mathrm{Cu}}_2{\mathrm{ZnSnS}}_4$ (CZTS) is a promising material as an absorber in photovoltaic applications. The measured efficiency, however, is far from the theoretically predicted value for the known CZTS phases. To improve the understanding of this discrepancy, we investigate the structural, dynamical, and dielectric properties of the three main phases of CZTS (kesterite, stannite, and PMCA) using density functional perturbation theory (DFPT). The effect of the exchange-correlation functional on the computed properties is analyzed. A qualitative agreement of the theoretical Raman spectrum with measurements is observed. However, none of the phases correspond to the experimental spectrum within the error bar that is usually to be expected for DFPT. This corroborates the need to consider cation disorder and other lattice defects extensively in this material.

We report extraordinarily large magnetic hysteresis loops in the iridates ${\mathrm{Sr}}_3{\mathrm{NiIrO}}_5$ and ${\mathrm{Sr}}_3{\mathrm{CoIrO}}_6$. We find coercive magnetic fields of up to 55 T with switched magnetic moments $\approx 1{\mu }_{\mathrm{B}}$ per formula unit in ${\mathrm{Sr}}_3{\mathrm{NiIrO}}_6$ and coercive fields of up to 52 T with switched moments $\approx 3{\mu }_{\mathrm{B}}$ per formula unit in ${\mathrm{Sr}}_3{\mathrm{CoIrO}}_6$. We propose that the magnetic hysteresis involves the field-induced evolution of quasi-one-dimensional chains in a frustrated triangular configuration. The striking magnetic behavior is likely to be linked to the unusual spin-orbit-entangled local state of the ${\mathrm{Ir}}^{4+}$ ion and its potential for anisotropic exchange interactions.

Bogoliubov transformations have been successfully applied in several condensed-matter contexts, e.g., in the theory of superconductors, superfluids, and antiferromagnets. These applications are based on bulk models where translation symmetry can be assumed, so that few degrees of freedom in Fourier space can be “diagonalized” separately, and in this way it is easy to find the approximate ground state and its excitations. As translation symmetry cannot be invoked when it comes to nanoscopic systems, the corresponding multidimensional Bogoliubov transformations are more complicated. For bosonic systems it is much simpler to proceed using phase-space variables, i.e., coordinates and momenta. Interactions can be accounted for by the self-consistent harmonic approximation, which is naturally developed using phase-space Weyl symbols. The spin-flop transition in a short antiferromagnetic chain is illustrated as an example. This approach, rarely used in the past, is expected to be generally useful to estimate quantum effects, e.g., on phase diagrams of ordered vs disordered phases.

We experimentally demonstrate magnon Kerr effect in a cavity-magnon system where magnons in a small yttrium iron garnet (YIG) sphere are strongly but dispersively coupled to the photons in a three-dimensional cavity. When the YIG sphere is pumped to generate considerable magnons, the Kerr effect yields a perceptible shift of the cavity's central frequency and more appreciable shifts of the magnon modes. We derive an analytical relation between the magnon frequency shift and the drive power for the uniformly magnetized YIG sphere and find that it agrees very well with the experimental results of the Kittel mode. Our study paves the way to explore nonlinear effects in the cavity-magnon system.

In this study, current pumping by an external potential is studied on the basis of the Keldysh Green's function method, and a pumping formula written in terms of retarded and advanced Green's functions is obtained. It is shown that pumping is essentially driven by a change of particle distribution before and after an external perturbation. The formula is used to study the spin pumping effect in the case of strong $s-d$ exchange interaction, and the driving field is identified to be the spin gauge field. At the lowest order in the precession frequency of magnetization, the spin gauge field works as a constant potential, and the system is shown to reduce to a static problem of spin current generation by a time-independent potential with off-diagonal spin components.