Polaron tunneling is a prominent example of a problem characterized by different energy scales, for which the standard quantum Monte Carlo methods face a slowdown problem. We propose a quantum-tunneling Monte Carlo (QTMC) method which is free from this issue and can be used for a wide range of tunneling phenomena. We apply it to study an impurity interacting with a one-dimensional Bose-Einstein condensate and simultaneously trapped in an external double-well potential. Our scheme works for an arbitrary coupling between the particle and condensate and, at the same time, allows for an account of tunneling effects. We discover two distinct quasiparticle peaks associated, respectively, with the phonon-assisted tunneling and the self-trapping of the impurity, which are in a crossover regime for the system modeled. We observe and analyze changes in the weights and spectral positions of the peaks (or, equally, effective masses of the quasiparticles) when the coupling strength is increased. Possible experimental realizations using cold atoms are discussed.

Very recent experiments have reported the tunneling between Yu-Shiba-Rusinov (YSR) bound states at the atomic scale. These experiments have been realized with the help of a scanning tunneling microscope where a superconducting tip is functionalized with a magnetic impurity and is used to probe another magnetic impurity deposited on a superconducting substrate. In this way it has become possible to study for the first time the spin-dependent transport between individual superconducting bound states. Motivated by these experiments, we present here a comprehensive theoretical study of the tunneling processes between YSR bound states in a system in which two magnetic impurities are coupled to superconducting leads. Our theory is based on a combination of an Anderson model with broken spin degeneracy to describe the impurities and nonequilibrium Green's function techniques to compute the current-voltage characteristics. This combination allows us to describe the spin-dependent transport for an arbitrary strength of the tunnel coupling between the impurities. We first focus on the tunnel regime and show that our theory naturally explains the experimental observations of the appearance of current peaks in the subgap region due to both the direct and thermal tunneling between the YSR states in both impurities. Then, we study in detail the case of junctions with increasing transparency, which has not been experimentally explored yet, and predict the occurrence of a large variety of (multiple) Andreev reflections mediated by YSR states that give rise to a very rich structure in the subgap current. In particular, we predict the occurrence of multiple Andreev reflections that involve YSR states in different impurities. These processes have no analog in single-impurity junctions, and they are manifested as current peaks with negative differential conductance for subgap voltages. Overall, our work illustrates the unique physics that emerges when the spin degree of freedom is added to a system with superconducting bound states.

Much effort has been devoted to the electronic properties of relatively thick $\mathrm{Zr}{\mathrm{Te}}_5$ crystals, focusing on their three-dimensional topological effects. Thin $\mathrm{Zr}{\mathrm{Te}}_5$ crystals, on the other hand, were much less explored experimentally. Here we present detailed magnetotransport studies of few-layer $\mathrm{Zr}{\mathrm{Te}}_5$ devices, in which electron-electron interactions and weak antilocalization are observed. The coexistence of the two effects manifests itself in corroborating evidence presented in the temperature and magnetic field dependence of the resistance. Notably, the temperature-dependent phase coherence length extracted from weak antilocalization agrees with strong electron-electron scattering in the sample. Meanwhile, universal conductance fluctuations have temperature and gate voltage dependence that is similar to that of the phase coherence length. Last, all the transport properties in thin $\mathrm{Zr}{\mathrm{Te}}_5$ crystals show strong two-dimensional characteristics. Our results provide insight into the highly intricate properties of topological material $\mathrm{Zr}{\mathrm{Te}}_5$.

In some $d$-electron oxides, the measured effective mass $m_{\mathrm{exptl}}^{*}$ has long been known to be significantly larger than the model effective mass $m_{\mathrm{model}}^{*}$ deduced from mean-field band theory, i.e., $m_{\mathrm{exptl}}^{*}=\beta m_{\mathrm{model}}^{*}$, where $\beta >1$ is the “mass-enhancement” or “mass-renormalization” factor. Previous applications of density functional theory (DFT), based on a symmetry-restricted structure with the smallest number of possible magnetic, orbital, and structural degrees of freedom, missed such mass enhancement. This fact has been taken as evidence of strong electronic correlation, often described via the symmetry-restricted dynamic mean-field approach of the many-body theory, being the exclusive enabling physics. This paper uses instead a static density functional approach that does not restrict positional or spin degrees of freedom (symmetry-broken structures). This approach analyzes the contributions of different symmetry-broken modalities to mass enhancement for a few nominally highly correlated $d$-electron perovskites as well as the nominally uncorrelated, closed-shell $s\text{−}p$ bonding perovskites. It shows that the energy-lowering symmetry-broken spin effects (e.g., nonzero local moment in the paramagnetic phase) and structural effects (e.g., atomic displacement) as described in mean-field DFT already manifest mass enhancement for both electrons and holes in a range of $d$-electron perovskites $\mathrm{SrV}{\mathrm{O}}_3, \mathrm{SrTi}{\mathrm{O}}_3, \mathrm{BaTi}{\mathrm{O}}_3$, and $\mathrm{LaMn}{\mathrm{O}}_3$, as well as $p$-electron perovskites $\mathrm{CsPb}{\mathrm{I}}_3$ and $\mathrm{SrBi}{\mathrm{O}}_3$, including both metals ($\mathrm{SrV}{\mathrm{O}}_3$) and insulators (the rest). This is revealed only when enlarged unit cells of the same parent global symmetry, which are large enough to allow for symmetry-breaking distortions and concomitant variations in spin order, are explored for their ability to lower the total energy. Positional symmetry breaking that leads to mass enhancement includes octahedral rotation in halide perovskites such as $\mathrm{CsPb}{\mathrm{I}}_3$, Jahn-Teller-like $Q_2^{+}$ distortion in $\mathrm{LaMn}{\mathrm{O}}_3$, and bond disproportionation in $\mathrm{SrBi}{\mathrm{O}}_3$, while magnetic symmetry breaking resulting in mass enhancement includes the formation of a distribution of local moments in $\mathrm{SrV}{\mathrm{O}}_3$ that averages to zero in the paramagnetic phase. Not all symmetry breaking leads to significant mass enhancement, e.g., the rather small octahedral rotations in the nearly perfectly cubic $\mathrm{SrTi}{\mathrm{O}}_3$ cause negligible mass enhancement, as do the paraelectric displacements in $\mathrm{BaTi}{\mathrm{O}}_3$. In principle, by ergodicity, the two descriptions, i.e., the symmetry-restricted dynamic approach with a single time-fluctuating magnetic moment and the symmetry-broken mean-field approach with a static distribution of spatially fluctuated local moments, are not mutually exclusive but are a choice of representation and consequently, a choice of computational efficiency. In approximate implementations, the symmetry-broken mean-field approach appears to remove much of what was strong correlation in dynamically correlated symmetry-restricted solutions, leaving smaller (“weak”) residual correlation with respect to the exact solution.

A nonzero-temperature technique for strongly correlated electron lattice systems, combining elements of both variational wave function (VWF) approach and expansion in the inverse number of fermionic flavors ($1/{\mathcal{N}}_f$), is developed. The departure point, VWF method, goes beyond the renormalized mean-field theory and provides semiquantitative description of principal equilibrium properties of high-$T_c$ superconducting cuprates. The developed here scheme of $\text{VWF}+1/{\mathcal{N}}_f$, in the leading order provides dynamical spin and charge responses around the VWF solution, generalizing the weak-coupling spin-fluctuation theory to the regime of strong correlations. Thermodynamic corrections to the correlated saddle-point state arise systematically at consecutive orders. Explicitly, $\text{VWF}+1/{\mathcal{N}}_f$ is applied to evaluate dynamical response functions for the hole-doped Hubbard model and compared with available determinant quantum Monte Carlo data, yielding a good overall agreement in the regime of coherent collective-mode dynamics. The emergence of well-defined spin and charge excitations from the incoherent continua is explicitly demonstrated and a nonmonotonic dependence of the charge-excitation energy on the interaction magnitude is found. The charge mode energy saturates slowly when approaching the strong-coupling limit, which calls for a reevaluation of the $t\text{−}J$-model approach to the charge dynamics in favor of more general $t\text{−}J\text{−}U$ and $t\text{−}J\text{−}U\text{−}V$ models. The results are also related to recent inelastic resonant x-ray and neutron-scattering experiments for the high-$T_c$ cuprates.

Controlling the spin dynamics and spin lifetimes is one of the main challenges in spintronics. To this end, the study of the spin diffusion in two-dimensional electron gases (2DEGs) shows that when the Rashba and Dresselhaus spin-orbit couplings (SOC) are balanced, a persistent spin helix regime arises. There, a striped spin pattern shows a long lifetime, limited only by the cubic Dresselhaus SOC, and its dynamics can be controlled by in-plane drift fields. Here, we derive a spin-diffusion equation for nondegenerate two-subband 2DEGs. We show that the intersubband scattering rate, which is defined by the overlap of the subband densities, enters as a new nob to control the spin dynamics, and can be controlled by electric fields, being maximum for symmetric quantum wells. We find that for large intersubband couplings the dynamics follows an effective diffusion matrix given by approximately half of the subband-averaged matrices. This extra $\frac{1}{2}$ factor arises from Matthiessen's rule summing over the intrasubband and intersubband scattering rates, and leads to a reduced diffusion constant and larger spin lifetimes. We illustrate our findings with numerical solutions of the diffusion equation with parameters extracted from realistic Schrödinger-Poisson calculations.

Materials with ring-shaped electronic bands are promising thermoelectric (TE) candidates since their unusual dispersion shape is predicted to give large power factors. In this paper, we use density functional theory to investigate single and double quintuple-layer ${\mathrm{Bi}}_2{\mathrm{Te}}_3,{\mathrm{Bi}}_2{\mathrm{Se}}_3$, and ${\mathrm{Sb}}_2{\mathrm{Te}}_3$ and to compare the TE properties using three scattering approximations: constant relaxation time, constant mean free path, and scattering rates proportional to the density of states (the so-called DOS model). Focus is placed on elucidating how these particular dispersion shapes influence the TE characteristics and on understanding how each scattering model impacts TE transport. The single quintuple-layer materials possess two ring-shaped valence-band maxima that provide an abrupt increase in conducting channels, which benefits the power factor. Below the band edge a ring-shaped minimum is found to further enhance TE performance within the DOS model due to a sharp drop in the DOS and, thus, scattering. An analytic “octic” dispersion model, designed to capture the observed characteristics of the band structure, is introduced and shown to qualitatively reproduce the first-principles results. The double quintuple-layer materials display notably worse TE properties since their dispersions are significantly modified compared to the single quintuple-layer case and lose much of the ring-shaped character. Our analysis shows that the benefits of ring-shaped bands are sensitive to the alignment of the two ring maxima and to the degree of ring anisotropy. Moreover, the predicted TE performance can vary significantly depending on the choice of scattering approximation, which could benefit from further study to assess the accuracy of these simple scattering models.

We demonstrate a large magnetoresistance (MR) in a $\mathrm{Co}/\mathrm{Mo}{\mathrm{S}}_2/\mathrm{graphene}/\mathrm{Mo}{\mathrm{S}}_2/\mathrm{Co}$ magnetic tunnel junction by means of ab initio transport calculations. A Co electrode turns out to be an excellent spin injector for a $\mathrm{Mo}{\mathrm{S}}_2/\mathrm{graphene}/\mathrm{Mo}{\mathrm{S}}_2$ barrier. The transmission spectrum, current-voltage characteristics, spin injection efficiency, and magnetoresistance are calculated for the modeled device at various bias voltages in the parallel and antiparallel magnetic configurations. A remarkable change in the transmission spectrum and a subsequent change in total current through the junction have been observed, when the relative magnetic orientations of the electrodes are altered. The huge change in current due to the change in the relative magnetic orientation of the Co electrodes produces a high magnetoresistance up to $1270\%$. The obtained values of the device parameters clearly indicate that a $\mathrm{Mo}{\mathrm{S}}_2/\mathrm{graphene}/\mathrm{Mo}{\mathrm{S}}_2$ heterostructure would be an excellent compound for highly efficient spin-valve device applications.

The associations between emergent physical phenomena (e.g., superconductivity) and orbital, charge, and spin degrees of freedom of $3d$ electrons are intriguing in transition metal compounds. Here, we successfully manipulate the superconductivity of spinel oxide ${\mathrm{Li}}_{1\pm x}{\mathrm{Ti}}_2{\mathrm{O}}_{4-\delta }$ (LTO) by ionic liquid gating. A dome-shaped superconducting phase diagram is established, where two insulating phases are disclosed both in heavily electron-doping and hole-doping regions. The superconductor-insulator transition (SIT) in the hole-doping region can be attributed to the loss of Ti valence electrons. In the electron-doping region, LTO exhibits an unexpected SIT instead of a metallic behavior despite an increase in carrier density. Furthermore, a thermal hysteresis is observed in the normal state resistance curve, suggesting a first-order phase transition. We speculate that the SIT and the thermal hysteresis stem from the enhanced $3d$ electron correlations and the formation of orbital ordering by comparing the transport and structural results of LTO with the other spinel oxide superconductor ${\mathrm{MgTi}}_2{\mathrm{O}}_4$ (MTO), as well as analyzing the electronic structure by first-principles calculations. Further comprehension of the detailed interplay between superconductivity and orbital ordering would contribute to the revealing of unconventional superconducting pairing mechanism.

We analyze the valley selection rules for optical transitions from impurity states to the conduction band in two-dimensional Dirac materials, taking a monolayer of ${\mathrm{MoS}}_2$ as an example. We employ the analytical model of a shallow impurity potential which localizes electrons described by a spinor wave function, and, first, find the system eigenstates taking into account the presence of two valleys in the Brillouin zone. Then, we find the spectrum of the absorbance and calculate the photon-drag electric current due to the impurity-band transitions, drawing the general conclusions regarding the valley optical selection rules for the impurity-band optical transitions in gapped Dirac materials.

The ab initio simulation of charged interfaces in the framework of density functional theory (DFT) is heavily employed for the study of electrochemical energy conversion processes. The capacitance is the primary descriptor for the response of the electrochemical interface. It is essentially equal to the inverse of the energy curvature as a function of electron number, and as such there appears a conflict with the fundamental principle of piecewise linearity in DFT that requires the energy curvature to be zero at fractional electron numbers, i.e., almost everywhere. To resolve this conflict, we derive an exact expression between the energy curvature and the Kohn-Sham density of states, the local density of states, and the Fukui potential. We find that the piecewise linearity requirement does not hold for the volume- or area-specific energy of extended systems and surfaces. Applied to the joint density functional theory of an electrode-electrolyte interface, including the ionic and dielectric response of the electrolyte, the same expression represents a rigorous basis for the partitioning of the total interfacial capacitance into contributions of the quantum capacitance, space-charge capacitance, and electrochemical double-layer capacitance. It provides insight into the influence of the electrode material, thickness, and temperature on the charging characteristics, as demonstrated by results for a bulk gold electrode, a single-layer gold electrode, and a single-layer graphene electrode.

We show here that a coherent perfect absorber and laser (CPAL) enabled by parity-time ($\mathcal{PT}$)-symmetry breaking may be exploited to build monochromatic amplifying devices for flexural waves. The fourth-order partial differential equation governing the propagation of flexural waves in thin-elastic piezoelectric plates leads to $4\times 4$ transfer matrices, and captures the interplay between propagating and evanescent waves that is translated into $\mathcal{PT}$-symmetry properties specific to elastic plate systems. We thus demonstrate the possibility of using CPAL for such systems and we argue the possibility of using this concept to detect extremely small-scale vibration perturbation with important outcomes in surface science (imaging of nanometer vibration) and geophysics (improving seismic sensors like velocimeters). The device can also generate finite signals using very low exciting intensities and/or alternatively can be used as a perfect absorber for flexural energy, by tailoring the left and right incident wave amplitude and phase, with evident energy harvesting applications.

Compared to isolated ${\mathrm{C}}_{60}^{3-}$ ions, characterized by a three-dimensional equipotential trough at the bottom of the lowest adiabatic potential energy surface (APES), the Jahn-Teller (JT) effect in cubic fullerides is additionally influenced by the interaction of JT distortions at ${\mathrm{C}}_{60}$ sites with vibrational modes of the lattice. This leads to modification of JT stabilization energy and to the warping of the trough at each fullerene site, as well as to the interaction of JT distortions at different sites. Here we investigate these effects in three fcc fullerides with $A=\mathrm{K}$, Rb, Cs and in ${\mathrm{Cs}}_3{\mathrm{C}}_{60}$ with bcc (A15) structure. DFT calculations of orbital vibronic coupling constants at ${\mathrm{C}}_{60}$ sites and of phonon spectra have been done for fully ordered lattices (1 ${\mathrm{C}}_{60}/\mathrm{u}.\mathrm{c}.$). Based on them the elastic response function for local JT distortions has been evaluated and the lowest APES investigated. To this end an expression for the latter as a function of trough coordinates of all sites has been derived. The results show that the JT stabilization energy slightly increases compared to an isolated ${\mathrm{C}}_{60}^{3-}$ and a warping of the trough of a few meV occurs. The interaction of JT distortions on nearest- and next-nearest-neighbor fullerene sites is of similar order of magnitude. These effects arise first of all due to the interaction of ${\mathrm{C}}_{60}$ sites with the displacements of neighbor alkali atoms and are more pronounced in fcc fullerides than in the A15 compound. The results of this study support the picture of weakly hindered independent rotations of JT deformations at ${\mathrm{C}}_{60}$ sites in cubic $A_3{\mathrm{C}}_{60}$.

Vanadium is reported to undergo a pressure-induced bcc-rhombohedral phase transition at 30–70 GPa, with a transition pressure that is sensitive to the hydrostaticity of the sample environment. However, the experimental evidence for the structure of the high-pressure phase being rhombohedral is surprisingly weak. We have restudied vanadium under pressure to 154 GPa using both polycrystalline and single-crystal samples, and a variety of different pressure transmitting media (PTM). We find that only when using single-crystal samples does one observe a rhombohedral high-pressure phase; the high-pressure diffraction profiles from the polycrystalline samples do not fit a rhombohedral lattice, irrespective of the PTM used. The single-crystal samples reveal two rhombohedral phases, with a continuous transition between them, and distortions from cubic symmetry are much smaller than previously calculated.

Atomistic simulations of $\left({\mathrm{Na}}_2\mathrm{O}{)}_x\right({\mathrm{SiO}}_2{)}_{1-x}$ glasses are carried out using the building-block method that uses copies of low-energy units, “building blocks,” to build large realistic structural models. The calculated pair-correlation functions show that the local structure of these glasses is in good agreement with diffraction experiments. The electronic density of states for the doped models reveals defects in the band gap close to the conduction tail that are localized around undercoordinated Na atoms. Thermal properties for the systems, including the thermal expansion coefficient, are studied within the quasiharmonic approximation, and compare favorably with experiment. The elastic properties of the glasses are studied by calculating bulk and shear modulus.

Both the length scale and the magnetic field strength dependence of the magnetic noise in ac magnetically driven garnets were measured optically using the Faraday effect. At fields in a limited region, roughly twice the coercive field, the noise exhibits a Lorentzian power spectral density (PSD) consistent with random telegraph noise (RTN). The RTN onset as a function of applied field is rather abrupt and then decreases slowly with further increases in the applied field. The PSD of the noise in the lowest and highest magnetic field regimes is white background noise and is about two decades smaller than that of the RTN. In the field regime of the RTN, the PSD is Lorentzian at all measured length scales. As the measurement length scale increases past the order of a few microns, the low-frequency plateau of the Lorentzian PSD diminishes with increasing area measured as expected for a collection of uncorrelated RTN oscillators with similar characteristic frequencies.

The crystal and magnetic structures of multiferroic $\mathrm{LiFe}{\left({\mathrm{WO}}_4\right)}_2$ were investigated by temperature and magnetic-field-dependent specific heat, susceptibility, and neutron diffraction experiments on single crystals. Considering only the two nearest-neighbor magnetic interactions, the system forms a $J_1, J_2$ magnetic chain, but more extended interactions are sizable. Two different magnetic phases exhibiting long-range incommensurate order evolve at $T_{\text{N}1}\approx 22.2\mathrm{K}$ and $T_{\text{N}2}\approx 19\mathrm{K}$. First, a spin-density wave develops with moments lying in the $ac$ plane. In its multiferroic phase below $T_{\text{N}2}, \mathrm{LiFe}{\left({\mathrm{WO}}_4\right)}_2$ exhibits a spiral arrangement with an additional spin component along $b$. Therefore, the inverse Dzyaloshinskii-Moriya mechanism fully explains the multiferroic behavior in this material. A partially unbalanced multiferroic domain distribution was observed even in the absence of an applied electric field. For both phases only a slight temperature dependence of the incommensurability was observed, and there is no commensurate phase emerging at low temperature or at finite magnetic fields up to $6\mathrm{T}$. $\mathrm{LiFe}{\left({\mathrm{WO}}_4\right)}_2$ thus exhibits a simple phase diagram with the typical sequence of transitions for a type-II multiferroic material.

Topological invariants play an important role in characterizing topological phases. However, the topological invariants of Hermitian systems usually fail to characterize non-Hermitian topological systems due to non-Hermiticity. In this work, we generalize the Majorana polarization, which is initially defined to describe Hermitian topological superconductors, as a topological edge invariant to characterize the non-Hermitian topological superconductors. The definition of the generalized Majorana polarization depends upon two inequivalent particle-hole symmetries in non-Hermitian systems. The spinless Kitaev chain model and topological superconductor model on the honeycomb lattice are considered to examine the reliability and validity of the generalized Majorana polarization. We find that the phase transitions obtained by using Majorana polarization are consistent with the commonly used complex-energy point gap descriptions, which indicates the Majorana polarization is a reliable topological invariant to characterize the topological phase transition in non-Hermitian topological superconductors. In addition, the non-Hermitian skin effect on Majorana bound states in non-Hermitian topological superconductors is also discussed.

Intrinsic triferroicity is essential and highly sought for novel device applications, such as high-density multistate data storage. So far, the intrinsic triferroicity has only been discussed in three-dimensional systems. Herein on the basis of first principles, we report the intrinsic triferroicity in a two-dimensional lattice. Being exfoliatable from the layered bulk, single-layer $\mathrm{Fe}{\mathrm{O}}_2\mathrm{H}$ is shown to be an intrinsically triferroic semiconductor, presenting antiferromagnetism, ferroelasticity, and ferroelectricity simultaneously. Moreover, the directional control of its ferroelectric polarization is achievable by 90 ° reversible ferroelastic switching. In addition, single-layer $\mathrm{Fe}{\mathrm{O}}_2\mathrm{H}$ is identified to harbor in-plane piezoelectric effect. The unveiled phenomena and mechanism of triferroics in this two-dimensional system not only broaden the scientific and technological impact of triferroics but also enable a wide range of nanodevice applications.

We derive analytical results on energy spectral phase transitions and deformations in the simplest model of a one-dimensional lattice displaying the non-Hermitian skin effect, namely, the Hatano-Nelson model with unidirectional hopping under on-site potential uncorrelated disorder on the complex energy plane. Although the energy spectrum under open boundary conditions (OBCs) exactly reproduces the distribution of on-site potential disorder, the energy spectrum under periodic boundary conditions (PBC) undergoes spectral deformations, from one or more closed loops in the fully delocalized phase with no overlap with the OBC spectrum to a mixed spectrum (closed loops and some OBC energies) in the mobility edge phase to a complete collapse toward the OBC spectrum in the bulk localized phase. Such transitions are observed as the strength of disorder is increased. Depending on the kind of disorder, different interesting behaviors are found. In particular, for continuous disorder with a radial distribution on the complex energy plane it is shown that in the delocalized phase the energy spectrum under PBC is locked and fully insensitive to disorder, whereas transition to the bulk localized phase is signaled by the change in a topological winding number. When the disorder is described by a discrete distribution, the bulk localization transition never occurs, whereas topological phase transitions associated with PBC energy spectral splittings can be observed.