We demonstrate a mechanism for realizing ferrimagnetic metal, antiferromagnetic half-metal, and paramagnetic metal phases by tuning the onsite Coulomb repulsion $U$ in a band insulator. In a simple model of a correlated band insulator, namely the ionic Hubbard model at half-filling, and in the presence of a second neighbor hopping, we show that as $U$ is increased, first he insulating band gap is suppressed to zero at a critical $U_1$ and remains zero within the paramagnetic metallic phase. Interestingly, at a larger $U_c$, ferrimagnetic order turns on, leading to a ferrimagnetic metallic phase for $U_c<U<U_2$. For $U>U_2$, the system exhibits an antiferromagnetic half-metallic phase, followed finally by a transition into an antiferromagnetic Mott insulator. Our results, based on dynamical mean-field theory, suggest alternate routes for achieving magnetically ordered metallic and half-metallic phases which have potential applications in spintronics.

We investigate the influence of thermal energy on the current flow and electron spin states in double quantum dots in series. The quadruplet Pauli spin blockade, which is caused by the quadruplet and doublet states, occurs at low temperatures affecting the transport properties. As the temperature increases, the quadruplet Pauli spin blockade occurs as a result of the thermal energy, even in regions where it does not occur at low temperatures. This is because the triplet state is formed in one dot as a result of the gradual change of the Fermi distribution function of the electrodes with increasing temperature. Moreover, the thermally assisted Pauli spin blockade results in coexistence of the Coulomb and Pauli spin blockades. Conversely, for the standard triplet Pauli spin blockade, which occurs as a result of the triplet and singlet states, the current through the double dots monotonously smears out as the temperature increases. Therefore, the thermally assisted Pauli spin blockade is not clearly observed. However, the coexistence of the Coulomb and triplet Pauli spin blockades as a result of the thermal energy is clearly obtained in the calculation of the probability of the spin state in the double dots.

Recently, the linear and nonlinear planar Hall effect (PHE) on the topological insulators (TIs) surface has been extensively studied in experiments. To explain this phenomenon, various microscopic mechanisms are proposed theoretically, and one has to employ different mechanisms to separately understand the linear and nonlinear PHE even for the same system. Here, we study the planar magnetic resistance effect in TI thin film and find that a peculiar anisotropic scattering and a spin valve structure with respect to the PHE can be caused by the tilt and shift of Dirac cones, respectively, which are induced by the combination of spin-momentum locking of surface states and an in-plane magnetic field. The tilt and shift effects can act as the origin of both the linear and nonlinear PHE by distorting the spin texture of surface states or forming the spin polarization. These two mechanisms interplay and dominate, respectively, in strong coupling (thin TI) and decoupling (thick TI) between bottom and top surfaces. For thick TI film, we show that both the linear and nonlinear PHEs induced by the tilt effect can recover the results observed in recent experiments. Our theory provides a perspective to understand the origin of both linear and nonlinear PHE observed in recent experiments.

We theoretically study the interaction of an ultrafast intense linearly polarized optical pulse with monolayers of transition metal dichalcogenides (TMDCs). Such a strong pulse redistributes electrons between the bands and generates femtosecond currents during the pulse. Due to the large bandwidth of the incident pulse, this process is completely an off-resonant. While in TMDCs, the time-reversal symmetry is conserved, the inversion symmetry is broken, and these monolayers have axial symmetry along the armchair direction but not along with the zigzag one. The pulse, polarized along with asymmetric directions of TMDC monolayer, generates both longitudinal, i.e., along the direction of polarization, and transverse, i.e., in the perpendicular direction, currents. Such currents result in charge transfer through the system. We study different TMDC materials and show how the femtosecond transport in TMDC monolayers depend on their parameters, such as lattice constant and bandgap.

Non-Hermitian Hamiltonians are relevant to describe the features of a broad class of physical phenomena, ranging from photonics and atomic and molecular systems to nuclear physics and mesoscopic electronic systems. An important question relies on the understanding of the influence of a curved background on the static and dynamical properties of non-Hermitian systems. In this work, we study the interplay of geometry and non-Hermitian dynamics by unveiling the existence of curvature-dependent non-Hermitian phase transitions. We investigate a prototypical model of Dirac fermions on a sphere with an imaginary mass term. This exactly solvable model admits an infinite set of curvature-dependent pseudo-Landau levels. We characterize these phases by computing an order parameter given by the pseudomagnetization and, independently, the non-Hermitian fidelity susceptibility. Finally, we probe the non-Hermitian phase transitions by computing the (generalized) Loschmidt echo and the dynamical fidelity after a quantum quench of the imaginary mass and find singularities corresponding to exceptional radii of the sphere.

Dispersionless bands, flat bands, provide an excellent test bed for novel physical phases due to the fine-tuned character of flat band tight-binding Hamiltonians. The accompanying macroscopic degeneracy makes any perturbation relevant, no matter how small. For short-range hoppings flat-bands support compact localized states, which allowed us to develop systematic flat-band generators in $d=1$ dimension in Phys. Rev. B 95, 115135 (2017) and Phys. Rev. B 99, 125129 (2019). Here we extend this generator approach to $d=2$ dimensions. The shape of a compact localized state turns into an important additional flat-band classifier. This allows us to obtain analytical solutions for classes of $d=2$ flat-band networks and to reclassify and reobtain known ones, such as the checkerboard, kagome, Lieb, and Tasaki lattices. Our generator can be straightforwardly generalized to three lattice dimensions as well.

We study the two-dimensional one-band Hubbard model on the square lattice through the spin rotation invariant Kotliar-Ruckenstein slave-boson method, with an emphasis on the electron-doped regime. The phase diagram determined within mean field approximation reveals three different magnetically ordered phases separated by first-order transitions, and a wide instability region as indicated by a negative compressibility. We show that this conundrum of the unstable mean field domain is resolved by spatially mixed phases of neighboring stable states. We also find charge density wave type instabilities in the antiferromagnetically ordered (AFM) state at wave vectors commensurate with the doping. The corresponding charge and spin density response functions are calculated from a Gaussian fluctuations formalism about a magnetic saddle point, for which we provide a detailed derivation. Further, we present density and spin excitation spectra in the AFM phase, allowing for a quantitative calculation of the spin wave parameters, the gap in the longitudinal spin excitation spectrum, and the density excitations in the upper Hubbard band.

We study the melting of long-range antiferromagnetic order in the Hubbard model after an interaction quench, using nonequilibrium dynamical mean-field theory. From previous studies, the system is known to quickly relax into a prethermal symmetry-broken state. Using a convergent truncation of the memory integrals in the Kadanoff Baym equations, we unravel the subsequent relaxation dynamics of this state over several orders of magnitude in time. At long times, the prethermal state can be characterized by a single slow variable, which is related to the conduction band population. The dynamics of this variable is highly nonlinear, with a pronounced speedup once the gap falls below a certain value. This behavior indicates that nonthermal order can be self-sustained on some timescale because of a thermalization bottleneck is provided by the gap opened by the long-range order itself. These results cannot be reproduced using simple Fermi's golden rule estimate for the evolution of the conduction band population, and are distinct from the conventional scenario for the relaxation of prethermal states, in which a slow thermalization follows a rapid prethermalization.

Excitons are neutral objects that, naively, should have no response to a uniform electric field. Could the Berry curvature of the underlying electronic bands alter this conclusion? In this work, we show that Berry curvature can indeed lead to anomalous transport for excitons in two-dimensional materials subject to a uniform in-plane electric field. By considering the constituent electron and hole dynamics, we demonstrate that there exists a regime for which the corresponding anomalous velocities are in the same direction. We establish the resulting center-of-mass motion of the exciton through both a semiclassical and fully quantum mechanical analysis, and elucidate the critical role of Bloch oscillations in achieving this effect. We identify transition metal dichalcogenide heterobilayers as candidate materials to observe the effect.

We explore the effect of valley-contrasting gaps in the optical response of two-dimensional anisotropic tilted Dirac systems. We study the spectrum of intraband and interband transitions through the joint density of states (JDOS), the optical conductivity tensor, and the Drude spectral weight. The energy bands present an indirect gap in each valley with a reduced magnitude with respect to the nominal gap of the untilted system. Thus, a new possibility opens for the position of the Fermi level (an “indirect zone”) and for the momentum space available for allowed transitions. The JDOS near each gap displays a set of three van Hove singularities which are in contrast to the case of gapped graphene (an absorption edge only) or 8-$Pmmn$ borophene (two interband critical points due the tilt). For the Fermi level lying within the gap the JDOS shows the usual linear dependence on frequency, while when lying above an indirect zone it looks similar to the borophene case. These spectral characteristics in each valley determine the prominent structure of the optical conductivity. The longitudinal conductivity illustrates the strong anisotropy of the optical response. Similarly, the Drude weight is anisotropic and shows regions of nonlinear dependence on the Fermi level. The breaking of valley symmetry leads to a finite Hall response and associated optical properties. The anomalous and valley Hall conductivities present graphenelike behavior with characteristic modifications due to the indirect zones. Almost perfect circular dichroism and valley polarization can be achieved by tuning the exciting frequency with an appropriate Fermi level position. We also calculate the spectra of optical opacity and polarization rotation, which can reach magnitudes of tenths of radians in some cases. The spectral features of the calculated response properties are signatures of the simultaneous presence of tilt and mass, and suggest optical ways to determine the formation of different gaps in such class of Dirac systems like 8-$Pmmn$ borophene, quinoid-type graphene, organic conductor $\alpha \text{−}{\left(\mathrm{BEDT}\text{−}\mathrm{TTF}\right)}_2{\mathrm{I}}_3$, or some transition-metal dichalcogenides.

We present a formalism to solve the phonon Boltzmann transport equation (BTE) for finite Knudsen numbers that supplies a hydrodynamic heat transport equation similar to the Navier-Stokes equation for general semiconductors. This generalization of Fourier's law applies in general cases, from systems dominated by momentum-preserving normal collisions, as is well known, to kinetic materials dominated by resistive collisions, where it captures nonlocal effects. The key feature of our framework is that the macrostate is described in terms of the heat flux and its first derivatives. We obtain explicit expressions for the nonequilibrium phonon distribution and for the geometry-independent macroscopic parameters as a function of phonon properties that can be calculated from first principles. Ab initio model predictions are found to agree with a wide range of experiments in silicon. In contrast to approaches directly based on the BTE, the hydrodynamic equation can be solved in arbitrary geometries, thus providing a powerful tool for nanoscale heat modeling at a low computational cost.

Spin-orbit torque (SOT) induced magnetization switching in CoFeB/Ta/CoFeB trilayer with two ferromagnetic CoFeB layers exhibiting in-plane and perpendicular magnetic anisotropy (PMA), respectively, is investigated. Interlayer exchange coupling (IEC), measured using the ferromagnetic resonance technique is modified by varying thickness of the heavy metal Ta spacer ($t_{\mathrm{Ta}}$). The evolution of the IEC with $t_{\mathrm{Ta}}$ leads to different orientations of the magnetic anisotropy axes of two CoFeB layers: for $t_{\mathrm{Ta}}>1.3\mathrm{nm}$, magnetization prefers antiferromagnetic alignment, whereas for $t_{\mathrm{Ta}}<1.3\mathrm{nm}$, ferromagnetic coupling is measured. The magnetization state of the CoFeB layer exhibiting PMA is controlled by the spin-polarized current originating from the Ta layer in micrometer sized Hall bars. The SOT induced critical current density with Ta spacer thickness is presented, showing an increase with decreasing $t_{\mathrm{Ta}}$, which coincides with the CoFeB coercive field dependence. In a narrow range of $t_{\mathrm{Ta}}$ corresponding to the ferromagnetic IEC, the field-free SOT induced magnetization switching is achieved.

We apply unsupervised learning techniques to classify the different phases of the $J_1\text{−}{J}_2$ antiferromagnetic Ising model on the honeycomb lattice. We construct the phase diagram of the system using convolutional autoencoders. These neural networks can detect phase transitions in the system via “anomaly detection” without the need for any label or a priori knowledge of the phases. We present different ways of training these autoencoders, and we evaluate them to discriminate between distinct magnetic phases. In this process, we highlight the case of high-temperature or even random training data. Finally, we analyze the capability of the autoencoder to detect the ground state degeneracy through the reconstruction error.

When a strongly disordered system of interacting quantum dipoles is locally excited, the excitation relaxes on some (potentially very long) timescale. We analyze this relaxation process, both for electron glasses with strong Coulomb interactions—in which particle-hole dipoles are emergent excitations—and for systems (e.g., quantum magnets or ultracold dipolar molecules) made up of microscopic dipoles. We consider both energy relaxation rates ($T_1$ times) and dephasing rates ($T_2$ times), and their dependence on frequency, temperature, and polarization. Systems in both two and three dimensions are considered, along with the dimensional crossover in quasi-two-dimensional geometries. A rich set of scaling laws is found.

We present a microscopic study of the behavior of the order parameters near the boundaries of a two-band superconducting material, described by the standard tight-binding Bardeen-Cooper-Schrieffer model. We find superconducting surface states. The relative difference between bulk and surface critical temperatures is a nontrivial function of the interband coupling strength. For superconductors with weak interband coupling, boundaries induce variations of the gaps with the presence of multiple length scales, despite nonzero interband Josephson coupling.

In the quest for accurate approximations of the noninteracting kinetic energy functional as a functional of the electronic density, two different paths are usually employed: semilocal functionals based on the derivatives of the electronic density (mainly the gradient and the Laplacian) or nonlocal functionals based on the linear response of the homogeneous electron gas, i.e., the Lindhard function. While the former are defined in real space but fail to reproduce the Lindhard function, the latter cannot be expressed exactly in real space (being defined only in the reciprocal space), so applications to finite systems are complicated. In this paper we introduce a nonlocal ingredient ($y_{\alpha }$) based on the Yukawa potential, i.e., the screened Hartree potential, which can be combined with other semilocal ingredients to obtain a more accurate description of the Lindhard function for both small and large wave vectors. We show and analyze the different properties of $y_{\alpha }$ and introduce a class of density functionals, the Yukawa-generalized gradient approximation (yGGA). We show that both the total energy and the first functional derivative (the kinetic potential) of yGGA functionals can be easily computed in real space. We present model yGGA functionals which well approximate the Lindhard function for both small and large wave vectors and can accurately describe jellium clusters and their perturbations.

We developed the theory of hydrodynamics of an isotropic Fermi liquid of electrons coupled to isotropic acoustic phonons, assuming that umklapp processes may be neglected. At low temperatures, the fluid is approximately Galilean invariant; at high temperatures, the fluid is nearly relativistic; at intermediate temperatures, there are seven additional temperature regimes with unconventional thermodynamic properties and hydrodynamic transport coefficients in a three-dimensional system. We predict qualitative signatures of electron-phonon fluids in incoherent transport coefficients, shear and Hall viscosity, and plasmon dispersion relations. Our theory may be relevant for numerous quantum materials where strong electron-phonon scattering has been proposed to underlie a hydrodynamic regime, including ${\mathrm{WTe}}_2, {\mathrm{WP}}_2$, and ${\mathrm{PtSn}}_4$.

We present two models for the atomic structure of inversion domain boundaries in wurzite GaN, that have not been discussed in existing literature. Using density functional theory, we find that one of these models has a lower formation energy than a previously proposed model known as Holt-IDB. Although this newly proposed model has a formation energy higher than the accepted lower-energy structure, known as IDB*, we argue that it can be formed under typical growth conditions. We present evidence that it may have been already observed in experiments, albeit misidentified as Holt-IDB. Our analysis was facilitated by a convenient notation, which we introduced, to characterize these models; it is based on the mismatch in crystal stacking sequence across the $\left\{10\overline{1}0\right\}$ plane. Additionally, we introduce an improved method to calculate energies of certain domain walls that challenge the periodic boundary conditions needed for plane-wave density functional theory methods. This new method provides improved estimations of domain wall energies.

We evaluate the microscopically relevant parameters for electrical transport of hybrid superconductor-semiconductor interfaces. In contrast to the commonly used geometrically constricted metallic systems, we focus on materials with dissimilar electronic properties like low-carrier density semiconductors combined with superconductors, without imposing geometric confinement. We find an intrinsic mode-selectivity, a directional momentum-filter, due to the differences in electronic band structure, which creates a separation of electron reservoirs each at the opposite sides of the semiconductor, while at the same time selecting modes propagating almost perpendicular to the interface. The electronic separation coexists with a transport current dominated by Andreev reflection and low elastic backscattering, both dependent on the gate-controllable electronic properties of the semiconductor.