The in-plane current profile within multilayers of the generic structure Ta/Pt/(CoNi)/Pt/Ta is investigated. A large set of samples where the thickness of each layer was systematically varied was grown, followed by the measurement of the sheet resistance of each sample. The data are analyzed by a series of increasingly elaborate models, from a macroscopic engineering approach to mesoscopic transport theory. Non-negligible variations of the estimated repartition of current between the layers are found. The importance of having additional structural data is highlighted.

Nonequilibrium Floquet topological phases due to periodic driving are known to exhibit rich and interesting features with no static analogs. Various known topological invariants usually proposed to characterize static topological systems often fail to fully characterize Floquet topological phases. This fact has motivated extensive studies of Floquet topological phases to better understand nonequilibrium topological phases and to explore their possible applications. Here we present a theoretically simple Floquet topological insulating system that may possess an arbitrary number of counterpropagating chiral edge states. Further investigation into our system reveals another related feature by tuning the same set of system parameters, namely, the emergence of almost flat (dispersionless) edge modes. In particular, we employ two-terminal conductance and dynamical winding numbers to characterize counterpropagating chiral edge states. We further demonstrate the robustness of such edge states against symmetry preserving disorder. Finally, we identify an emergent chiral symmetry at certain subregimes of the Brillouin zone that can explain the presence of almost flat edge modes. Our results have exposed more interesting possibilities in Floquet topological matter.

We develop a nonequilibrium auxiliary quantum master equation dual-boson method (aux-DB) and argue that it presents a convenient way to describe steady states of correlated impurity models (such as single-molecule optoelectronic devices) where electron and energy transport should be taken into account. The aux-DB is shown to provide high accuracy with relatively low numerical cost. Theoretical analysis is followed by illustrative simulations within generic junction models, where the scheme is benchmarked against numerically exact results.

Inspired by the experimental findings of an exotic ferromagnetic insulating state in ${\mathrm{LaMnO}}_3/{\mathrm{SrTiO}}_3$ (LMO/STO) heterostructures, we calculate the electronic and magnetic state of ${\mathrm{LaMnO}}_3/{\mathrm{SrTiO}}_3$ superlattices with comparable thicknesses employing ab initio dynamical mean-field theory. Projecting on the low-energy subspace of Mn $3d$ and Ti $3d$ states, and solving a multi-impurity problem, our approach emphasizes local correlations at the Mn and Ti sites. We find that a ferromagnetic insulating state emerges due to the intrinsic effects of strong correlations in the system, in agreement with experimental studies. We also predict that, due to electronic correlations, the emerging two-dimensional electron gas is located at the LMO side of the interface. This is in contrast to density-functional theory results that locate the electron gas on the STO side. We estimate the transition temperature for the paramagnetic-to-ferromagnetic phase transition, which may be verified experimentally. Importantly, we also clarify that the epitaxial strain is a key ingredient for the emergence of the exotic ferromagnetic insulating state. This becomes clear from calculations on a strained ${\mathrm{LaMnO}}_3$ system, also showing ferromagnetism which is not seen in the unstrained bulk material.

In this joint experimental and theoretical study we demonstrate coherent control of the optical field emission and electron transport in plasmonic gaps subjected to intense single-cycle laser pulses. Our results show that an external THz field or a minor dc bias, orders of magnitude smaller than the optical bias owing to the laser field, allows one to modulate and direct the electron photocurrents in the gap of a connected nanoantenna operating as an ultrafast nanoscale vacuum diode for lightwave electronics. Using time-dependent density functional theory calculations we elucidate the main physical mechanisms behind the observed effects and show that an applied dc field significantly modifies the optical field emission and quiver motion of photoemitted electrons within the gap. The quantum many-body theory reproduces the measured net electron transport in the experimental device, which allows us to establish a paradigm for controlling nanocircuits at petahertz frequencies.

We present a semiclassical treatment of one-dimensional many-body quantum systems in equilibrium, where quantum corrections to the classical field approximation are systematically included by a renormalization of the classical field parameters. Our semiclassical approximation is reliable in the limit of weak interactions and high temperatures. As a specific example, we apply our method to the interacting Bose gas and study experimentally observable quantities, such as correlation functions of bosonic fields and the full counting statistics of the number of particles in an interval. Where possible, our method is checked against exact results derived from integrability, showing excellent agreement.

A bulk photovoltaic effect was observed in insulating carbon-doped gallium nitride crystals by investigating the light-induced change of the contact potential difference with a Kelvin probe as a function of the carbon concentration in the crystal. Bulk polarization was caused by excitation of defects setting in at photon energies of 0.95–1.05 eV (C1) and 2.5–2.6 eV (C3). From the dependence on the carbon concentration, C1 can be assigned to isolated carbon defects and C3 to tricarbon defects.

We present the results of large-scale numerical simulations of the mobility of a two-dimensional electron liquid on the helium surface in the presence of a one-dimensional periodic potential. Even where the potential is much weaker than the electron-electron interaction, it can strongly change the mobility. The effect depends on the interrelation between the potential period and the mean interelectron distance. It is most pronounced where the period is close to the period of the Wigner crystal that would form if the liquid were cooled to a lower temperature. The results suggest, in particular, that the correlation length in the electron liquid can be found by measuring the mobility in a weak periodic potential. The simulations are based on the microscopic model of the electron scattering by the excitations in helium.

In this work we carry out a theoretical study of the phonon-induced resistivity in twisted double bilayer graphene (TDBG), in which two Bernal-stacked bilayer graphene devices are rotated relative to each other by a small angle $\theta$. We show that at small twist angles ($\theta \sim 1^{\circ }$) the effective mass of the TDBG system is greatly enhanced, leading to a drastically increased phonon-induced resistivity in the high-temperature limit where phonon scattering leads to a linearly increasing resistivity with increasing temperature. We also discuss possible implications of our theory on superconductivity in such a system and provide an order of magnitude estimation of the superconducting transition temperature.

It is shown that a surface acoustic wave (SAW) can induce spin resonance of a spin accumulation in a paramagnetic material. The high-frequency mechanical motion of the SAW produces an effective AC magnetic field. If it is oriented orthogonal to the spins, spin precession is induced if an additional DC magnetic field ${\mathbf{B}}_0$ is applied along the spin axis and the corresponding Larmor frequency ${\omega }_0$ matches the SAW frequency. The spin accumulation is then resonantly suppressed. We describe this SAW-induced spin resonance quantitatively and analyze the example of a nonlocal spin-transport device with a silicon channel, in which the spin accumulation is created by electrical spin injection from a ferromagnetic contact. The analysis demonstrates that the (nonlocal) electrical detection of SAW-induced spin resonance is feasible.

Tensor renormalization group (TRG) constitutes an important methodology for accurate simulations of strongly correlated lattice models. Facilitated by the automatic differentiation technique widely used in deep learning, we propose a uniform framework of differentiable TRG ($\partial \mathrm{TRG}$) that can be applied to improve various TRG methods, in an automatic fashion. $\partial \mathrm{TRG}$ systematically extends the essential concept of second renormalization [Phys. Rev. Lett. 103, 160601 (2009)] where the tensor environment is computed recursively in the backward iteration. Given the forward TRG process, $\partial \mathrm{TRG}$ automatically finds the gradient of local tensors through backpropagation, with which one can deeply “train” the tensor networks. We benchmark $\partial \mathrm{TRG}$ in solving the square-lattice Ising model, and we demonstrate its power by simulating one- and two-dimensional quantum systems at finite temperature. The global optimization as well as GPU acceleration renders $\partial \mathrm{TRG}$ a highly efficient and accurate many-body computation approach.

Weyl semimetals (WSMs) have unusual optical responses originating from unique topological properties of their bulk and surface electron states. Their third-order optical nonlinearity is expected to be very strong, especially at long wavelengths, due to linear dispersion and high Fermi velocity of three-dimensional Weyl fermions. Here we derive the third-order nonlinear optical conductivity of WSMs in the long-wavelength limit and calculate the intensity of the nonlinear four-wave mixing signal as it is transmitted through the WSM film or propagates away from the surface of the material in the reflection geometry. All results are analytic and show the scaling of the signal intensity with variation of all relevant parameters. The nonlinear generation efficiency turns out to be surprisingly high for a lossy material, of the order of several mW per ${\mathrm{W}}^3$ of the incident pump power. Optimal conditions for maximizing the nonlinear signal are realized in the vicinity of bulk plasma resonance. This indicates that ultrathin WSM films of the order of skin depth in thickness could find applications in compact optoelectronic devices.

The anharmonicity of the acoustic phonon dispersion of graphene has been studied by the harmonic linear response (HLR) approach at finite temperature. This is a nonperturbative method based on the linear response of the system to applied forces, as derived from equilibrium computer simulations. Anharmonic shifts are analyzed in the long-wavelength limit at room temperature, with emphasis on the effect of applied tensile or compressive in-plane stress. The simulation results are compared with available analytical models, based either on first-order perturbation theory or on a description by anomalous exponents. The simulations show better agreement to the expectations of the perturbational approach. The effect of temperature and zero-point vibrations on the acoustic out-of-plane anharmonic shifts of graphene are briefly reviewed.

We develop a real-space theory of the voltage bias driven transition from a Mott insulator to a correlated metal. Within our Keldysh mean-field approach the problem reduces to a self-consistency scheme for the charge and spin profiles in this open system. We solve this problem for a two dimensional antiferromagnetic Mott insulator at zero temperature. The charge and spin magnitude is uniform over the system at zero bias, but a bias $V$ leads to spatial modulation over a length scale $\xi \left(V\right)$ near the edges. $\xi \left(V\right)$ grows rapidly and becomes comparable to system size as $V$ increases towards a threshold scale $V_c$. The linear response conductance of the insulator is zero with the current being exponentially small for $V\ll V_c$. The current increases rapidly as $V\to V_c$. Beyond $V_c$, we observe an inhomogeneous low moment antiferromagnetic metal, and at even larger bias, a current saturated paramagnetic metal. We suggest an approximate scheme for the spectral features of this nonequilibrium system.

Plutonium is a critically important material as the behavior of its $5f$ electrons stands midway between the metalliclike itinerant character of the light actinides and localized atomic-core-like character of the heavy actinides. The $\delta$ phase of plutonium ($\delta$-Pu), whereas still itinerant, has a large coherent Kondo peak and strong electronic correlations coming from its near-localized character. Using sophisticated Gutzwiller wave function and dynamical mean-field theory correlated theories, we study the Fermi surface and associated mass renormalizations of $\delta$-Pu together with calculations of the de Haas–van Alphen frequencies. We find a large ($\sim 200\%$) correlation induced volume expansion in both the hole and the electron pockets of the Fermi surface in addition to an intermediate mass enhancement. All of the correlated electron theories predict, approximately, the same hole pocket placement in the Brillouin zone, which is different from that obtained in conventional density-functional band-structure theory, whereas the electron pockets from all theories are in, roughly, the same place.

We demonstrate that the finite-difference time-domain (FDTD) method can be used to study the formation of laser-induced periodic surface structures (LIPSSs) under oblique incidence and arbitrary polarization states. The inhomogeneous energy absorption below a rough surface at oblique incident angle is studied by the FDTD method and compared to the analytical efficacy theory developed by Sipe et al. [Phys. Rev. B 27, 1141 (1983)]. The two approaches show excellent agreement. The surface topographies of both low-spatial-frequency LIPSSs (LSFLs) and high-spatial-frequency LIPSSs (HSFLs) at oblique incidence and various polarization states (linear, circular, radial, and azimuthal) are simulated by taking into account topography-driven interpulse feedback effects. For $s$- and mixed $s$- and $p$-polarized beams at large incident angles, the simulated LSFLs show orientations that are neither perpendicular nor parallel to the laser polarization. Their physical origin is explained by a nonzero angle between the in-plane wave vector of the incident light and the scattered light. By using circularly-polarized beams, the simulated surface topographies are further shown to be in excellent agreement with the triangular LSFLs and the fine-dot nanoscaled structures reported in the literature. In addition, the simulation of HSFLs suggests that their periods have almost no dependence on the incident angle nor the polarization angle, while their surface topographies resemble that of blazed gratings at large incident angles for $p$-polarized and mixed $s$- and $p$-polarized beams. The origin is explained by the appearance of asymmetry in the scattered near-field light. The extension to oblique incidence and arbitrary polarization states demonstrates that the FDTD approach on LIPSSs is a highly versatile and powerful technique which has the potential to be one of the foundations for a complete modeling and understanding of LIPSSs under various irradiation conditions.

We theoretically study the quench dynamics of induced anisotropy of a large-spin magnetic molecule coupled to spin-polarized ferromagnetic leads. The real-time evolution is calculated by means of the time-dependent density-matrix numerical renormalization group method implemented within the matrix product states framework, which takes into account all correlations in a very accurate manner. We determine the system response to a quench in the spin-dependent coupling to ferromagnetic leads. In particular, we focus on the transient dynamics associated with crossing from the weak- to the strong-coupling regime, where the Kondo correlations become important. The dynamics is examined by calculating the time-dependent expectation values of the spin-quadrupole moment and the associated spin operators. We identify the relevant timescales describing the quench dynamics and determine the influence of the effective exchange coupling of molecules and spin polarization of the leads on the dynamical behavior of the system. Furthermore, the generalization of our predictions for large spin values of molecules is considered. Finally, we analyze the effect of finite temperature and show that it gives rise to a reduction of magnetic anisotropy by strong suppression of the time-dependent spin-quadrupole moment due to thermal fluctuations.

We investigate the influence of chirality on the $\mathcal{PT}$-symmetric and $\mathcal{PT}$-broken phase of $\mathcal{PT}$-symmetric chiral systems. Starting from the point that transverse magnetic (TM) and transverse electric (TE) waves have different exceptional points, we show that with circularly polarized waves (which are linear combinations of TM and TE waves) mixed $\mathcal{PT}$-symmetric phases can be realized and the extent of these phases can be highly controlled by either or both the chirality and the angle of incidence. Additionally, while the transmission of both TM and TE waves in nonchiral $\mathcal{PT}$-symmetric systems is the same for forward and backward propagation, we show that with chirality this symmetry can be broken. As a result, it is possible to realize asymmetric, i.e., side-dependent, rotation, and ellipticity in the polarization state of the transmitted wave. Our results constitute a simple example of a chiral $\mathcal{PT}$-symmetric optical system in which the various phases (full $\mathcal{PT}$, mixed, broken) and the asymmetric effects can be easily tuned by adjusting the chirality parameter and/or the angle of incidence.