We discuss a joint microscopic theory for the laser-induced magnetization dynamics and spin transport in magnetic heterostructures based on the $s\text{−}d$ interaction. Angular momentum transfer is mediated by scattering of itinerant $s$ electrons with the localized ($d$ electron) spins. We use the corresponding rate equations and focus on a spin-$\frac{1}{2}$ $d$ electron system, leading to a simplified analytical expression for the dynamics of the local magnetization that is coupled to an equation for the nonequilibrium spin accumulation of the $s$ electrons. We show that this description converges to the microscopic three-temperature model in the limit of a strong $s\text{−}d$ coupling. The equation for the spin accumulation is used to introduce diffusive spin transport. The presented numerical solutions show that during the laser-induced demagnetization in a ferromagnetic metal, a short-lived spin accumulation is created that counteracts the demagnetization process. Moreover, the spin accumulation leads to the generation of a spin current at the interface of a ferromagnetic and nonmagnetic metal. Depending on the specific magnetic system, both local spin dissipation and interfacial spin transport are able to enhance the demagnetization rate by providing relaxation channels for the spin accumulation that is built up during demagnetization in the ferromagnetic material.

Using quantum Monte Carlo simulations, we study effects of disorder on the $S=1/2$ Heisenberg model with exchange constant $J$ on the square lattice supplemented by multispin interactions $Q$. It was found recently [L. Liu et al., Phys. Rev. X 8, 041040 (2018)] that the ground state of this $J-Q$ model with random couplings undergoes a quantum phase transition from the Néel antiferromagnetic state into a randomness-induced spin-liquid-like state that is a close analog to the well known random-singlet (RS) state of the Heisenberg chain with random couplings. This 2D RS state arises from a spontaneously symmetry-broken fourfold degenerate columnar valence-bond solid that is broken up by the disorder into finite domains, with spinons localized at topological defects. The interacting spinons form a critical collective many-body state without magnetic long range order but with the mean spin-spin correlations decaying with distance $r$ as $r^{-2}$, as in the one-dimensional RS state. The dynamic exponent $z\ge 2$, varying continuously with the model parameters. In this work, we further investigate the properties of the RS state in the $J-Q$ model with random $Q$ couplings. We study the temperature dependence of the specific heat and various susceptibilities for large enough systems to reach the thermodynamic limit. We also analyze the size dependence of the critical magnetic order parameter and its susceptibility in the ground state. For all these quantities, we find consistency with the conventional quantum-critical scaling laws when the condition implied by the $r^{-2}$ form of the spin correlations is imposed. In particular, all the different quantities can be explained by the same value of the dynamic exponent $z$ at fixed model parameters. We argue that the RS state identified in the $J-Q$ model corresponds to a generic renormalization group fixed point that can be reached in many quantum magnets with random couplings and that it has already been observed experimentally.

We study the out-of-equilibrium dynamics of one-dimensional quantum Ising-like systems, arising from sudden quenches of the Hamiltonian parameter $g$ driving quantum transitions between disordered and ordered phases. In particular, we consider quenches to values of $g$ around the critical value $g_c$, and mainly address the question whether, and how, the quantum transition leaves traces in the evolution of the transverse and longitudinal magnetizations during such a deep out-of-equilibrium dynamics. We shed light on the emergence of singularities in the thermodynamic infinite-size limit, likely related to the integrability of the model. Finite systems in periodic and open boundary conditions develop peculiar power-law finite-size scaling laws related to revival phenomena, but apparently unrelated to the quantum transition, because their main features are generally observed in quenches to generic values of $g$. We also investigate the effects of dissipative interactions with an environment, modeled by a Lindblad equation with local decay and pumping dissipation operators within the quadratic fermionic model obtainable by a Jordan-Wigner mapping. Dissipation tends to suppress the main features of the unitary dynamics of closed systems. We finally address the effects of integrability breaking, due to further lattice interactions, such as in anisotropic next-to-nearest-neighbor Ising (ANNNI) models. We show that some qualitative features of the post-quench dynamics persist, in particular, the different behaviors when quenching to quantum ferromagnetic and paramagnetic phases, and the revival phenomena due to the finite size of the system.

We study the dynamical quantum phase transition of a critical quantum quench, in which the prequenched Hamiltonian, or the postquenched Hamiltonian, or both of them are set to be the critical points of equilibrium quantum phase transitions. We find a half-quantized or unquantized dynamical topological order parameter and dynamical Chern number; these results and also the existence of dynamical quantum phase transition are all closely related to the singularity of the Bogoliubov angle at the gap-closing momentum. The effects of the singularity may also be canceled out if both the prequenched and postquenched Hamiltonians are critical; then the dynamical topological order parameter and dynamical Chern number restore to integer ones. Our findings show that the widely accepted definitions of the dynamical topological order parameter and dynamical Chern number are problematic for the critical quenches in the perspective of topology, which call for new definitions of them.

A formulation of the phase field crystal model is presented that is consistent with the necessary microscopic independence between the phase field, reflecting the broken symmetry of the phase, and both mass density and elastic distortion. Although these quantities are related in equilibrium through a macroscopic equation of state, they are independent variables in the free energy and can be independently varied in evaluating the dissipation functional that leads to some of the model governing equations. The equations obtained describe dislocation motion in an elastically stressed solid and serve as an extension of the equations of dislocation mechanics to the phase field crystal setting. Both finite and small deformation theories are considered, and the corresponding kinetic equations for the fields are derived.

Magnetic soliton crystals with layered structures that host periodic chiral helimagnetic ordering are promising candidates for spintronic nanodevices. Among them, helimagnetic $\mathrm{Cr}{\mathrm{Nb}}_3{\mathrm{S}}_6$ is unique owing to its crystallographic chirality and monoaxial Dzyaloshinskii-Moriya interaction. It is crucial to explore its magnetic configurations and properties with respect to the temperature and thickness, especially in reduced dimensions. Here, the chiral helimagnetic ground state in $\mathrm{Cr}{\mathrm{Nb}}_3{\mathrm{S}}_6$ nanostructures is investigated using off-axis electron holography in the transmission electron microscope. The period of the helical state is found to be independent of both temperature and specimen thickness, while the temperature dependence of the saturation magnetization is shown to follow a classical Heisenberg spin model. Monte Carlo simulations based on a discrete classical Heisenberg model reproduce the experimental observations closely, confirming the applicability of a three-dimensional Heisenberg model even in a confined specimen geometry.

We consider the out-of-equilibrium behavior of a general class of mesoscopic devices composed of several superconducting or/and normal metal leads separated by quantum dots. Starting from a microscopic Hamiltonian description, we provide a nonperturbative approach to quantum electronic transport in the tunneling amplitudes between dots and leads: using the equivalent of a path integral formulation, the lead degrees of freedom are integrated out in order to compute both the current and the current correlations (noise) in this class of systems, in terms of the dressed Green's function matrix of the quantum dots. In order to illustrate the efficiency of this formalism, we apply our results to the “all superconducting Cooper pair beam splitter,” a device composed of three superconducting leads connected via two quantum dots, where crossed Andreev reflection operates Cooper pair splitting. Commensurate voltage differences between the three leads allow us to obtain expressions for the current and noise as a function of the Keldysh Nambu Floquet dressed Green's function of the dot system. This voltage configuration allows the occurrence of nonlocal processes involving multiple Cooper pairs which ultimately lead to the presence of nonzero DC currents in an out-of-equilibrium situation. We investigate in detail the results for the noise obtained numerically in the specific case of opposite voltages, where the transport properties are dominated by the so called “quartet processes,” involving the coherent exchange of two Cooper pairs among all three superconducting terminals. We show that these processes are noiseless in the nonresonant case, and that this property is also observed for other voltage configurations. When the dots are in a resonant regime, the noise characteristics change qualitatively, with the appearance of giant Fano factors.

Based on the two-fluid hydrodynamics, an analog of the famous Rayleigh-Plesset equation for the dynamics of a spherical vapor bubble in superfluid helium is derived. The two-fluid nature of He II and the specific form of the momentum flux density tensor give rise to a number of effects in the evolution of the boundary position $R\left(t\right)$, absent in ordinary fluids. One of them is the abnormal attenuation of the boundary oscillation, which exceeds the usual viscous damping by several orders of magnitude. There is also an additional term proportional to the squared velocity of the normal component, which is independent of the derivative $dR/dt$, and therefore can be included in the pressure drop. Its physical meaning is related to the dependence of pressure on the relative velocity between the normal and superfluid components. One more effect renormalizes the coefficient in front of ${(dR/dt)}^2$. The dissipative part of the momentum flux tensor is also being upgraded to take into account the two-fluid hydrodynamics. As an illustration of the stated theory, a numerical solution of the obtained master equation for the evolution of a vapor film on spherical heaters in He II is presented. The obtained results declare that some early issues and conclusions on the dynamics of the cavity in superfluid helium should be reviewed.

We propose an improved approach to carry out the imaginary time evolution of infinite projected entangled-pair states, especially for systems with criticality. A cyclic optimal truncation is introduced to update the tensors along a closed loop, aiming to remove the redundant internal correlations. We demonstrate the algorithm by considering an elaborate evolution based on a simple update on a small plaquette. This scheme can also be applied to a full update strategy. We demonstrate their performances on simulating the ground states of the spin-$1/2$ antiferromagnetic Heisenberg model and the transverse field Ising model on a square lattice.

Quantum interference on the kagome lattice generates electronic bands with narrow bandwidth, called flat bands. Crystal structures incorporating this lattice can host strong electron correlations with nonstandard ingredients, but only if these bands lie at the Fermi level. In the six compounds with the CoSn structure type (FeGe, FeSn, CoSn, NiIn, RhPb, and PtTl) the transition metals form a kagome lattice. The two iron variants are robust antiferromagnets so we focus on the latter four and investigate their thermodynamic and transport properties. We consider these results and calculated band structures to locate and characterize the flat bands in these materials. We propose that CoSn and RhPb deserve the community's attention for exploring flat-band physics.

The continuous coupling function in quantum impurity problems is exactly partitioned into a part represented by a finite-size Wilson chain and a part represented by a set of additional reservoirs, each coupled to one Wilson chain site. These additional reservoirs represent high-energy modes of the environment neglected by the numerical renormalization group and are required to restore the continuum limit of the original problem. We present a hybrid time-dependent numerical renormalization group approach which combines an accurate numerical renormalization group treatment of the nonequilibrium dynamics on the finite-size Wilson chain with a Bloch-Redfield formalism to include the effect of these additional reservoirs. Our approach overcomes the intrinsic shortcoming of the time-dependent numerical renormalization group approach induced by the bath discretization with a Wilson parameter $\mathrm{\Lambda }>1$. We analytically prove that for a system with a single chemical potential, the thermal equilibrium reduced density operator is the steady-state solution of the Bloch-Redfield master equation. For the numerical solution of this master equation, a Lanczos method is employed which couples all energy shells of the numerical renormalization group. The presented hybrid approach is applied to the real-time dynamics in correlated fermionic QISs. An analytical solution of the resonant-level model serves as a benchmark for the accuracy of the method which is then applied to nontrivial models, such as the interacting resonant-level model and the single-impurity Anderson model.

We investigate the Landau-Zener-Stückelberg-Majorana interferometry of a superconducting qubit in a semi-infinite transmission line terminated by a mirror. The transmon-type qubit is at the node of the resonant electromagnetic (EM) field, “hiding” from the EM field. We modulate the resonant frequency of the qubit by applying a sinusoidal flux pump. We perform spectroscopy by measuring the reflection coefficient of a weak probe in the system. Remarkable interference patterns emerge in the spectrum, which can be interpreted as multiphoton resonances in the dressed qubit. Our calculations agree well with the experiments.

Understanding and controlling heat transfer across interfaces has become an important issue for the performance of micro- and nanoscale electronics, as well as achieving a high figure of merit for thermoelectrics. Intrinsic and extrinsic defects can have a significant impact on thermal transport in bulk materials and across interfaces, but the mechanism is not well understood. In this work, nonequilibrium molecular dynamics simulations are used to determine the impact of interfacial atomic vacancies on thermal transport across a Cu/Si junction. In contrast to the reduction in thermal transport typically seen with bulk defects, we find that by introducing atomic vacancies at a concentration of 6.3% near the interface in either or both materials, the interfacial thermal conductance can be increased by up to 76%. By controlling the initial positions of the vacancies and keeping track of their movements and population, we find that interfacial thermal transport is dependent on temperature, vacancy concentration, and distribution, and a positive correlation between the conductance and point defect activities (extent of vacancy migration, rate of Frenkel defect creation and annihilation) is observed. Further calculations based on the phonon density of states and normal mode decomposition reveal that the increase in interfacial thermal conductance originates primarily from high-frequency phonons, supported by enhanced inelastic phonon transport which contributes to more than 60% of the increase. Our findings suggest a practical way to manipulate inelastic phonon conversion through the presence of defects, which provides an alternative perspective on improving thermal transport between materials with a large lattice mismatch.

Organic semiconductors exhibit properties of individual molecules and extended crystals simultaneously. The strongly bound excitons they host are typically described in the molecular limit, but excitons can delocalize over many molecules, raising the question of how important the extended crystalline nature is. Using accurate Green's function based methods for the electronic structure and nonperturbative finite difference methods for exciton-vibration coupling, we describe exciton interactions with molecular and crystal degrees of freedom concurrently. We find that exciton delocalization controls these interactions, with thermally activated crystal phonons predominantly coupling to delocalized states, and molecular quantum fluctuations predominantly coupling to localized states. Based on this picture, we quantitatively predict and interpret the temperature and pressure dependence of excitonic peaks in the acene series of organic semiconductors, which we confirm experimentally, and we develop a simple experimental protocol for probing exciton delocalization. Overall, we provide a unified picture of exciton delocalization and vibrational effects in organic semiconductors, reconciling the complementary views of finite molecular clusters and periodic molecular solids.

We calculate the component of the photovoltage in bismuth ferrite that is generated by ferroelectric domain walls, using first-principles methods, in order to compare its magnitude to the experimentally measured photovoltage. We find that excitons at the ferroelectric domain walls form an electric dipole layer resulting in a domain-wall driven photovoltage. This is of the same order of magnitude as the experimentally measured one, but only if the carrier lifetimes and diffusion lengths are larger than previously assumed.

We study the electronic properties of dual-gated electron-hole bilayers in which the two layers are separated by a perfectly opaque tunnel barrier. Combining an electrostatic and thermodynamic analysis with mean-field theory estimates of interacting system chemical potentials, we explain the dependence of the electron and hole densities on the two gate voltages. Because chemical potential jumps occur for both electrons and holes at neutrality, there is a finite area in gate voltage parameter space over which electron and hole densities are equal. In that regime the electron-hole pair density depends only on the sum of the two gate voltages. We are able to explain a recent experimental study of electrically controlled bilayers by allowing for interlayer tunneling and using a nonequilibrium steady-state transport picture.

We present a computational study of resonant inelastic x-ray scattering (RIXS) and x-ray absorption in a representative rare-earth nickelate ${\mathrm{LuNiO}}_3$. We study the changes in the spectra across the metal-insulator/site-disproportionation transition. In particular, we address the question of site selectivity of the two methods in the disproportionated insulating phase and the signature of metal-insulator transition in the fluorescencelike feature of the RIXS spectra. To this end we use the local-density approximation $+$ dynamical mean-field theory approach combined with the configuration integration method to compute the core-level spectra.

The honeycomb lattice iridates $A_2\mathrm{Ir}{\mathrm{O}}_3$ ($A=\mathrm{Na}$, Li) are spin-orbit assisted Mott insulators proximate to Kitaev's quantum spin liquid. The insulating state as well as the magnetic properties are believed to arise due to a delicate balance of several energy scales. We report on high-pressure electrical transport and x-ray-diffraction measurements on $A_2\mathrm{Ir}{\mathrm{O}}_3$ ($A=\mathrm{Na}$, Li) in an attempt to study their structural and electronic evolution with pressure. We found that while $\mathrm{L}{\mathrm{i}}_2\mathrm{Ir}{\mathrm{O}}_3$ undergoes a structural phase transition into the dimerized state at a pressure of $P$ ∼ 4 GPa, in $\mathrm{N}{\mathrm{a}}_2\mathrm{Ir}{\mathrm{O}}_3$ the conservation of the original $C2/m$ structure up to at least 58 GPa is observed. In addition, $\mathrm{L}{\mathrm{i}}_2\mathrm{Ir}{\mathrm{O}}_3$ undergoes a sluggish structural rearrangement at the pressure range 20–40 GPa coinciding with a significant decrease in resistance. Despite dissimilar structural evolution and different mechanisms of the electrical conductivity, Arrhenius conductivity for $\mathrm{N}{\mathrm{a}}_2\mathrm{Ir}{\mathrm{O}}_3$ and Mott variable-range hopping in $\mathrm{L}{\mathrm{i}}_2\mathrm{Ir}{\mathrm{O}}_3$, both systems show a very similar $R$($P$) behavior. Namely, after a nonmonotonic decrease of the resistance $R$ and the charge gap Δ, the Δ stabilizes at about 45 GPa and even increases slightly with pressure; the $R$($T$) shows insulating behavior up to the highest pressure measured, 80 and 55 GPa, respectively. This resilient nonmetallic behavior of the studied iridates suggests a formation close to a localized-itinerant crossover of unusual electronic states, whose possible features are discussed. Unforeseeably, the $R$($P$) behavior is not dependent on the buffer element $A$, which seems essential for understanding the nature of the electrical conductivity in iridates.

We carried out a comprehensive study of magnetic critical behavior in single crystals of ternary chalcogenide $\mathrm{Fe}{\mathrm{Cr}}_2{\mathrm{Te}}_4$ that undergoes a ferrimagnetic transition below $T_c\sim$ 123 K. Detailed critical behavior analysis and scaled magnetic entropy change indicate a second-order ferrimagnetic transition. Critical exponents $\beta =0.30\left(1\right)$ with $T_c=122.4\left(5\right)\mathrm{K}, \gamma =1.22\left(1\right)$ with $T_c=122.8\left(1\right)\mathrm{K}$, and $\delta =4.24\left(2\right)$ at $T_c\sim 123\mathrm{K}$ suggest that the spins approach the three-dimensional Ising ($\beta =0.325, \gamma =1.24$, and $\delta =4.82$) model coupled with attractive long-range interactions between spins that decay as $J\left(r\right)\approx r^{-4.88}$. Our results suggest that the ferrimagnetism in $\mathrm{Fe}{\mathrm{Cr}}_2{\mathrm{Te}}_4$ is due to itinerant ferromagnetism among the antiferromagnetically coupled Cr-Fe-Cr trimers.