We study optical harmonic generation on the $1S$ exciton-polariton in the semiconductor ZnSe. Intense and spectrally narrow exciton resonances are found in optical second (SHG), third (THG), and fourth (FHG) harmonic generation spectra. The resonances are shifted to higher energy by 3.2 meV from the exciton energy in the linear reflectivity spectrum. Additional resonances are observed in the THG and FHG spectra and assigned to combinations of incident and backscattered photons in the crystal. Rotational anisotropy diagrams are measured and further information on the origin of the optical harmonic generation and the involved exciton states is obtained by a symmetry analysis using group theory in combination with a microscopic consideration.

We investigate the nonequilibrium spin transport through an electron-phonon coupled quantum dot, on which an external magnetic field $B_0$ and a rotating magnetic field $[B_{1}cos\left({\omega }_{1}t\right),B_{1}sin\left({\omega }_{1}t\right)]$ are applied. It is found that the spin current is significantly affected by the destructive interference between electron tunneling waves through different spin channels. As a result, dips appear in the spin current as a function of ${\omega }_1$ every time the Rabi frequency is tuned to be integral numbers of the phonon frequency. It is also found that the main spin current peak does not always exist at the resonant rotating frequency ${\omega }_1=g{\mu }_B{B}_0$.

We generalize the celebrated semiclassical wave-packet approach from the adiabatic to the nonadiabatic regime. A unified description covering both of these regimes is particularly desired for systems with spatially varying band structures where band gaps of various sizes are simultaneously present, e.g., in moiré patterns. For a single wave packet, alternative to the previous derivation by Lagrangian variational approach, we show that the same semiclassical equations of motion can be obtained by introducing a spatial-texture-induced force operator similar to the Ehrenfest theorem. For semiclassically computing the current, the ensemble of wave packets based on adiabatic dynamics is shown to well correspond to a phase-space fluid for which the fluid's mass and velocity are two distinguishable properties. This distinction is not inherited to the ensemble of wave packets with the nonadiabatic dynamics. We extend the adiabatic kinetic theory to the nonadiabatic regime by taking into account decoherence, whose joint action with electric field favors certain forms of interband coherence. The steady-state density matrix as a function of the phase-space variables is then phenomenologically obtained for calculating the current. The result, applicable with a finite electric field, expectedly reproduces the known adiabatic limit by taking the electric field to be infinitesimal, and therefore attains a unified description from the adiabatic to the nonadiabatic situations.

Understanding the effect of electric fields on defect creation and diffusion in metal oxides is of fundamental importance for developing accurate models of oxide degradation in electronic devices and dielectric breakdown. We use the Berry phase operator method within density functional theory to calculate how an applied electric field affects barriers for the creation of oxygen vacancy-interstitial defect pairs (DPs) and diffusion of interstitial O ions in monoclinic ($m$-)${\mathrm{HfO}}_2$. The results demonstrate that even close to breakdown fields, barriers for DP generation exceed 6 eV in the perfect $m-{\mathrm{HfO}}_2$ lattice. Simulated injection of extra electrons from electrodes significantly lowers barriers for the creation of DPs, which are further reduced by the field to around 1 eV. Thus, bias application facilitates the injection of electrons into the oxide; these extra electrons reduce energy barriers for the creation of O vacancies, and these barriers as well as those for O ion diffusion are further lowered by the field. We find that, within a linear regime, the electric field modulates the barrier height by a dot product between the electric field and the electric dipole at the zero-field transition state to good accuracy.

String breaking is a central dynamical process in theories featuring confinement, where a string connecting two charges decays at the expense of the creation of new particle-antiparticle pairs. Here, we show that this process can also be observed in quantum Ising chains where domain walls get confined either by a symmetry-breaking field or by long-range interactions. We find that string breaking occurs, in general, as a two-stage process. First, the initial charges remain essentially static and stable. The connecting string, however, can become a dynamical object. We develop an effective description of this motion, which we find is strongly constrained. In the second stage, which can be severely delayed due to these dynamical constraints, the string finally breaks. We observe that the associated timescale can depend crucially on the initial separation between domain walls and can grow by orders of magnitude by changing the distance by just a few lattice sites. We discuss how our results generalize to one-dimensional confining gauge theories and how they can be made accessible in quantum simulator experiments such as Rydberg atoms or trapped ions.

The interplay of synchronization and topological band structures with symmetry-protected midgap states under the influence of driving and dissipation is largely unexplored. Here, we consider a trimer chain of electron shuttles, each consisting of a harmonic oscillator coupled to a quantum dot positioned between two electronic leads. Each shuttle is subject to thermal dissipation and undergoes a bifurcation toward self-oscillation with a stable limit cycle if driven by a bias voltage between the leads. By mechanically coupling the oscillators together, we observe synchronized motion at the ends of the chain, which can be explained using a linear stability analysis. Because of the inversion symmetry of the trimer chain, these synchronized states are topologically protected against local disorder. Furthermore, with current experimental feasibility, the synchronized motion can be observed by measuring the dot occupation of each shuttle. Our results open another avenue to enhance the robustness of synchronized motion by exploiting topology.

Motivated by recent experiments on interacting bosons in a quasi-one-dimensional optical lattice [M. Rispoli et al., Nature (London) 573, 385 (2019)], we analyze theoretically properties of the system in the crossover between delocalized and localized regimes. Comparison of time dynamics for uniform and density-wave-like initial states enables a demonstration of the existence of the mobility edge. To this end we define a different observable, the mean speed of transport at long times. It gives us an efficient estimate of the critical disorder for the crossover. We also show that the mean velocity growth of occupation fluctuations close to the edges of the system carries similar information. Using the quantum quench procedure, we show that it is possible to probe the mobility edge for different energies.

Several previous studies reported that a one-dimensional Heisenberg chain model is inadequate in describing the magnetic properties of the low-dimensional quantum antiferromagnet $\alpha \text{−}{\mathrm{CuV}}_2{\mathrm{O}}_6$, but the origin for this observation has remained unclear. We have reinvestigated the magnetic properties of $\alpha \text{−}{\mathrm{CuV}}_2{\mathrm{O}}_6$ and found that our anisotropic magnetic susceptibility, neutron-powder diffraction, and electron paramagnetic spin-resonance measurements are in good agreement with extensive density-functional theory ($\mathrm{DFT}+U$) total energy calculations which indicate that the correct spin lattice model for $\alpha \text{−}{\mathrm{CuV}}_2{\mathrm{O}}_6$ is rather a $S=1/2$ 2D-Heisenberg antiferromagnetic lattice. The magnetic susceptibility data are well described by a rectangular Heisenberg antiferromagnet with anisotropy ratio $\alpha \sim$ 0.7 consistent with the DFT results. Quantum Monte Carlo simulations of the magnetic susceptibilities for a rectangular lattice Heisenberg antiferromagnet were performed in the anisotropy range 0.5 $\le \alpha \le$ 1.0. The results of the Quantum Monte Carlo calculations were cast into a Padé approximant which was used to fit the temperature-dependent magnetic susceptibility data. Neutron-powder-diffraction measurements were used to conclusively solve the collinear antiferromagnetic structure of $\alpha \text{−}{\mathrm{CuV}}_2{\mathrm{O}}_6$ below the Néel temperature of $\sim 22.4$ K.

Phase-coherent spin waves can be generated by magnetic field pulse, spin current pulse, or optical pulse. Here we use optical spin-orbit torque, originating from the conversion of an optical pulse into a spin-polarized current pulse, to excite spin waves in the frequency range from GHz to THz. We investigate the frequency, amplitude, and damping of the spin waves of Co thin films. From the frequency analysis, we determine the stiffness of Co spin waves to be $5\mathrm{meV}\mathrm{n}{\mathrm{m}}^2$. From the amplitude analysis, we show that the Co layer acts as a cavity for spin waves. From the damping analysis, we observe that the damping enhancement due to the spin pumping effect is about two times larger in spin waves than in uniform precession.

The ultrafast electron transport is central to the requirement of all-electrically controlled spintronics in metallic collinear antiferromagnetic (AFM) ${\mathrm{Mn}}_2\mathrm{Au}$ thin films within the terahertz (THz) range. Here we use the THz time-domain spectroscopy to measure the conductivity spectra of ${\mathrm{Mn}}_2\mathrm{Au}$ thin films with different thicknesses and temperatures. We demonstrate an onset of carrier localization in ${\mathrm{Mn}}_2\mathrm{Au}$ with decreasing the film thickness from 15 to 5 nm. By using optical pump-THz probe spectroscopy, we found the surface trapping states in the thinnest ${\mathrm{Mn}}_2\mathrm{Au}$ film limit its photoconductivity significantly. The DC conductivity and scattering time become larger and longer with decreasing temperature for all samples, which suggests that ${\mathrm{Mn}}_2\mathrm{Au}$ behaves in a typical metallic fashion. The THz optical and conductivity properties of ${\mathrm{Mn}}_2\mathrm{Au}$ films will advance our understanding of ultrafast electronic transport processes in AFM ${\mathrm{Mn}}_2\mathrm{Au}$ thin films.

We performed nuclear magnetic resonance (NMR) and muon spin relaxation (μSR) experiments to identify the magnetic ground state of the frustrated quantum $A$-site spinel, $\mathrm{Cu}{\mathrm{Al}}_2{\mathrm{O}}_4$. Our results verify that the ground state does not exhibit a long-range magnetic ordering, but a glasslike transition manifests at $T^{*}=2.3\mathrm{K}$. However, the Gaussian shape and the weak longitudinal field dependence of μSR spectra below $T^{*}$ show that the ground state has dynamic spin fluctuations, distinct from those of conventional spin glasses.

Magnetic skyrmions with a topological particle nature are considered as potential information carriers for future spintronics memory and logic devices. The stabilization of magnetic skyrmions at zero magnetic field in nanostructured components is a prerequisite for incorporating them into advanced nonvolatile memory devices. Here, using a real-space phase field model based on Ginzburg-Landau theory, we demonstrate that ferroelectric polarization can stabilize magnetic skyrmions at zero magnetic field in multiferroic heterostructures composed of MnSi, ${\mathrm{PbTiO}}_3$, and ${\mathrm{SrTiO}}_3$. The stabilization of magnetic skyrmions in multiferroic heterostructures essentially depends on the directions of spontaneous polarizations and the size ratios of different constituents, in which polarization-induced inhomogeneous strain plays an important role. By applying an electric field, the polarization switching takes place in the ferroelectric constituent and the polarization-induced strain changes in the multiferroic heterostructures, resulting in a transition from skyrmion to helical phase in the ferromagnetic constituent. In addition, the skyrmion and helical phases can coexist in the absence of an external field and be switched reversibly by a local magnetic field with a small magnitude. Stabilization and control of magnetic skyrmions by spontaneous polarization at zero magnetic field may create additional opportunities for nonvolatile skyrmion memory devices.

The high-$T_c$ phase of lanthanum hydrides deserves special attention because this phase displays the highest observed critical temperature. We focus on the evaluation of the critical temperature, the isotope coefficient, and their pressure dependence in this phase ($Fm\overline{3}m$ phase of the $\mathrm{La}{\mathrm{H}}_{10}$ compound). By combining the method of two coupling constants, ${\lambda }_{\mathrm{opt}}$ and ${\lambda }_{\mathrm{ac}}$, and two characteristic frequencies with first-principle calculations we are able to analyze the role of light hydrogen in the amplitude of the critical temperature in this unique compound. Specifically, the critical temperature of 254 K and the isotope coefficient of 0.45 are evaluated at high pressure, and the results are in good agreement with the experimental data. The peculiar decrease in $T_c$ upon further increase in pressure is explained by the presence of a flat region on the Fermi surface and the appearance of a two-gap structure, which should be experimentally observable by tunneling spectroscopy.

It has been proposed that the deconfined criticality in $(2+1)d$—the quantum phase transition between a Néel antiferromagnet and a valence-bond solid (VBS)—may actually be pseudocritical, in the sense that it is a weakly first-order transition with a generically long correlation length. The underlying field theory of the transition would be a slightly complex (nonunitary) fixed point as a result of fixed points annihilation. This proposal was motivated by existing numerical results from large scale Monte Carlo simulations as well as a conformal bootstrap. However, an actual theory of such a complex fixed point, incorporating key features of the transition such as the emergent SO(5) symmetry, is so far absent. Here we propose a Wess-Zumino-Witten (WZW) nonlinear sigma model with level $k=1$, defined in $2+\epsilon$ dimensions, with target space $S^{3+\epsilon }$ and global symmetry $\text{SO}(4+\epsilon )$. This gives a formal interpolation between the deconfined criticality at $d=3$ and the $\text{SU}{\left(2\right)}_1$ WZW theory at $d=2$ describing the spin-$1/2$ Heisenberg chain. The theory can be formally controlled, at least to leading order, in terms of the inverse of the WZW level $1/k$. We show that at leading order there is a fixed point annihilation at $d^{*}\approx 2.77$, with complex fixed points above this dimension including the physical $d=3$ case. The pseudocritical properties such as correlation length, scaling dimensions, and the drifts of scaling dimensions as the system size increases, calculated crudely to leading order, are qualitatively consistent with existing numerics.

The interaction of energetic ions with the electronic and ionic system of target materials is an interesting but challenging multiscale problem, and understanding of the early stages after impact of heavy, initially charged ions is particularly poor. At the same time, energy deposition during these early stages determines later formation of damage cascades. We address the multiscale character by combining real-time time-dependent density functional theory for electron dynamics with molecular dynamics simulations of damage cascades. Our first-principles simulations prove that core electrons affect electronic stopping and have an unexpected influence on the charge state of the projectile. We show that this effect is absent for light projectiles, but dominates the stopping physics for heavy projectiles. By parametrizing an inelastic energy loss friction term in the molecular dynamics simulations using our first-principles results, we also show a qualitative influence of electronic stopping physics on radiation-damage cascades.

Radiative heat transfer (RHT) and radiative thermal energy (RTE) for two-dimensional (2D) nanoparticle ensembles are investigated in the framework of many-body radiative heat transfer theory. We consider nanoparticles made of different materials: metals (Ag), polar dielectrics (SiC), or insulator-metallic phase-change materials $\left({\mathrm{VO}}_2\right)$. We start by investigating the RHT between two parallel 2D finite-size square-lattice nanoparticle ensembles, with particular attention to many-body interactions (MBI) effects. We fix the particle radius $\left(a\right)$ as the smallest length scale, and we describe the electromagnetic scattering from particles within the dipole approximation. Depending on the minimal distance between the in-plane particles (the lattice spacing $p$ for periodic systems), on the separation $d$ between the two lattice and on the thermal wavelength ${\lambda }_T=\hslash c/k_{B}T$, we systematically analyze the different physical regimes characterizing the RHT. Four regimes are identified, rarefied regime, dense regime, non-MBI regime, and MBI regime, respectively. When $p\ll {\lambda }_T^{}$, a multiple scattering of the electromagnetic field inside the systems gives rise to a MBI regime. MBI effects manifest themselves in different ways, depending on the separation $d$: (a) If $d>{\lambda }_T$, due to the pure intra-ensemble MBI inside each 2D ensemble, the total heat conductance is less affected, and the thermal conductance spectrum manifests a single peak which is nonetheless shifted with respect to the one typical of two isolated nanoparticles. (b) If $d<{\lambda }_T$, there is a strong simultaneous intra-ensemble and inter-ensemble MBI. In this regime there is a direct quantitative effect on the heat conductance, in addition to a qualitative effect on the thermal conductance spectrum, which now manifests a new second peak. As for the RTE, to correctly describe the radiation emitted by metallic nanoparticles, we derive an expression of the Poynting vector including also magnetic contribution, in addition to the electric one. By analyzing both periodic and nonperiodic ensembles, we show that the RTE emitted by a single 2D nanoparticle ensemble is sensitive to the particle distribution. As instance, we see that the RTE emitted by 2D concentric-ring-configuration ensemble has an inhibition feature near the center of the ensemble.

We consider the disorder-induced correction to the minimal conductance of an anisotropic two-dimensional Dirac node or a three-dimensional Weyl node. An analytical expression is derived for the correction $\delta G$ to the conductance of a finite-size sample by an arbitrary potential, without taking the disorder average, in second-order perturbation theory. Considering a generic model of a short-range disorder potential, this result is used to compute the probability distribution $P\left(\delta G\right)$, which is compared to the numerically exact distribution obtained using the scattering matrix approach. We show that $P\left(\delta G\right)$ is Gaussian when the sample has a large width-to-length ratio and study how the expectation value, the standard deviation, and the probability of finding $\delta G<0$ depend on the anisotropy of the dispersion.

Nonequilibrium magnetic dynamics has been investigated in the Co/Au bimetallic nanoparticle system. The system exhibits typical superparamagnetic behavior at higher temperatures and unequivocal hallmarks of magnetic relaxation process at $T\sim 7\mathrm{K}$. Since different scenarios of magnetic transition can be hypothesized here, detailed analysis of magnetization ac and dc experimental data has been performed. Specific methods have been employed in order to reveal the nature of the examined process. The observation of critical dynamics [power-law divergence of the relaxation time at $T_g$; dynamic scaling of ${\chi }^{\prime \prime }(T,f)$ data] and aging effects in the presence of strong interparticle interactions provide evidence that the system undergoes a transition to a super-spin-glass state rather than to a super-spin-blocked state.

Using a combination of hysteresis loop, Kerr microscope, and x-ray magnetic circular dichroism measurements, we investigated the antiferromagnetic (AFM) domain switching process modulated by the sub-nm thick Co inserting layer in a single crystalline Fe/Co/CoO/MgO(001). The CoO AFM domain switching occurs at lower temperature for the thicker Co interlayer, and the activation energy barrier of CoO AFM domain switching decreases as the Co interlayer thickness increases. The exchange coupling strength between the AFM spins in CoO layer and the ferromagnetic spins in the Fe/Co bilayer is found to be independent of the Co layer thickness. Our results suggest an approach to modulate the dynamic properties of AFM domains with an interfacial modification.

Investigation on terahertz (THz) emission is important not only for fundamental research but also for practical application. A rapidly developing approach to generate THz emission is using a heterostructure composed of ferromagnetic metal (FM) and nonmagnetic metal (NM) which is pumped by femtosecond laser pulse. Previous works in this regard mainly focused on bilayer with heavy NM (such as Pt) with a large spin Hall angle. Here we present a comprehensive investigation on THz emission from Co/Al heterostructures stemming from the inverse spin Hall effect. It is surprising to find that although the spin Hall angle of Al is two orders of magnitude smaller than that of Pt, the measured THz signals are close to one-third of that of the typical Co/Pt heterostructures. To explore the underlying physics, theoretical models are employed to investigate the spin-related properties of Al. We obtain that the upper limit of the spin Hall angle of Al is 0.55% and the spin-diffusion length is 2.2 ± 0.2 nm; the diffusion length is comparable to that of Pt (2.3 ± 0.1 nm). This work shows that it is worth revisiting light metals for the spintronic THz emitter.