We study the role of the onset of Shockley states $D_s$ belonging to (111) surfaces of Cu, Ag, and Au in the Kondo effect when a magnetic impurity is deposited on them. When $D_s$ approaches the Fermi level $E_F$, which can be done by compressing (stretching) the metallic sample, we find that most of the thermodynamic and dynamic properties of the impurity are affected in a nontrivial way. We model the system by a generic Anderson impurity model and solve it by using the numerical renormalization group technique. In particular, the impurity contribution to magnetic susceptibility and entropy as a function of temperature exhibit negative values and go to zero slowly in a logarithmic shape. Furthermore, we found suppression of the spectral density weight at the Fermi level when $D_s\sim E_F$ even in the Kondo regime. As a consequence, the conductance through the impurity is strongly reduced by nearly $25\%$ of the unitary value $2e^2/h$. Finally, we analyze these features in realistic systems like Co on Ag(111) reported in the literature.

Electron spins in silicon have long coherence times and are a promising qubit platform. However, electric field noise in semiconductors poses a challenge for most single- and multiqubit operations in quantum-dot spin qubits. We investigate the dependence of low-frequency charge noise spectra on temperature and aluminum-oxide gate dielectric thickness in Si/SiGe quantum dots with overlapping gates. We find that charge noise increases with aluminum-oxide thickness. We also find strong dot-to-dot variations in the temperature dependence of the noise magnitude and spectrum. These findings suggest that each quantum dot experiences noise caused by a distinct ensemble of two-level systems, each of which has a nonuniform distribution of thermal activation energies. Taken together, our results suggest that charge noise in Si/SiGe quantum dots originates at least in part from a nonuniform distribution of two-level systems near the surface of the semiconductor.

A dissipative-driven system can generate steady-state entanglement robust to control parameters, while a quantum plasmonic system has the advantages of being strongly coupled to qubits and manipulation of electromagnetic waves in the subwavelength regime. We propose an effective multiqubit dissipative-driven entanglement model for a plasmonic system which takes into account coupling of the plasmonic cavity and qubits as well as the direct coupling between any two qubits and develop a numerical solution of the effective model on a scalable composite plasmonic structure. In this structure, the multiqubit entanglement under asymmetric detuning, especially for those combinations containing pairs of antisymmetric detuning qubits, achieves stronger steady-state entanglement measured by negativity and pairwise concurrence. The robustness of multiqubit entanglement against variations of the control parameters is also presented. In addition, the phenomenon of entanglement sudden death caused by the driving field in entanglement dynamics is investigated. Our model shows evidence that quantum plasmonics has great potential in multiqubit entanglement.

We report the study of ballistic transport in normal metal/graphene/superconductor junctions in edge-contact geometry. While in the normal state, we have observed Fabry-Pérot resonances suggesting that charge carriers travel ballistically, the superconducting state shows that the Andreev reflection at the graphene/superconductor interface is affected by these interferences. Our experimental results in the superconducting state have been analyzed and explained with a modified Octavio-Tinkham-Blonder-Klapwijk model taking into account the magnetic pair-breaking effects and the two different interface transparencies, i.e., between the normal metal and graphene, and between graphene and the superconductor. We show that the transparency of the normal metal/graphene interface strongly varies with doping at large scale, while it undergoes weaker changes at the graphene/superconductor interface. When a cavity is formed by the charge transfer occurring in the vicinity of the contacts, we see that the transmission probabilities follow the normal state conductance highlighting the interplay between the Andreev processes and the electronic interferometer.

Using a combination of picosecond acoustics and synchrotron x-ray diffraction, the sound velocities and equations of state of Fe-Si alloys have been determined over a wide range of compositions (10–29 at. % Si) and pressures (up to 65 GPa) under quasihydrostatic conditions. We observe marked variation in the elastic properties of the alloys depending on synthesis method and degree of Si ordering. In particular, it is observed that there is a sharp change in the density dependence of the sound velocities which coincides with the observation of long-range ordering by diffraction methods. This change in elasticity is probably due to a change in bond character of the Fe-Si alloy. Furthermore, the bcc-hcp transition in these alloys has been mapped to high pressures and Si contents. We observed that the onset of the transition changes by more than 6 GPa per at. % for alloys containing more than $\sim 15$ at. % Si.

${\mathrm{Ba}}_3{\mathrm{NbFe}}_3{\mathrm{Si}}_2{\mathrm{O}}_{14}$ (langasite) is structurally and magnetically single-domain chiral with the magnetic helicity induced through competing symmetric exchange interactions. Using neutron scattering, we show that the spin waves in antiferromagnetic langasite display directional anisotropy. On applying a time-reversal symmetry breaking magnetic field along the $c$ axis, the spin-wave energies differ when the sign is reversed for either the momentum transfer $\pm \vec{Q}$ or applied magnetic field $\pm {\mu }_0\mathrm{H}$. When the field is applied within the crystallographic $ab$ plane, the spin-wave dispersion is directionally isotropic and symmetric in $\pm {\mu }_0\mathrm{H}$. However, a directional anisotropy is observed in the spin-wave intensity. We discuss this directional anisotropy in the dispersion in langasite in terms of a field-induced precession of the dynamic unit cell staggered magnetization resulting from a broken twofold symmetry. Directional anisotropy, often referred to as nonreciprocal responses, can occur in antiferromagnetic phases in the absence of the Dzyaloshinskii-Moriya interaction or other effects resulting from spin-orbit coupling.

An inelastic neutron scattering study has been performed in an $S=3/2$ bilayer honeycomb lattice compound ${\mathrm{Bi}}_3{\mathrm{Mn}}_4{\mathrm{O}}_{12}\left({\mathrm{NO}}_3\right)$ at ambient and high magnetic fields. Relatively broad and monotonically dispersive magnetic excitations were observed at ambient field, where no long-range magnetic order exists. In the magnetic-field-induced long-range ordered state at 10 T, the magnetic dispersions become slightly more intense, albeit still broad as in the disordered state, and two excitation gaps, probably originating from an easy-plane magnetic anisotropy and intrabilayer interactions, develop. Analyzing the magnetic dispersions using the linear spin-wave theory, we estimated the intraplane and intrabilayer magnetic interactions, which are almost consistent with those determined by ab initio density functional theory calculations [M. Alaei et al., Phys. Rev. B 96, 140404(R) (2017)], except the third and fourth neighbor intrabilayer interactions. Most importantly, as predicted by the theory, there is no significant frustration in the honeycomb plane but frustrating intrabilayer interactions probably give rise to the disordered ground state.

We simulate the current-voltage relation of short layered superconductors, which we model as stacks of capacitively coupled Josephson junctions. The system is driven by external laser fields, in order to optically control the voltage drop across the junction. We identify parameter regimes in which supercurrents can be stabilized against thermally induced phase slips, thus reducing the effective voltage across the superconductor. Furthermore, single driven Josephson junctions are known to exhibit phase-locked states, where the superconducting phase is locked to the driving field. We numerically observe their persistence in the presence of thermal fluctuations and capacitive coupling between adjacent Josephson junctions. Our results indicate how macroscopic material properties can be manipulated by exploiting the large optical nonlinearities of Josephson plasmons.

While lattice thermal conductivity is an important parameter for many technological applications, its calculation is a time-consuming task, especially for compounds with a complex crystal structure. In this paper, we solve this problem using machine learning interatomic potentials. These potentials trained on the density functional theory results and provide an accurate description of lattice dynamics. Additionally, active learning was applied to significantly reduce the number of expensive quantum-mechanical calculations required for training and increases reliability of the potential. The ${\mathrm{CoSb}}_3$ skutterudite was considered as an example, and the solution of the Boltzmann transport equation for phonons was compared with the Green-Kubo method. We demonstrated that accurate and reliable potentials can be obtained by performing just a few hundred quantum-mechanical calculations. The potentials reproduce not only the vibrational spectrum, but also the lattice thermal conductivity, as calculated by various methods.

We analyze the effects of disorder on the correlation functions of one-dimensional quantum models of fermions and spins with long-range interactions that decay with distance $\ell$ as a power law $1/{\ell }^{\alpha }$. Using a combination of analytical and numerical results, we demonstrate that power-law interactions imply a long-distance algebraic decay of correlations within disordered-localized phases, for all exponents $\alpha$. The exponent of algebraic decay depends only on $\alpha$, and not, e.g., on the strength of disorder. We find a similar algebraic localization for wave functions. These results are in contrast to expectations from short-range models and are of direct relevance for a variety of quantum mechanical systems in atomic, molecular, and solid-state physics.

Electric fields are increasingly used for coherently manipulating spin states in semiconductor and molecular systems. Here we discuss the spin manipulation allowed by the modulation of the main parameters entering the Hamiltonians of molecular spin clusters. In particular, we focus on transitions between states that differ in terms of scalar quantities, such as the total- or the partial-spin sums, but have vanishing expectation values of both the total- and the single-spin projections. These conditions supposedly define subspaces that are immune to the main decoherence mechanisms and that cannot be identified in individual spins, where the total- and the partial-spin sums are fixed or cannot be defined. We show that the desired transitions can in principle be realized in systems as simple as a spin dimer, by modulating the $g$-factor, the axial anisotropy, or, under suitable conditions, the exchange interaction.

Low-energy impurity diffusion in a host material is often regarded as an adiabatic process, characterized by its adiabatic potential energy barrier. Here we show that the diffusion process in semiconductors can involve nonadiabatic electron excitations, rending it to be a more complicated process. Impurity diffusion in a device at working temperature can pump one electron up from localized impurity state into the host conduction band and causes the impurity to be a dynamic donor since it temporarily loses its electron to the host. This nonadiabatic process, against a common belief, fundamentally change the diffusion behavior, including its barrier height and diffusion path. Although we mainly demonstrate this process with Au metal impurity in bulk Si through time-dependent density functional theory simulations, we believe this could be a rather common phenomenon as it is shown that the similar phenomena also exist in Zn, Cd impurities diffusion in bulk Si, and Ti diffusion in $\mathrm{Ti}{\mathrm{O}}_2$. We believe this study can open up a new direction of inquiry for such diffusion behavior in semiconductor.

In this paper we investigate the electronic and optical dielectric properties of lateral and vertical heterostructures composed of two-dimensional (2D) $\mathrm{Zr}{\mathrm{S}}_2$ and $\mathrm{Hf}{\mathrm{S}}_2$ monolayers based on density-functional theory. First, we show that the bulk and monolayer $\mathrm{Zr}{\mathrm{S}}_2$ and $\mathrm{Hf}{\mathrm{S}}_2$ as well as the vertical ${\left(\mathrm{Zr}{\mathrm{S}}_2\right)}_m/{\left(\mathrm{Hf}{\mathrm{S}}_2\right)}_n$ heterostructures are indirect band-gap semiconductors, while the lateral heterostructures exhibit an indirect to direct band-gap transition. Then we demonstrate that the optical properties of the bulk and monolayer $\mathrm{Hf}{\mathrm{S}}_2$ and $\mathrm{Zr}{\mathrm{S}}_2$ are strongly anisotropic; for the bulk $\mathrm{Hf}{\mathrm{S}}_2$ and $\mathrm{Zr}{\mathrm{S}}_2$, the in-plane components of the dielectric function are negative in a certain frequency band, where they can work as naturally hyperbolic metamaterials. Interestingly, the vertical heterostructures also possess a hyperbolic region, whose position and width can be tunable with the thickness ratio of constituents. It is also found that the $\left(\mathrm{Zr}{\mathrm{S}}_2\right)/\left(\mathrm{Hf}{\mathrm{S}}_2\right)$ vertical heterostructures can enhance spontaneous emission and about 100-fold improvement of the Purcell factor is obtained. These results prove the feasibility of 2D material heterostructures to realize tunable hyperbolic metamaterials; the heterostructures present a promising opportunity for the practical applications in light-generation technologies.

Raman spectroscopy is used to control the kinetics of the magnetostructural transition in ${\mathrm{Li}}_2{\mathrm{RuO}}_3$ crystals from the low-temperature dimerized phase ($P{2}_1/m$) to the high-temperature phase ($C2/m$) at $T_c\sim 550$ K accompanied by the destruction of the spin singlets on the Ru-Ru bonds. The theoretical group analysis and the results of the density functional theory calculations of the lattice dynamics were used to identify phonon modes that are sensitive to the transition. The observed anomalies in the behavior of the self-energies of such phonons near $T_c$ suggest their connection either with possible dynamic disordering of Ru atomic positions or with spin-phonon interaction.

Single crystal samples of the frustrated quasi-one-dimensional quantum magnet ${\mathrm{Rb}}_2{\mathrm{Cu}}_2{\mathrm{Mo}}_3{\mathrm{O}}_{12}$ are investigated by magnetic, thermodynamic, and electron spin resonance (ESR) measurements. Quantum phase transitions between the gapped, magnetically ordered, and fully saturated phases are observed. Surprisingly, the former has a distinctive three-dimensional character, while the latter is dominated by one-dimensional quantum spin fluctuations. The entire $H-T$ phase diagram is mapped out and found to be substantially anisotropic. In particular, the lower critical fields differ by over 50% depending on the direction of applied field, while the upper ones are almost isotropic, as is the magnetization above saturation. The ESR spectra are strongly dependent on field orientation and point to a helical structure with a rigidly defined spin rotation plane.

Based on the Korringa-Kohn-Rostoker Green's function technique, we present a computational scheme for calculating the electronic structure of layered systems with homogeneous spin-spiral magnetic state. From the self-consistent nonrelativistic calculations, the total energy of the spin-spiral states is determined as a function of the wave vector, while a relativistic extension of the formalism in first order of the spin-orbit coupling gives an access to the effect of the Dzyaloshinskii-Moriya interactions. We demonstrate that the newly developed method properly describes the magnetic ground state of a Mn monolayer on W(001) and that of a Co monolayer on Pt(111). The obtained spin-spiral energies are mapped to a classical spin model, the parameters of which are compared to those calculated directly from the relativistic torque method. In case of the Co/Pt(111) system, we find that the isotropic interaction between the Co atoms is reduced and the Dzyaloshinskii-Moriya interaction is increased when capped by a Ru layer. In addition, we perform spin-spiral calculations on Ir/Fe/Co/Pt and Ir/Co/Fe/Pt multilayer systems and find a spin-spiral ground state with very long wavelength due to the frustrated isotropic couplings between the Fe atoms, whereas the Dzyaloshinskii-Moriya interaction strongly depends on the sequence of the Fe and Co layers.

We use inelastic neutron scattering to study spin excitation anisotropy in mechanically detwinned $\mathrm{Ba}{\left({\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x\right)}_2{\mathrm{As}}_2$ with $x=0.048$ and 0.054. Both samples exhibit a tetragonal-to-orthorhombic structural transition at $T_s$, a collinear static antiferromagnetic order at wave vector ${\mathbf{Q}}_1={\mathbf{Q}}_{\mathrm{AF}}=(1,0)$ below the $\mathrm{N}\acute{\mathrm{e}}\mathrm{el}$ temperature $T_N$, and superconductivity below $T_c$ ($T_s>T_N>T_c$). In the high-temperature paramagnetic tetragonal phase ($T\gg T_s$), spin excitations centered at ${\mathbf{Q}}_1$ and ${\mathbf{Q}}_2=(0,1)$ are gapless and have fourfold ($C_4$) rotational symmetry. On cooling to below $T_N$ but above $T_c$, spin excitations become highly anisotropic, developing a gap at ${\mathbf{Q}}_2$ but still are gapless at ${\mathbf{Q}}_1$. Upon entering into the superconducting state, a neutron spin resonance appears at ${\mathbf{Q}}_1$ with no magnetic scattering at ${\mathbf{Q}}_2$. By comparing these results with those from angle-resolved photoemission spectroscopy experiments, we conclude that the anisotropic shift of the $d_{yz}$ and $d_{xz}$ bands in detwinned $\mathrm{Ba}{\left({\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x\right)}_2{\mathrm{As}}_2$ below $T_s$ is associated with the spin excitation anisotropy, and the superconductivity-induced resonance arises from the electron-hole Fermi surface nesting of quasiparticles with the $d_{yz}$ orbital characters.

We study the combined effects of fluctuations and frustration in frustrated antiferromagnets described by the spin-1/2 easy-axis XXZ model with transverse magnetic fields, and propose its promising quantum simulation with a coherently coupled binary mixture of Fermi atoms in a triangular optical lattice. We perform a cluster mean-field plus scaling analysis to show that the ground state exhibits several nontrivial magnetic phases and a spin reorientation transition caused by the quantum order-by-disorder mechanism. Moreover, we find from Monte Carlo simulations that thermal fluctuations induce an unexpected coexistence of Berezinskii-Kosterlitz-Thouless physics and long-range order in different correlators. These predictions, besides being relevant to present and future experiments on triangular antiferromagnetic materials, can be tested in the laboratory with the combination of the currently available techniques for cold atoms.

Antiferromagnetic (AF) domain walls have recently attracted revived attention, not only in the emerging field of AF spintronics, but also more specifically for offering fast domain wall velocities and dynamic excitations up to the terahertz frequency regime. Here, we introduce an approach to nucleate and stabilize an AF domain wall in a synthetic antiferromagnet (SAF). We present experimental and micromagnetic studies of the magnetization reversal in ${\left[{\left(\mathrm{Co}/\mathrm{Pt}\right)}_{X-1}/\mathrm{Co}/\mathrm{Ir}\right]}_{N-1}{\left(\mathrm{Co}/\mathrm{Pt}\right)}_X$ SAFs, where interface-induced perpendicular magnetic anisotropy (PMA) and AF interlayer exchange coupling (IEC) are completely controlled via the individual layer thicknesses within the multilayer stack. By combining strong PMA with even stronger AF-IEC, the SAF reveals a collective response to an external magnetic field applied normal to the surface, and we stabilize the characteristic surface spin-flop (SSF) state for an even number $N$ of AF-coupled ${\left(\mathrm{Co}/\mathrm{Pt}\right)}_{X-1}/\mathrm{Co}$ multilayer blocks. In the SSF state our system provides a well-controlled and fully tunable vertical AF domain wall, easy to integrate as no single-crystal substrates are required and with uniform two-dimensional magnetization in the film plane for further functionalization options, such as lateral patterning via lithography.