We discuss the temperature-dependent thermoelectric transport properties of semiconductor nanostructures comprising a quantum dot coupled to quantum wires: the thermal dependence of the electrical conductance, thermal conductance, and thermopower. We explore the universality of the thermoelectric properties in the temperature range associated with the Kondo crossover. In this thermal range, general arguments indicate that any equilibrium property's temperature dependence should be a universal function of the ratio $T^{*}=T/T_K$, where $T_K$ is the Kondo temperature. Considering the particle-hole symmetric, spin-degenerate Anderson model, the zero-bias electrical conductance has already been shown to map linearly onto a universal conductance through a quantum dot embedded or side-coupled to a quantum wire. Employing rigorous renormalization-group arguments, we calculate universal thermoelectric transport coefficients that allow us to extend this result to the thermopower and the thermal conductance. We present numerical renormalization-group results to illustrate the physics in our findings. Applying the universal thermoelectric coefficients to recent experimental results of the electrical conductance and thermovoltages versus $V_{\mathrm{gate}}$, at different temperatures in the Kondo regime, we calculate all the thermoelectric properties and obtain simple analytical fitting functions that can be used to predict the experimental results of these properties. However, we cannot check all of them, due to the lack of available experimental results over a broad temperature range.

We performed ambivalent doping study on single crystals of two sets, $\mathrm{Sn}{\mathrm{Se}}_{1-x}{\mathrm{As}}_x$ and ${\mathrm{Sn}}_{1-x}{\mathrm{As}}_x\mathrm{Se}$, with the aim to explore the interaction of doping species with intrinsic defects. We found that As atoms substitute preferentially for Se atoms in both sets forming the extrinsic substitutional point defect ${\mathrm{As}}_{\mathrm{Se}}$. In the first set, As lowers the concentration of Sn vacancies, $V_{\mathrm{Sn}}$, by an order of magnitude compared to undoped stoichiometric SnSe crystal. The remaining Sn vacancies are preferentially coordinated with As atoms. Importantly, a very low concentration of As led to healing process of hosting structure in terms of intrinsic point defects and eventual $\mathrm{Sn}{\mathrm{Se}}_2$ inclusions. This is reflected in an increase of the Hall mobility and drop of the Hall concentration. In the second set, the concentration of Sn vacancies markedly increases upon doping in contrast to the first set. Additionally, the coordination of Sn vacancies by As atoms is less evident due to the high concentration of vacancies. The substitutional defect ${\mathrm{As}}_{\mathrm{Se}}$ is a deep-level defect that produces no free carriers at room temperature. Moreover, the coupling of $V_{\mathrm{Sn}}$ to ${\mathrm{As}}_{\mathrm{Se}}$ defects increases their activation energy. This results in an unprecedentedly low Hall concentration in SnSe which stays below ${10}^{16}\mathrm{c}{\mathrm{m}}^{–3}$ for $x=0.0075$. The present study indicates that doping of SnSe is a rather complex process that generally includes a strong interaction of doping atoms with the hosting structure. On the other hand, such doping allows adjustment of the type and concentration of defects. The present study reveals a general tendency of point defects to clustering, which modifies the properties of point defects markedly.

Improving the control of heat flow at the nanoscale is essential for promoting its applications in many fields, such as energy conversion, thermal informatics, and communication technologies. Here we perform a systematic study on the synergistic effect of screw dislocations and surface resonators on thermal transport of Si nanowires and the corresponding mechanisms based on molecular dynamics simulations. We uncover that screw dislocations reduce the thermal conductivity by enhancing the anharmonicity of nanowires due to the nonhomogeneous stress field. For resonant structures, we demonstrate that the suppression of relaxation time is the main mechanism for thermal conductivity reduction. The suppression of relaxation time by more than two orders of magnitude below 4 THz dramatically reduces the resonant structure thermal conductivity, while the previously proposed group velocity reduction mechanism can only impede phonon transport beyond 4 THz slightly. By comparing the mechanisms produced by dislocations and resonators, we find that the resonators have a stronger effect over screw dislocations in impeding the phonon transport at low frequencies while it becomes opposite at high frequencies. As a result, they can be combined together to manipulate phonon transport synergistically at all frequencies. Our findings not only provide insights into the mechanisms of thermal conductivity engineering by screw dislocations and surface resonators, but they also illustrate a paradigm for ultralow thermal conductivity design through the tailoring of the entire frequency range of phonon transport.

Considering von Neumann expression for reduced density matrix as thermodynamic entropy of a system strongly coupled to baths, we use nonequilibrium Green's function (NEGF) techniques to derive bath and energy resolved expressions for entropy, entropy production, and information flows. The consideration is consistent with dynamic (quantum transport) description and expressions reduce to expected forms in limiting cases of weak coupling or steady state. Formulation of the flows in terms of only system degrees freedom is convenient for simulation of thermodynamic characteristics of open nonequilibrium quantum systems. We utilize standard NEGF for derivations in noninteracting systems, Hubbard NEGF is used for interacting systems. Theoretical derivations are illustrated with numerical simulations within generic junction models.

We have studied the monoatomic-layer superconductors formed on semiconducting substrates and investigated the role of steps in superconducting properties. In the present study we found that the steps of the Si(111)-√3×√43 Pb reconstructed structure significantly disrupt the electronic states of the superconducting atomic layer and hold Josephson-like vortices under the out-of-plane magnetic field. Because of the strong decoupling, narrow terraces whose width is less than three times the coherence length behave like superconducting nano stripes and exhibit large critical field against the out-of-plane magnetic field. Since monolayer superconductors are intrinsically strong against in-plane magnetic field, superconducting materials highly tolerant against magnetic fields in all directions might be tailored by controlling the step arrangement in the atomic-layer superconductors.

We study the effects of a periodically driven electric field applied to a variety of tight-binding models in one dimension. We first consider a noninteracting system with or without a staggered on-site potential, and we find that periodic driving can generate states localized completely or partially near the ends of a finite-sized system. Depending on the system parameters, such states have Floquet eigenvalues lying either outside or inside the continuum of eigenvalues of the bulk states. In the former case, we find that these states are completely localized at the ends and are true edge states, while in the latter case, the states are not completely localized at the ends although the localization can be made almost perfect by tuning the driving parameters. We then consider a system of two bosonic particles which have an on-site Hubbard interaction and show that a periodically driven electric field can generate two-particle states which are localized at the ends of the system. We show that many of these effects can be understood using a Floquet perturbation theory which is valid in the limit of a large staggered potential or large interaction strength. Some of these effects can also be understood qualitatively by considering time-independent Hamiltonians which have a potential at the sites at the edges; Hamiltonians of these kinds effectively appear in a Floquet-Magnus analysis of the driven problem. Finally, we discuss how the edge states produced by periodic driving of a noninteracting system of fermions can be detected by measuring the differential conductance of the system.

Two opposite chiralities of Dirac electrons in a two-dimensional (2D) graphene sheet modify the Friedel oscillations strongly: electrostatic potential around an impurity in graphene decays much faster than in 2D electron gas. At distances $r$ much larger than the de Broglie wavelength, it decays as $1/r^3$. Here we show that a weak uniform magnetic field affects the Friedel oscillations in an anomalous way. It creates a field-dependent contribution which is dominant in a parametrically large spatial interval $p_0^{-1}\lesssim r\lesssim k_F{l}^2$, where $l$ is the magnetic length, $k_F$ is Fermi momentum, and $p_0^{-1}={\left(k_{F}l\right)}^{4/3}/k_F$. Moreover, in this interval, the field-dependent oscillations do not decay with distance. The effect originates from a spin-dependent magnetic phase accumulated by the electron propagator. The obtained phase may give rise to novel interaction effects in transport and thermodynamic characteristics of graphene and graphene-based heterostructures.

Direct laser interference patterning (DLIP) with ultrashort laser pulses (ULPs) represents a precise and fast technique to produce tailored periodic submicrometer structures on various materials. In this work, an experimental and theoretical approach is presented to investigate the fundamental mechanisms for the formation of unprecedented laser-induced topographies on stainless steel following proper combinations of DLIP with ULPs. The combined spatial and temporal shaping of the pulse increases the level of control over the structure while it brings insights into the structure formation process. The aim of DLIP is to determine the initial conditions of the laser-matter interaction by defining an ablated region while double ULPs are used to control the reorganization of the self-assembled laser-induced submicrometer sized structures by exploiting the interplay of different absorption and excitation levels coupled with the melt hydrodynamics induced by the first of the double pulses. A multiscale physical model is presented to correlate the interference period, polarization orientation, and number of incident pulses with the induced morphologies. Special emphasis is given to electron excitation, relaxation processes, and hydrodynamical effects that are crucial to the production of complex morphologies. Results are expected to derive knowledge of laser-matter interaction in combined DLIP and ULP conditions and enable enhanced fabrication capabilities of complex hierarchical submicrometer sized structures for a variety of applications.

$\mathrm{NdGa}{\mathrm{O}}_3$ (NGO) is an important substrate material often used in depositing functional oxide thin films. We have carried out temperature-dependent polarized Raman and infrared spectroscopy studies along with theoretical calculations on single-crystal NGO, which revealed a structural phase transition from centrosymmetric space group Pbnm to noncentrosymmetric space group $Pbn{2}_1$ at ∼200 K. The microscopic origin of the phase transition is extracted using a unique protocol involving polarized Raman spectroscopy (PRS) studies carried out as a function of crystallographic orientation. Here, we demonstrate the power of PRS in probing directional atomic rearrangements across the phase transition.

We propose a ferromagnetic Heusler alloy that can switch between a metal and a half-metal. This effect can provide tunable spintronics properties. Using the density functional theory with reliable implementations of the electron correlation effects, we find ${\mathrm{Mn}}_2\mathrm{ScSi}$ total energy curves consisting of distinct branches with a very small energy difference. The phase at low lattice crystal volume is a low magnetic half-metallic state while the phase at high lattice crystal volume is a high magnetic metallic state. We suggest that the transition between half-metallic and metallic states can be triggered by a triaxial contraction/expansion of the crystal lattice or by an external magnetic field if we assume that the lattice is cubic and remains cubic under expansion/contraction. However, the phase at high volume can also undergo an austenite-martensite phase transition because of the presence of Jahn-Teller active $3d$ electrons on the Mn atoms.

We study the momentum dependence of static magnetic susceptibility $\chi \left(q\right)$ in the paramagnetic phase of Heisenberg magnets and its relation to critical behavior within the nonlinear sigma model (NLSM) at arbitrary dimension $2<d<4$. In the first order of $1/N$ expansion, where $N$ is the number of spin components, we find $\chi \left(q\right)\propto {\{q^2+{\xi }^{-2}[1+f\left(q\xi \right)]\}}^{-1+\eta /2}$, where $\xi$ is the correlation length, $q$ is the momentum, measured from the magnetic wave vector, and the universal scaling function $f\left(x\right)$ describes the deviation from the standard Landau-Ginzburg momentum dependence. In agreement with previous studies at large $x$ we find $f(x\gg 1)\simeq (2B_4/N)x^{4-d}$; the absolute value of the coefficient $B_4$ increases with $d$ at $d>5/2$. Using NLSM, we obtain the contribution of the “anomalous” term ${\xi }^{-2}f\left(q\xi \right)$ to the critical exponent $\nu$, comparing it to the contribution of the nonanalytical dependence, originating from the critical exponent $\eta$ (the obtained critical exponents $\nu$ and $\eta$ agree with previous studies). In the range $3\le d<4$ we find that the former contribution dominates and fully determines the $1/N$ correction to the critical exponent $\nu$ in the limit $d\to 4$.

The optical Hall response is theoretically studied for spin-orbit coupled metals with ferroic orders of cluster-type magnetic multipoles. We find that different magnetic multipoles give rise to distinct spectra in the optical Hall conductivity. In the cases of monopole and quadrupole orders, the optical Hall response appears predominantly in high- and low-energy regions, which correspond to the energy scales of electron correlation and kinetic energy, respectively, while the response is dispersed and rather weak in the case of toroidal order. By decomposing the spectra into different interband contributions, we reveal selection rules stemming from the interplay between the antisymmetric spin-orbit coupling and the underlying multipoles. Our results suggest that the optical Hall measurement is useful to detect and distinguish the cluster-type magnetic multipole orders.

We study many-body localization (MBL) in a one-dimensional system of spinless fermions with a deterministic aperiodic potential in the presence of random interactions $V_{ij}$ decaying as power-law $V_{ij}/{\left(r_{ij}\right)}^{\alpha }$ with distance $r_{ij}$. We demonstrate that MBL survives even for $\alpha <1$ and is preceded by a broad nonergodic subdiffusive phase. Starting from parameters at which the short-range interacting system shows an infinite temperature MBL phase, turning on random power-law interactions results in many-body mobility edges in the spectrum with a larger fraction of ergodic delocalized states for smaller values of $\alpha$. Hence, the critical disorder $h_c^r$, at which ergodic to nonergodic transition takes place, increases with the range of interactions. Time evolution of the density imbalance $I\left(t\right)$, which has power-law decay $I\left(t\right)\sim t^{-\gamma }$ in the intermediate to large time regime, shows that the critical disorder $h_c^I$, above which the system becomes diffusionless (with $\gamma \sim 0$) and transits into the MBL phase, is much larger than $h_c^r$. In between $h_c^r$ and $h_c^I$ there is a broad nonergodic subdiffusive phase, which is characterized by the Poissonian statistics for the level spacing ratio, multifractal eigenfunctions, and a nonzero dynamical exponent $\gamma \ll 1/2$. The system continues to be subdiffusive even on the ergodic side ($h<h_c^r$) of the MBL transition, where the eigenstates near the mobility edges are multifractal. For $h<h_0<h_c^r$, the system is superdiffusive with $\gamma >1/2$. The rich phase diagram obtained here is unique to the random nature of long-range interactions. We explain this in terms of the enhanced correlations among local energies of the effective Anderson model induced by random power-law interactions.

The intrinsic antiferromagnetic (AFM) interlayer coupling in the two-dimensional magnetic topological insulator $\mathrm{Mn}{\mathrm{Bi}}_2{\mathrm{Te}}_4$ places a restriction on realizing a stable quantum anomalous Hall effect (QAHE) [Y. Deng et al., Science 367, 895 (2020)]. Through density functional theory calculations, we demonstrate the possibility of tuning the AFM coupling to the ferromagnetic coupling in $\mathrm{Mn}{\mathrm{Bi}}_2{\mathrm{Te}}_4$ films by alloying about 50% V with Mn. As a result, the QAHE can be achieved without alternation with the even or odd septuple layers. This provides a practical strategy to get a robust QAHE in ultrathin $\mathrm{Mn}{\mathrm{Bi}}_2{\mathrm{Te}}_4$ films, rendering them attractive for technological innovations.

We report synthesis and magnetic properties of quasi-one-dimensional spin-$\frac{1}{2}$ Heisenberg antiferromagnetic chain compound ${\mathrm{BaNa}}_2\mathrm{Cu}{\left({\mathrm{VO}}_4\right)}_2$. This orthovanadate has a centrosymmetric crystal structure, $C2/c$, where the magnetic ${\mathrm{Cu}}^{2+}$ ions form spin chains. These chains are arranged in layers, with the chain direction changing by ${62}^{\circ }$ between the two successive layers. Alternatively, the spin lattice can be viewed as anisotropic triangular layers upon taking the interchain interactions into consideration. Despite this potential structural complexity, temperature-dependent magnetic susceptibility, heat capacity, electron spin resonance intensity, and nuclear magnetic resonance (NMR) shift agree well with the uniform spin-$1/2$ Heisenberg chain model with an intrachain coupling of $J/k_{\mathrm{B}}\simeq 5.6$ K. The saturation field obtained from the magnetic isotherm measurement consistently reproduces the value of $J/k_{\mathrm{B}}$. Further, the ${}^{51}\mathrm{V}$ NMR spin-lattice relaxation rate mimics the one-dimensional character in the intermediate temperature range, whereas magnetic long-range order sets in below $T_{\mathrm{N}}\simeq 0.25$ K. The effective interchain coupling is estimated to be $J_{\perp }/k_{\mathrm{B}}\simeq 0.1$ K. The theoretical estimation of exchange couplings using band-structure calculations reciprocate our experimental findings and unambiguously establish the one-dimensional character of the compound. Finally, the spin lattice of ${\mathrm{BaNa}}_2\mathrm{Cu}{\left({\mathrm{VO}}_4\right)}_2$ is compared with the chemically similar but not isostructural compound ${\mathrm{BaAg}}_2\mathrm{Cu}{\left({\mathrm{VO}}_4\right)}_2$.

Motivated by recent experimental results for ${\mathrm{GdRu}}_2{\mathrm{Si}}_2$ [Khanh et al., Nat. Nanotechnol. 15, 444 (2020)], in which a nanometric square skyrmion lattice was observed, we propose a simple analytical mean-field description of the high-temperature part of the phase diagram of centrosymmetric tetragonal frustrated antiferromagnets with dipolar interaction in the external magnetic field. Dipolar forces provide momentum-dependent biaxial anisotropy in reciprocal space. It is shown that in a tetragonal lattice, in the large part of the Brillouin zone, for mutually perpendicular modulation vectors in the $ab$ plane this anisotropy has mutually perpendicular easy axes and collinear middle axes, which leads to double-$Q$ modulated spin structure stabilization. In the large part of its stability region, the latter turns out to be a square skyrmion lattice with a topological charge of $\pm 1$ per magnetic unit cell, which is determined by the frustrated exchange coupling and thus nanometer sized. Easy and middle axes can be swapped in the presence of additional single-ion easy-axis anisotropy. This results in the different phase diagram. It is argued that the latter case is relevant to ${\mathrm{GdRu}}_2{\mathrm{Si}}_2$.

Reported here are measurements and analysis of the magnetization ($M$) versus temperature (1.9–400 K) in magnetic fields $H$ up to 90 kOe for a polycrystalline sample of Ising chain ferromagnet $\mathrm{Co}{\mathrm{Nb}}_2{\mathrm{O}}_6$ with $T_C=2.9\mathrm{K}$. For $T>T_C$, the fit of magnetic susceptibility $\chi =M/H$ ($H=300\mathrm{Oe}$) to $\chi ={\chi }_0+C/\left(T-\mathrm{\Theta }\right)$ with ${\chi }_0=0.0009\mathrm{emu}\mathrm{mo}{\mathrm{l}}^{–1}{\mathrm{Oe}}^{–1}$ determined from high-$T$ data yields $\mathrm{\Theta }=11\mathrm{K}$ and magnetic moment $\mu =4.994{\mu }_B$ per ${\mathrm{Co}}^{2+}$ ion calculated from experimental $C=3.12\mathrm{emu}\mathrm{K}\mathrm{mo}{\mathrm{l}}^{–1}{\mathrm{Oe}}^{–1}$. Values of $g$ obtained from ${\mu }^2/{{\mu }_B}^2=g^{2}S\left(S+1\right)$ for spin $S=1/2$ and 3/2 are used to determine ${\mu }_Z=gS{\mu }_B$ and compared with ${\mu }_Z$ obtained from saturation magnetization and neutron diffraction for $T\ll T_C$. This analysis of the data for both above and below $T_C$ shows that the ground state of ${\mathrm{Co}}^{2+}$ in $\mathrm{Co}{\mathrm{Nb}}_2{\mathrm{O}}_6$ has the effective spin $S=1/2$ and not $S=3/2$ expected from Hund's rules, the $S=1/2$ ground state resulting from the combined effects of a noncubic crystalline field and spin-orbit coupling. The fit of the data for $T>T_C$ to $\chi ={\chi }_0+\left(C/T\right)\exp \left(J_0/2k_{B}T\right)$ valid for an Ising chain with $S=1/2$ yields the intrachain ferromagnetic exchange constant $J_0/k_B=6.2\mathrm{K}$, whereas the $g$ value with $S=1/2$ and the experimental critical fields for spin flips yields interchain antiferromagnetic exchange constants $J_1/k_B=-0.42\mathrm{K}$ and $J_2/k_B=-0.67\mathrm{K}$.

We investigate the superconducting phase transition in a superconductor (S)/ferromagnet (F) bilayer with Rashba spin-orbit interaction at the S/F interface. This spin-orbit coupling produces spontaneous supercurrents flowing inside the atomic-thickness region near the interface, which are compensated by the screening Meissner currents [Mironov and Buzdin, Phys. Rev. Lett. 118, 077001 (2017)]. In the case of a thin superconducting film the emergence of the spontaneous surface currents causes an increase of the superconducting critical temperature, and we calculate the actual value of the critical temperature shift. We also show that in the case of a type-I superconducting film this phase transition can be of the first order. In the external magnetic field the critical temperature depends on the relative orientation of the external magnetic field and the exchange field in the ferromagnet. Also we predict the in-plane anisotropy of the critical current, which may open an alternative way for the experimental observation of the spontaneous supercurrents generated by the spin-orbit coupling.

Recent exploration of the commensurate structure in turbostratic double-layer graphene shows that large-angle twisting can be treated by the decrease of the effective velocity within the energy spectra of single-layer graphene. Within our work, we use this result as a starting point, aiming towards understanding the physics of large-angle twisted double-layer graphene (i.e., moiré) quantum dot systems. We show that within this simple approach, using the language of the first quantization, an unrealized (to our knowledge), illustrative property of the commutation relation appears in graphene physics. Intriguingly, large twisting angles are shown to be suitable for tuning the position symmetry in graphene systems. A complete overview of large-angle twisting of the considered dot systems is provided.

We address the question of different representations of Bloch states for lattices with a basis, with a focus on topological systems. The representations differ in the relative phase of the Wannier functions corresponding to the diffferent basis members. We show that the phase can be chosen in such a way that the Wannier functions for the different sites in the basis both become eigenstates of the position operator in a particular band. A key step in showing this is the extension of the Brillouin zone. When the distance between sites within a unit cell is a rational number, $p/q$, the Brillouin extends by a factor of $q$. For irrational numbers, the Brillouin zone extends to infinity. In the case of rational distance, $p/q$, the Berry phase lives on a cyclic curve in the parameter space of the Hamiltonian, on the Brillouin zone extended by a factor of $q$. For irrational distances, the most stable way to calculate the polarization is to approximate the distance as a rational sequence and use the formulas derived here for rational numbers. The use of different bases are related to unitary transformations of the Hamiltonian, as such, the phase diagrams of topological systems are not altered, but each phase can acquire different topological characteristics when the basis is changed. In the example we use, an extended Su-Schrieffer-Heeger model, the use of the diagonal basis leads to toroidal knots in the Hamiltonian space, whose winding numbers give the polarization.