Antiferromagnetic systems with rich spin dynamics and pronounced interplay between magnetic and lattice degrees of freedom are of great interest in the emerging field of terahertz and subterahertz magnonics and spintronics. Here we report on spin and lattice dynamics of single crystals of ${\mathrm{Ni}}_2{\mathrm{NbBO}}_6$ studied with the use of polarized Raman spectroscopy in the temperature range of 10–300 K which includes the antiferromagnetic transition at $T_N=23.5$ K. Well-defined and highly polarized magnetic excitations were observed below and partly above $T_N$. Three magnetic modes at 30, 84, and 113 ${\mathrm{cm}}^{-1}$ (at $T=10$ K) were detected. The two high-energy magnetic excitations were identified as two-magnon modes. In total, 57 out of the expected 60 Raman-active phonons were registered and identified. Several modes revealed a nontrivial spin-phonon coupling manifesting either hardening or softening mostly below $T_N$. The possible magnetic space groups were determined through symmetry analysis. Spin-wave spectra and density of states were calculated within the linear spin-wave theory for supporting the experimental findings.

Nonlinear relaxation between spin waves (magnons) and the crystal lattice (phonons) in an insulating ferromagnet is investigated theoretically. Magnons and phonons are described by equilibrium Bose-Einstein distributions with different temperatures, $T_s>T_l$. The magnon temperature is assumed to be much lower than the Debye temperature $T_s\ll {\mathrm{\Theta }}_D$, which is justified at low temperatures. The nonlinear heat current from magnons to phonons is calculated microscopically in terms of the Cherenkov radiation of phonons by magnons. The results are discussed in comparison with the well-known theoretical results on nonlinear electron-phonon relaxation in metals [M. Kaganov, I. M. Lifshitz, and I. V. Tanatarov, J. Exp. Theor. Phys. 31, 232 (1956)]. The elaborated theoretical description is relevant for spin-pumping experiments and thermoelectric devices in which the magnon temperature is essentially higher than the phonon one. The derived expression for the heat current can be used for calculation of the nonlinear heat boundary resistance in spin-caloritronic heterostructures.

We investigate the coherent energy and thermal transport in a temperature-biased long Josephson tunnel junction, when a Josephson vortex, i.e., a soliton, steadily drifts driven by an electric bias current. We demonstrate that thermal transport through the junction can be controlled by the bias current, since it determines the steady-state velocity of the drifting soliton. We study the effects on thermal transport of the damping affecting the soliton dynamics. In fact, a soliton locally influences the power flowing through the junction and can cause the variation of the temperature of the device. When the soliton speed increases approaching its limiting value, i.e., the Swihart velocity, we demonstrate that the soliton-induced thermal effects significantly modify. Finally, we discuss how the appropriate material selection of the superconductors forming the junction is essential, since short quasiparticle relaxation times are required to observe fast thermal effects.

We present theoretical results for the optical conductivity and the nonresonant Raman susceptibilities for three principal polarization geometries relevant to the square lattice. The susceptibilities are obtained using the recently developed extremely correlated Fermi liquid theory for the two-dimensional $t-t^{\prime }-J$ model, where $t$ and $t^{\prime }$ are the nearest- and second-neighbor hopping. Our results sensitively depend on $t,t^{\prime }$. By studying this quartet of related dynamical susceptibilities, and their dependence on $t,t^{\prime }$, doping, and temperature, we provide a useful framework of interpreting and planning future Raman experiments on strongly correlated matter.

We show that the RKKY interaction in the two-impurity Anderson model comprise two contributions: a ferromagnetic part stemming from the symmetrized hybridization functions and an antiferromagnetic part. We demonstrate that this antiferromagnetic contribution can also be generated by an effective local tunneling term between the two impurities. This tunneling can be analytically calculated for particle-hole symmetric impurities. Replacing the full hybridization functions by the symmetric part and this tunneling term leads to the identical low-temperature fixed point spectrum in the numerical renormalization group. Compensating this tunneling term allows us to restore the Varma-Jones quantum critical point between a strong-coupling phase and a local singlet phase even in the absence of particle-hole symmetry in the hybridization functions. We analytically investigate the spatial frequencies of the effective tunneling term based on the combination of the band dispersion and the shape of the Fermi surface. Numerical renormalization group calculations provide a comparison of the distance-dependent tunneling term and the local spin-spin correlation function. Deviations between the spatial dependency of the full spin-spin correlation function and the textbook RKKY interaction are reported.

Starting from the single-particle Dirac Hamiltonian for charge carriers in monolayer transition metal dichalcogenides (TMDs), we construct a four-band Hamiltonian describing interlayer excitons consisting of an electron in one TMD layer and a hole in the other TMD layer. An expression for the electron-hole interaction potential is derived, taking into account the effect of the dielectric environment above, below, and between the two TMD layers as well as polarization effects in the transition metal layer and in the chalcogen layers of the TMD layers. We calculate the interlayer exciton binding energy and average in-plane interparticle distance for different TMD heterostructures. The effect of different dielectric environments on the exciton binding energy is investigated and a remarkable dependence on the dielectric constant of the barrier between the two layers is found, resulting from competing effects as a function of the in-plane and out-of-plane dielectric constants of the barrier. The polarization effects in the chalcogen layers, which in general reduce the exciton binding energy, can lead to an increase in binding energy in the presence of strong substrate effects by screening the substrate. The excitonic absorbance spectrum is calculated and we show that the interlayer exciton peak depends linearly on a perpendicular electric field, which agrees with recent experimental results.

We develop a theory for spontaneous decay of a quantum emitter (QE) situated near metal-dielectric structure supporting localized surface plasmons. If plasmon resonance is tuned close to the QE emission frequency, the emission is enhanced due to energy transfer from the QE to a localized plasmon mode followed by photon emission by plasmonic antenna. The emission rate is determined by intimate interplay between the plasmon coupling to radiation field and the Ohmic losses in metal. Here we develop plasmon Green's function approach that includes plasmon's interaction with radiation to obtain explicit expressions for radiative decay rate and optical polarizability of a localized plasmon mode in arbitrary plasmonic nanostructure. Within this approach, we provide consistent definition of plasmon mode volume by relating it to plasmon mode density, which characterizes the plasmon field confinement, and recover the standard cavity form of the Purcell factor, but now for plasmonic systems. We show that, for a QE placed at a “hot spot” near a sharp tip of a small metal nanostructure, the plasmon mode volume scales with the metal volume while being very sensitive to the proximity to the tip. Finally, we derive the enhancement factor for radiated power spectrum for any nanoplasmonic system and relate it to the Purcell factor for spontaneous decay rate. We illustrate our results by numerical example of a QE situated near a gold nanorod tip.

We have investigated the interface formation at room temperature between Fe and ${\mathrm{TiO}}_2$-terminated ${\mathrm{SrTiO}}_3\left(001\right)$ surface using x-ray photoelectron spectroscopy. Oxygen vacancies within the ${\mathrm{SrTiO}}_3$ lattice in the first planes beneath the $\mathrm{Fe}\text{/}{\mathrm{SrTiO}}_3$ interface are induced by the Fe deposition. Through a detailed analysis of the Fe $2p$, Sr $3d$, and Ti $2p$ core-level line shapes we propose a quantitative description of the impact of the vacancies on the electronic properties of the $\mathrm{Fe}\text{/}{\mathrm{SrTiO}}_3$ system. While for an abrupt $\mathrm{Fe}\text{/}{\mathrm{SrTiO}}_3$ junction the Schottky barrier height for electrons is expected to be about 1 eV, we find that the presence of oxygen vacancies leads to a much lower barrier height value of 0.05 eV. The deposition of a fraction of Fe monolayer also pushes the surface conduction band edge of the ${\mathrm{SrTiO}}_3$ below the Fermi level in favor of the formation of a surface electron accumulation layer. This change in the band bending stems from the incorporation of oxygen vacancies in the near-surface region of ${\mathrm{SrTiO}}_3\left(001\right)$. We deduce the conduction band profile as well as the carrier density in the accumulation layer as a function of the surface potential by solving the one-dimensional Poisson equation within the modified Thomas-Fermi approximation. Owing to the electric-field dependence of the dielectric permittivity, the ${\mathrm{SrTiO}}_3$ with oxygen vacancies at the surface shows original electronic properties. In particular, our simulations reveal that variations of a few percent of the vacancies concentration at the surface can cause changes of several tenths of an eV in the band bending that can lead to important lateral surface inhomogeneities for the potential. We also find through our modeling that the defect states density related to oxygen vacancies at the ${\mathrm{SrTiO}}_3$ surface cannot exceed, at room temperature, a critical value of $\sim 8\times {10}^{13}/\mathrm{c}{\mathrm{m}}^2$.

In this paper, we investigate the effect of the stacking sequence in ${\mathrm{MoS}}_2$ multilayer systems on their electron transport properties, through first-principles simulations of structural and electron transport properties. We show that interlayer electron transport is highly sensitive to the stacking sequence of the multilayers, with specific sequences producing a much higher electron transmission due to larger orbital interactions and band-structure effects. These results explain contrasting experimental evidence on interlayer transport measurements as due to imperfect structural control, provide insight on modeling, and suggest ways to improve the performance of electron devices based on ${\mathrm{MoS}}_2$ multilayer systems via multilayer structure engineering.

In this paper, we present a method to numerically study transverse Hall voltages using an alternative quantity in two-terminal setups. Using nonlinear transport concepts, we find that the Hall voltage dependence on the model parameters can be investigated from the difference between the injectivities of each terminal. The method is suitable to work with nonequilibrium Green's functions as well as for scattering matrix approaches. We illustrate the proposed idea by studying the quantum spin Hall effect in graphene with disordered spin-orbit scattering centers induced by adatoms. We use two distinct models: a finite-difference implementation of the Dirac Hamiltonian and a tight-binding Hamiltonian combined with the scattering matrix approach and the nonequilibrium Green's functions approach, respectively.

Two-dimensional monochalcogenides (MX) have been identified as a unique and promising class of layered materials in recent years. The valence band of single-layer MX, as predicted by theory, is inverted into a bow-shaped (often referred to as an inverted sombrero) and relatively flat dispersion, which is expected to give rise to strongly correlated effects. The inversion leads to an indirect band gap, which is consistent with photoluminescence (PL) experiments, but PL provides no direct evidence of the band inversion in the valence band. Here we demonstrate for a hexagonal MX crystal, gallium selenide (GaSe), using a combination of angle-resolved photoemission spectroscopy (ARPES) and density functional theory (DFT), that the valence band of monolayer (ML) GaSe exhibits a robust inversion of the valence dispersion at the Γ point forming a bow-shaped dispersion with a depth of 120 ± 10 meV between the double valence band maximum along the Γ$K$ direction. We also demonstrate that the deeper-lying bands detected in the ARPES spectrum are consistent with DFT calculations only if spin-orbit coupling is considered. The presented ARPES evidence that spin-orbit coupling leads to the splitting of two fourfold-degenerate states into four Kramers doublets is of significance for PL measurements, as the change in energy of the second highest valence state at the Γ point has a measurable effect on the PL energies in high-energy luminescence. We predict the optical absorption coefficients for the principal transitions in ML GaSe using a four-band $\mathbf{k}·\mathbf{p}$ model parametrized from first-principles theory with spin-orbit effects considered.

The spectrum of a Coulomb impurity in bilayer graphene is investigated as function of the strength of a perpendicular magnetic field for different values of the angular quantum number $m$ and for different values of the gate voltage. We point out fundamental differences between the results from the two-band and four-band model. The supercritical instability and fall-to-center phenomena are investigated in the presence of a magnetic field. We find that in the four-band model the fall-to-center phenomenon occurs as in monolayer graphene, while this is not the case in the two-band model. We find that in a magnetic field the supercritical instability manifests itself as a series of anticrossings in the hole part of the spectrum for states coming from the low-energy band. However, we also find very distinct anticrossings in the electron part of the spectrum that continue into the hole part, which are related to the higher energy band of the four-band model. At these anticrossings, we find a very sharp peak in the probability density close to the impurity, reminiscent for the fall-to-center phenomenon. In this paper, these peculiar and interesting effects are studied for different magnetic field, interlayer coupling, and bias potential strengths.

The design of time-independent effective Hamiltonians that describe periodically modulated systems provides a promising approach to realize new forms of matter. This so-called Floquet engineering approach is currently limited to the description of wave-packet dynamics. Here, we utilize the notion of effective Hamiltonians and develop a Floquet engineering scattering formalism that relies on a systematic high-frequency expansion of the scattering matrix. The method unveils the critical role of micromotion. An application to the case of nonreciprocal transport is presented.

The type-II Dirac fermions that are characterized by a tilted Dirac cone and anisotropic magnetotransport properties have been recently proposed theoretically and confirmed experimentally. Here, we predict the emergence of two-dimensional (2D) type-II Dirac fermions in $\mathrm{LaAl}{\mathrm{O}}_3/\mathrm{LaNi}{\mathrm{O}}_3/\mathrm{LaAl}{\mathrm{O}}_3$ quantum-well structures. Using first-principles calculations and model analyses, we show that the Dirac points are formed at the crossing between the $d_{x^2-y^2}$ and $d_{z^2}$ bands protected by the mirror symmetry. The energy position of the Dirac points can be tuned to appear at the Fermi energy by changing the quantum-well width. For the quantum-well structure with a two-unit-cell-thick $\mathrm{LaNi}{\mathrm{O}}_3$ layer, we predict the coexistence of the type-II Dirac point and the closed nodal line. The results are analyzed and interpreted using a tight-binding model and symmetry arguments. Our findings offer a practical way to realize 2D type-II Dirac fermions in oxide heterostructures.

We investigate a possibility of pair electron-electron ($e-e$) collisions in a ballistic wire with spin-orbit coupling and only one populated mode. Unlike in a spin-degenerate system, a combination of spin splitting in momentum space with a momentum-dependent spin precession opens up a finite phase space for pair $e-e$ collisions around three distinct positions of the wire's chemical potential. For a short wire, we calculate the corresponding resonant contributions to the conductance, which have different power-law temperature dependencies, and, in some cases, vanish if the wire's transverse confinement potential is symmetric. Our results may explain the recently observed feature at the lower conductance plateau in InAs wires.

Density-functional theory is used to probe the spin structure of the Si(553)-Au surface. A diamagnetic $s{p}^2+p$ rehybridized structure, where the dangling bonds are either filled with two spin-paired electrons or are empty, is more favorable and in better agreement with experiment than the generally accepted spin-chain model. The shallow potential energy surface of Si(553)-Au, together with the ordered array of empty dangling bonds, suggests this surface as susceptible for spin polarization by doping, however.

Quantum oscillations of response functions in high magnetic fields tend to reveal some of the most interesting properties of metals. In particular, the oscillation phase shift is sensitive to topological band features, thereby helping to identify the presence of Weyl fermions. In this work, we predict a characteristic parameter dependence of the phase shift for Weyl fermions with tilted and overtilted dispersion (type-I and type-II Weyl fermions) and an arbitrary topological charge (multi-Weyl fermions). For type-II Weyl fermions our calculations capture the case of magnetic breakthrough between the electron and the hole part of the dispersion. Here, the phase shift turns out to depend only on the quantized topological charge due to the cancellation of nonuniversal contributions from the electron and the hole part.

We study state-sum constructions of $G$-equivariant spin topological quantum field theory (TQFTs) and their relationship to matrix product states. In the Neveu-Schwarz, Ramond, and twisted sectors, states of the TQFT are generalized matrix product states. Our results are applied to the classification of fermionic short-range-entangled phases with a unitary symmetry $G$ to determine the group law on the set of such phases. Interesting subtleties appear when the total symmetry group is a nontrivial extension of $G$ by fermion parity.