The impact of an applied electric field on exchange coupling parameters has been investigated based on first-principles electronic structure calculations by means of the Korringa-Kohn-Rostoker Green function method. The calculations have been performed for a Fe film, free-standing and deposited on two different substrates, having 1 monolayer (ML) thickness to minimize the effect of screening of the electric field typical for metallic systems. By comparing the results for the free-standing Fe ML with those for Fe on the various substrates, we could analyze the origin of the field-induced change of the exchange interactions. Compared to the free-standing Fe ML, in particular rather pronounced changes have been found for the Fe/Pt(111) system due to the localized electronic states at the Fe/Pt interface, which are strongly affected by the electric field and which play an important role for the Fe-Fe exchange interactions.

Rich exchange bias (EB) behaviors were previously observed when ferromagnetic (FM) materials contacted a spin glass, demonstrating magnetic degrees of freedom of the coupling between the glass and FM spins. However, the correlation between the degree of magnetic spin frustration and the strength of the resulting EB is far from being understood. Here, we systematically investigate the dependency of EB on interfacial spin frustration in magnetic complex oxide heterostructures including ${\mathrm{La}}_{0.7}{\mathrm{Ca}}_{0.3}\mathrm{Mn}{\mathrm{O}}_3\text{/}\mathrm{CaMn}{\mathrm{O}}_3$ (LCMO/CMO) systems. The experimental analysis revealed that the extent of interfacial spin frustration is determined by the subtle competition between different types of magnetic orders related to the glassy spin behaviors at the interface. Such spin frustration can be manipulated through strain engineering through changes in the Mn $e_g$ orbital by alternating the stacking sequence of the heterostructures. A highly tunable EB field with 95% change of strength between the highly and weakly frustrated heterostructures has been achieved. Magnetic depth profiles of the heterostructures provide convincing evidence that a magnetically depressed region always occurs in the LCMO layer at the LCMO/CMO interfaces irrespective of the stacking sequence. Finally, EB is established at the magnetic interface in the LCMO layer.

The magnetic state of exchanged biased $\mathrm{CoO}\left(20\mathrm{nm}\right)/\mathrm{Co}\left(d_F\right)$ bilayers ($d_F=5-20\mathrm{nm}$) was studied by means of polarized neutron reflectometry. By introducing a Nb(20 nm) spacer layer between the CoO/Co bilayer and the ${\mathrm{Al}}_2{\mathrm{O}}_3$ substrate, we designed a resonator structure with significantly enhanced intensity of the spin-flip (SF) scattering at the position of the optical resonances. For the trained sample with thinnest Co layer ($d_F=5\mathrm{nm}$), we detected strong SF scattering at the resonance position to the amount of 30% of the incoming intensity, pointing to a high degree of non-collinearity of the magnetization. With increasing $d_F$, the intensity of the SF scattering decreases linearly. Furthermore, an unconventional asymmetry of up-down and down-up scattering channels at the resonance positions was observed, which we ascribe to the out-of-plane magnetic stray field generated by chiral Bloch domain walls. This field leads to Zeeman splitting of the neutron energies depending on the initial neutron spin polarization. The chirality of the domain walls is assigned to the Dzyaloshinskii-Moriya interaction emerging at the CoO/Co interface. Our observations might prove useful for the design of spintronic devices based on the exchange bias effect.

We study interskyrmion interactions and stable spin configurations in a two-dimensional chiral magnet, in particular, focusing on the effects of two in-plane anisotropies: a tilted magnetic field and the magnetocrystalline anisotropy on a (011) thin film. Using analytical and micromagnetic calculations with ignoring the dipole-dipole interactions, we find that a slight deformation of a skyrmion shape makes the interskyrmion interaction anisotropic and weakly attractive. Furthermore, when the magnetocrystalline anisotropy is comparable to the Zeeman energy, skyrmions embedded in a uniform magnetization are tightly bound by creating a magnetic domain between them. The formation of the magnetic domain, and thus the strength of the interskyrmion interaction, can be controlled by the direction of an external magnetic field. The anisotropic interaction also affects the skyrmion alignment in the skyrmion crystal (SkX) phase. By employing the Monte Carlo simulation and the Landau-Lifshitz-Gilbert equation, we obtain an elongated triangular SkX phase. In particular, in the presence of a strong magnetocrystalline anisotropy, magnetic domains appear in the background of the lattice structure. We also find a parameter region that the SkX phase is stabilized due to the interskyrmion attraction.

In this work we study the influence of the eccentricity on the topological and dynamic properties of 2D systems with monodispersed isotropically distributed overlapping ellipses. This type of system has been proposed in the literature as an adequate model for composites of conductive nanoparticles dispersed in insulating matrices. We extend the available information on the influence of the eccentricity on the percolation threshold and the electrical conductivity in such composites. By applying Monte Carlo simulations, we have modeled the dynamics of 2D disordered systems and found, for ellipses, an eccentricity invariant dynamic behavior that is also present in the widthless-stick limit case and in rectangles. This behavior appears upon the use of a particular definition of the normalized proximity to the percolation threshold. Our study suggests that this invariance might also arise in systems with other particle geometries having the widthless stick as a limit case. We further suggest how the presence of this invariance may facilitate the numerical and experimental understanding of conductor-insulator nanocomposites.

We analyze theoretically the effects of electron-phonon interaction in the dynamics of an electron that can be trapped to a localized state and detrapped to an extended band state of a small quantum dot (QD) using a simple model system. It consists of a one-dimensional tight-binding linear chain of a few sites, having a discrete set of energy levels mimicking the discrete levels of the conduction band of the QD that is connected at its end to another site, the trap, having a single energy level well below the conduction band, where the electron is allowed to interact with a local phonon of a single frequency. In spite of its simplicity, the time-dependent model has no analytical solution but a numerically exact one can be found, producing a rich dynamics. The electronic motion is quasiperiodic in time, with oscillations around a mean value that are basic characteristics of the weak and strong coupling regimes of electron-phonon interaction and set the timescales of the system. Using values of the parameters appropriate for defects in semiconductor QDs, we find these timescales to range typically from tenths of picoseconds to a few picoseconds. The values of the time-averaged trap occupancy strongly depend on the the strength of the electron-phonon interaction and can be as large as $40\%$ when the coupling is most efficient, independently of other parameters. An interesting result of the present paper is the formation of resonances at specific values of the electron-phonon coupling parameter that only exist when several levels are allowed to coherently cooperate in the filling of the trap. They are characterized by a trap occupancy that is a periodic function of time with large amplitude and period picturing an electron that is periodically trapped and detrapped. We conclude that the formation of these resonances is a robust consequence of electron-phonon interaction in small systems. Electron-phonon interaction is an efficient mechanism that can provide ca. $50\%$ filling of a deep trap state on a subpicosecond to picosecond timescale, much faster than radiative decay occurring in timescales of tens of picoseconds to nanoseconds, while the occupancy of this state will be smaller than ca. $1\%$ in the absence of electron-phonon coupling.

Stannides ${\mathrm{TmMn}}_{6\text{−}x}{\mathrm{Fe}}_x{\mathrm{Sn}}_6$, with hexagonal ${\mathrm{HfFe}}_6{\mathrm{Ge}}_6$-type structures, were studied by x-ray and neutron diffraction, magnetic measurements, and by ${}^{119}\mathrm{Sn}$ Mössbauer spectroscopy. Transition metal atoms form kagome networks in (001) planes with an intraplanar ferromagnetic coupling of their moments. At temperatures >∼20 to 50 K, Mn-rich stannides ($x=0.4$, 0.6, 1.2) have an AFII easy-plane antiferromagnetic (AFM) structure, with interplanar couplings + + − − along the $c$ axis. The thulium sublattice orders magnetically at low temperature. The complexity of the resultant neutron diffraction patterns arises from a mixture of several magnetic phases, some being incommensurate. The Tm sublattice does not order >1.6 K in Fe-rich stannides ($x=4.25$, 4.5, 5.0). Fe-rich stannides have an AFI AFM structure, this time with interplanar couplings + − + −. The $M=\mathrm{Mn}/\mathrm{Fe}$ magnetic moments are directed along the $c$ axis for $x=5$ at any temperature. Further, neutron diffraction shows that moments rotate from the $c$ axis toward the basal plane >4.2 K with maximum rotation angles of ∼70° and ∼55° reached at ∼50 and ∼80 K for $x=4.25$ and 4.50, respectively. Transferred hyperfine fields at tin sites of ${\mathrm{TmMn}}_{6\text{−}x}{\mathrm{Fe}}_x{\mathrm{Sn}}_6$ stannides are the moduli of ${}^{119}\mathrm{Sn}$ vectorial hyperfine magnetic fields that are modeled for the AFI and AFII magnetic structures, assuming a random substitution of Mn with Fe. Models involve sums of dipolar-type and of isotropic vector components with simple assumptions about the model parameters. The transferred hyperfine magnetic fields at ${}^{119}\mathrm{Sn}$ nuclei are measured by Mössbauer spectroscopy. The hyperfine magnetic fields of Sn atoms, whose six transition metal nearest neighbors are ferromagnetically coupled, are predicted and observed to vary linearly with their number of Fe first nearest neighbors. The hyperfine magnetic fields on Sn atoms sandwiched between two AFM coupled kagome planes are expected proportional to $|p_2-p_1|$, where one of the two kagome planes contains $p_1$ Fe first nearest neighbors and the other $p_2$. Tin atoms with a ferromagnetic first nearest neighbor shell experience negative hyperfine fields in Mn-rich stannides, whereas they experience positive hyperfine fields in the Fe-rich stannides. This change is explained by a change of sign of the isotropic hyperfine magnetic fields which occurs when going from Mn- to Fe-rich stannides.

We present a microscopic electronic description of how antiskyrmions can be stabilized in a Dresselhaus spin-orbit-coupled magnetic metal. Furthermore, we show that the antiskyrmions can be tuned into Bloch skyrmions via a change in sign of the hopping integral. The results are based on the state-of-the-art hybrid Monte Carlo simulations of lattice models explicitly involving itinerant electrons. The origin of such topological textures is understood via an effective spin-only model. Our results uncover an important and useful connection between generic features of the band structure and topological spin textures of immense current interest, and present a microscopic explanation of skyrmion formation reported recently in certain magnetic Weyl semimetals.

A family of titanium oxypnictide materials $\mathrm{Ba}{\mathrm{Ti}}_2{Pn}_2\mathrm{O}$ ($Pn=\mathrm{pnictogen}$) becomes superconducting when a charge and/or spin-density wave is suppressed. With hole doping, isovalent doping and pressure, a whole range of tuning parameters is available. We investigate how charge doping controls the superconducting transition temperature $T_{\mathrm{c}}$. To this end, we use experimental crystal structure data to determine the electronic structure and Fermi surface evolution along the doping path. We show that a naive approach to calculating $T_{\mathrm{c}}$ via the density of states at the Fermi level and the McMillan formula systematically fails to yield the observed $T_{\mathrm{c}}$ variation. On the other hand, spin-fluctuation theory pairing calculations allow us to consistently explain the $T_{\mathrm{c}}$ increase with doping. All alkali-doped materials ${\mathrm{Ba}}_{1-x}{A}_x{\mathrm{Ti}}_2{\mathrm{Sb}}_2\mathrm{O}$ ($A=\mathrm{Na},\mathrm{K},\mathrm{and}\mathrm{Rb}$) are described by a sign-changing $s$-wave order parameter. Susceptibilities also reveal that the physics of the materials is controlled by a single Ti $3d$ orbital.

We study the exciton states in the symmetrically biased dice model, the electronic structures of which have an isolated flat band between two dispersive bands. At 1/3 or 2/3 filling, the model describes a two-dimensional semiconductor with the band edge at two degenerate valleys. Because of qualitative changes in the eigenvectors resulting from the bias term, the interband transition between the flat band and a dispersive band is valley contrasting under circularly polarized light. In terms of an effective-mass model and a realistic electron-hole interaction, we numerically calculate the spectrum and wave functions of the intravalley excitons, which are treated as Wannier-Mott excitons. We also discuss the fine structures of the exciton spectrum induced by the intravalley and intervalley exchange interactions. The symmetrically biased dice model thereby proves to be a new platform for valley-contrasting optoelectronics.

We investigate the proximity-induced spin-orbit coupling in heterostructures of twisted graphene and monolayers of transition metal dichalcogenides (TMDCs) ${\mathrm{MoS}}_2, {\mathrm{WS}}_2, {\mathrm{MoSe}}_2$, and ${\mathrm{WSe}}_2$ from first principles. We identify strain, which is necessary to define commensurate supercells, as the key factor affecting the band offsets and thus magnitudes of the proximity couplings. We establish that for biaxially strained graphene the band offsets between the Dirac point and conduction (valence) TMDC bands vary linearly with strain, regardless of the twist angle. This relation allows us to identify the apparent zero-strain band offsets and find a compensating transverse electric field correcting for the strain. The resulting corrected band structure is then fitted around the Dirac point to an established spin-orbit Hamiltonian. This procedure yields the dominant, valley-Zeeman, and Rashba spin-orbit couplings. The magnitudes of these couplings do not vary much with the twist angle, although the valley-Zeeman coupling vanishes for ${30}^{\circ }$ and Mo-based heterostructures exhibit a maximum of the coupling at around ${20}^{\circ }$. The maximum for W-based stacks is at $0^{\circ }$. The Rashba coupling is in general weaker than the valley-Zeeman coupling, except at angles close to ${30}^{\circ }$. We also identify the Rashba phase angle which measures the deviation of the in-plane spin texture from tangential, and find that this angle is very sensitive to the applied transverse electric field. We further discuss the reliability of the supercell approach with respect to atomic relaxation (rippling of graphene), relative lateral shifts of the atomic layers, and transverse electric field.

In a Josephson junction between two exciton condensates, the tunneling of charge-neutral electron-hole pairs has gained primary attention in the form of a supercurrent, which otherwise is challenging to detect experimentally. Here we design excitonic Josephson junctions based on graphene heterostructures that allow us to selectively sustain charge-neutral or charged supercurrent, offering unprecedented opportunities for revealing exotic physics of exciton condensates. In our schemes, each exciton condensate consists of a graphene monolayer vertically coupled with another graphene monolayer (GML), bilayer, or trilayer, shown to have characteristically different quantum phase transition temperatures. When two such identical condensates are connected, a neutral supercurrent always dominates within the GML/GML scheme, while in the other two schemes, a carrier-density induced transition can take place between the neutral and charged supercurrents. More strikingly, in the charged regime, both the electron or hole dominance and the DC or AC nature can be tuned by the chemical potential differences of the junction. These findings are also expected to be applicable to excitonic Josephson junctions beyond the graphene-based architecture.

Near-gate plasmons are a new type of plasma oscillations emerging in homogeneous two-dimensional electron systems where a gate provides partial screening of electron-electron interaction. Here we develop a theory of the near-gate plasmons in van der Waals heterostructures comprising a conducting layer separated by a thin insulator from an uncharged disk-shaped gate. We show that in these structures the near-gate plasmons form gate-size-quantized quasistationary discrete modes even in the collisionless limit. Belonging to continuum spectrum of two-dimensional plasmons outside of the disk-gate, the near-gate plasmons are manifested as Fano-like resonances in frequency and magnetodispersions of scattering cross section of the former scattered off the region under the gate. This enables to recover spectrum of the near-gate plasmons in the van der Waals heterostructures using near-field imaging techniques.

We have investigated the spectrum of two-dimensional (2D) plasmon polaritons over the full range of magnetic fields. In our study, we investigate a disk-shaped two-dimensional electron system (2DES) with a metallic gate on the backside of the substrate. Importantly, we show that 2D plasmon polaritons hybridize with the ${\mathrm{TM}}_0$ photonic mode of a dielectric waveguide formed by a sample substrate. We have developed a theory for plasmon-polaritons in an infinite 2DES. We find the experimental data to be in good agreement with the developed theory.

We demonstrate that the presence of a localized state at the corner of an insulating domain is not always a predictor nor a direct consequence of a certain nontrivial higher-order topological invariant, even though they appear to coexist in the same Hamiltonian parameter space. Our analysis of $C_n$-symmetric crystalline insulators and their multilayer stacks reveals that topological corner states are not necessarily correlated with other well-established higher-order boundary observables, such as fractional corner charge or filling anomaly. In a $C_3$-symmetric breathing Kagome lattice, for example, we show that the bulk polarization, which successfully predicts the fractional corner anomaly, fails to be the relevant topological invariant for zero-energy corner states; instead, these corner states are explained by the decoration of topological edges. Also, while the corner states at the interface between $C_4$-symmetric topological crystalline insulators and their trivial counterpart have long been reported to be the result of the bulk polarization of the lowest band, we reveal that such embedded corner states are trivial defect states. By refining several bulk-corner correspondences in two-dimensional topological crystalline insulators, our work motivates further development of rigorous theoretical grounds for associating the existence of corner states with higher-order topology of host materials.

We show how a strongly driven single-mode oscillator coupled to a first-order dynamical system gives rise to induced absorption or gain of a weak probe beam and associated fast or slow light depending on the detuning conditions. We derive the analytic solutions to the dynamic equations of motion, showing that the electromagnetically induced transparency- (EIT-) like response is a general phenomenology, potentially occurring in any nonlinear oscillator coupled to first-order dynamical systems. The resulting group delay (or advance) of the probe is fundamentally determined by the system damping rate. To illustrate the practical impact of this general theoretical framework, we quantitatively assess the observable consequences of either thermo-optic or free-carrier dispersion effects in conventional semiconductor microcavities in control/probe experiments, highlighting the generality of this physical mechanism and its potential for the realization of EIT-like phenomena in integrated and cost-effective photonic devices.

Classical XY model is a lattice model of statistical mechanics notable for its universality in the rich hierarchy of the optical, laser, and condensed matter systems. We show how to build complex structures for machine learning based on the XY model's nonlinear blocks. The final target is to reproduce the deep learning architectures, which can perform complicated tasks usually attributed to such architectures: speech recognition, visual processing, or other complex classification types with high quality. We developed a robust and transparent approach for the construction of such models, which has universal applicability (i.e., does not strongly connect to any particular physical system) and allows many possible extensions, while at the same time preserving the simplicity of the methodology.

We show that the onset of quantum chaos at infinite temperature in two many-body one-dimensional lattice models, the perturbed spin-1/2 XXZ and Anderson models, is characterized by universal behavior. Specifically, we show that the onset of quantum chaos is marked by maxima of the typical fidelity susceptibilities that scale with the square of the inverse average level spacing, saturating their upper bound, and that the strength of the integrability- or localization-breaking perturbation at these maxima decreases with increasing system size. We also show that the spectral function below the “Thouless” energy (in the quantum-chaotic regime) diverges when approaching those maxima. Our results suggest that, in the thermodynamic limit, arbitrarily small integrability- or localization-breaking perturbations result in quantum chaos in the many-body quantum systems studied here.

In this paper we present a detailed derivation of the stress tensor for the nonrelativistic full potential linearized augmented plane wave (LAPW) method. The formalism has been implemented into the wien2k code and has been thoroughly tested for the equilibrium lattice parameters in various solids. Hydrostatic and nonhydrostatic conditions have been applied and the accuracy of individual stress components has been tested at finite strain. We also tested the convergence of the stress tensor with respect to the basis set and found it necessary to increase the basis set a bit as compared to total energy calculations. The effect of the tetrahedron and Fermi-Dirac (FD) methods on the calculated stress is studied for aluminum and found that the FD method improves the results. Finally, a brief comparison with previous attempts in the literature on this topic is given.