We show that two coupled, stacked plasmonic scatterers exhibit surprisingly complex excitation spectra as soon as retardation plays a role in the coupling. At a certain stacking distance and a certain frequency, the amplitude and phase exhibit sharp features, and the phase difference between the scatterers shows a singularity, which constitutes a vortex. Above this singularity, the antisymmetric oscillation that arises from plasmon hybridization ceases to exist and is replaced by a second, symmetric oscillation. We examine the distance-dependent behavior of these phenomena by an analytical coupled-dipole model as well as by numerical simulations of stacked split-ring resonators and show that the singularity can be explained as an antiresonance. Furthermore, we present a simple necessary and sufficient condition under which the oscillations have a phase difference of exactly zero or $\pi$ in case of arbitrary retardation and therefore exhibit the intrinsic symmetry of the system. We show that this condition implies that the spectral positions of (anti)symmetric oscillations do not coincide with the spectral positions of the (anti)symmetric mode. The vortices that are present at the phase singularities might be used to generate light with angular optical momentum in plasmonic metasurfaces.

The Kapitza or interfacial thermal resistance at the boundary of two different insulating solids depends on the transmission of phonons across the interface and the phonon dispersion of either material. We extend the existing atomistic Green's function (AGF) method to compute the probability for individual phonon modes to be transmitted across the interface. The extended method is based on the concept of the Bloch matrix and allows us to determine the wavelength and polarization dependence of the phonon transmission as well as to analyze efficiently the contribution of individual acoustic and optical phonon modes to interfacial thermal transport. The relationship between the phonon transmission probability and dispersion is explicitly established. A detailed description of the method is given and key formulas are provided. To illustrate the role of the phonon dispersion in interfacial thermal conduction, we apply the method to study phonon transmission and thermal transport at the armchair interface between monolayer graphene and hexagonal boron nitride. We find that the phonon transmission probability is high for longitudinal (LA) and flexural (ZA) acoustic phonons at normal and oblique incidence to the interface. At room temperature, the dominant contribution to interfacial thermal transport comes from the transverse-polarized phonons in graphene (45.5%) and longitudinal-polarized phonons in boron nitride (47.4%).

Interfacial effects on spin and symmetry filtering in single-crystal $\mathrm{Fe}(001)/\mathrm{MgO}/\mathrm{Fe}$ magnetic tunnel junctions are investigated with the insertion of a Fe monoatomic step at the bottom MgO interface. After annealing, the atomically flat bottom electrode is covered by a fractional part of a Fe monoatomic layer resulting in two-dimensional Fe islands that are separated for low coverages and percolated above around half a monolayer. The magnetotransport properties of the junctions are studied as a function of this Fe sublayer coverage that is varied from 0 to 1 monolayer. Surprisingly, the magnetoresistance ratio exhibits a maximum for a coverage around half a monolayer. Tunneling spectroscopy experiments performed at low temperature allow relating this result to the decrease of the contribution of the interfacial resonance state to the conductance of the junction.

We have investigated single crystals and polycrystals from the series ${\mathrm{Nb}}_{1-y}{\mathrm{Fe}}_{2+y}, -0.004\le y\le 0.018$, by electron spin resonance, muon spin relaxation, and Mössbauer spectroscopy. Our data establish that at lowest temperatures all samples exhibit bulk magnetic order. Slight Fe excess induces low-moment ferromagnetism, consistent with bulk magnetometry, while Nb-rich and stoichiometric ${\mathrm{NbFe}}_2$ display spin density wave order with small magnetic moment amplitudes of order $\sim 0.001–0.01{\mu }_B/\mathrm{Fe}$. This provides microscopic evidence for a modulated magnetic state on the border of ferromagnetism in ${\mathrm{NbFe}}_2$.

The magnetic field-dependent transport of ${\mathrm{Pr}}_{0.6}{\mathrm{Ca}}_{0.4}{\mathrm{MnO}}_3$ has been investigated to expose the field-induced melting of orbital ordering. The critical field and critical temperature for the melting of the orbital ordered state depend on the orientation of the applied magnetic field, seemingly independent of the nature of the magnetic phase. The crystalline $c$ axis is the hard axis of the field-induced melting, but in-plane anisotropy also exists, correlating with the anisotropic charge-exchange-(CE) type orbital-ordered structure. These findings demonstrate that the anisotropic melting coincides with the orbital ordering anisotropy, thus indicating a strong spin-orbital coupling.

Solid-state amorphization of crystalline copper nanolayers embedded in a ${\mathrm{Cu}}_{64}{\mathrm{Zr}}_{36}$ metallic glass is studied by molecular dynamics simulations for different orientations of the crystalline layer. We show that solid-state amorphization is driven by a reduction of interface energy, which compensates the bulk excess energy of the amorphous nanolayer with respect to the crystalline phase up to a critical layer thickness. A simple thermodynamic model is derived, which describes the simulation results in terms of orientation-dependent interface energies. Detailed analysis reveals the structure of the amorphous nanolayer and allows a comparison to a quenched copper melt, providing further insights into the origin of excess and interface energy.

We derive a method to determine the effective local elasticity tensor by combining the local stress, local strain, and global strain in conjunction with a linear least-squares solver. This method reduces to the standard stress-strain fluctuation method for the estimation of global moduli if the local stress and the local strain are substituted by global counterparts. The quality of the developed method is verified by the analysis of an fcc Lennard-Jones single crystal. By using this method, surface elastic behaviors of three nanoplates are investigated. Simulation results prove that both softening and stiffening effects could be detected, depending on the surface orientations and loading directions. This approach could be especially valuable for complicated morphologies where the physical properties of the local material are challenging or impractical to obtain.

We study the relaxation dynamics of strongly interacting quantum systems that display a kind of many-body localization in spite of their translation-invariant Hamiltonian. We show that dynamics starting from a random initial configuration is nonperturbatively slow in the hopping strength, and potentially genuinely nonergodic in the thermodynamic limit. In finite systems with periodic boundary conditions, density relaxation takes place in two stages, which are separated by a long out-of-equilibrium plateau whose duration diverges exponentially with the system size. We estimate the phase boundary of this quantum glass phase, and discuss the role of local resonant configurations. We suggest experimental realizations and methods to observe the discussed nonergodic dynamics.

Understanding the role of spin-orbit coupling (SOC) has been crucial for controlling magnetic anisotropy in magnetic multilayer films. It has been shown that electronic structure can be altered via interface SOC by varying the superlattice structure, resulting in spontaneous magnetization perpendicular or parallel to the plane. In lieu of magnetic thin films, we study the similarly anisotropic helimagnet ${\mathrm{Cr}}_{1/3}{\mathrm{NbS}}_2$ where the spin-polarization direction, controlled by the applied magnetic field, can modify the electronic structure. As a result, the direction of spin polarization can modulate the density of states and in turn affect the in-plane electrical conductivity. In ${\mathrm{Cr}}_{1/3}{\mathrm{NbS}}_2$, we found an enhancement of in-plane conductivity when the spin polarization is out-of-plane as compared to in-plane spin polarization. This is consistent with the increase in density of states near the Fermi energy at the same spin configuration, found from first-principles calculations. We also observe unusual field dependence of the Hall signal in the same temperature range. This is unlikely to originate from the noncollinear spin texture but rather further indicates strong dependence of electronic structure on spin orientation relative to the plane.

A combination of ultraviolet and inverse photoemission is often used to determine the position of the transport levels of semiconductors. Although data from direct methods like photoemission appear advantageous at first glance, large discrepancies between thus-derived band gaps and optically measured band gaps have led to fundamentally different evaluation methods of the data from ultraviolet photoelectron spectroscopy (UPS)/inverse photoelectron spectroscopy (IPS) experiments, the essential alternatives being the maxima or the onsets of the frontier peaks. In this paper, we review published data as well as present new experimental data for a few representative II-VI and III-V compound and element semiconductors. New data from silicon are utilized as examples for evaluating details of such combined UPS and IPS spectra and for answering the question of how surface effects, especially the consequences of surface reconstruction, can adequately be taken into account. The results clearly indicate that, for all three types of semiconductors, only peak onsets represent the correct band positions. Possible reasons for this finding are discussed, and an explanation in the framework of relaxation (i.e., dynamical screening) is suggested.

We provide a quasi-one-dimensional model which can support a pair-density-wave (PDW) state, in which the superconducting (SC) order parameter modulates periodically in space, with gapless Bogoliubov quasiparticle excitations. The model consists of an array of strongly interacting one-dimensional systems, where the one-dimensional systems are coupled to each other by local interactions and tunneling of the electrons and Cooper pairs between them. Within the interchain mean-field theory, we find several SC states from the model, including a conventional uniform SC state, PDW SC state, and a coexisting phase of the uniform SC and PDW states. In this quasi-1D regime we can treat the strong correlation physics essentially exactly using bosonization methods and the crossover to the 2D system by means of interchain mean-field theory. The resulting critical temperatures of the SC phases generically exhibit a power-law scaling with the coupling constants of the array, instead of the essential singularity found in weak-coupling BCS-type theories. Electronic excitations with an open Fermi surface, which emerge from the electronic Luttinger liquid systems below their crossover temperature to the Fermi liquid, are then coupled to the SC order parameters via the proximity effect. From the Fermi surface thus coupled to the SC order parameters, we calculate the quasiparticle spectrum in detail. We show that the quasiparticle spectrum can be fully gapped or nodal in the uniform SC phase and also in the coexisting phase of the uniform SC and PDW parameters. In the pure PDW state, the excitation spectrum has a reconstructed Fermi surface in the form of Fermi pockets of Bogoliubov quasiparticles.

Hybrid lateral superlattices composed of a square array of antidots and a periodic one-dimensional magnetic modulation are prepared in $\mathrm{Ga}\left[\mathrm{Al}\right]\mathrm{As}$ heterostructures. The two-dimensional electron gases exposed to these superlattices are characterized by magnetotransport experiments in vanishing average perpendicular magnetic fields. Despite the absence of closed orbits, the diagonal magnetoresistivity in the direction perpendicular to the magnetic modulation shows pronounced classical resonances. They are located at magnetic fields where snake trajectories exist which are quasicommensurate with the antidot lattice. The diagonal magnetoresistivity in the direction of the magnetic modulation increases sharply above a threshold magnetic field and shows no fine structure. The experimental results are interpreted with the help of numerical simulations based on the semiclassical Kubo model.

We present measurements of tunneling magnetoresistance (TMR) in single-wall carbon nanotubes attached to ferromagnetic contacts in the Coulomb blockade regime. Strong variations of the TMR with gate voltage over a range of four conductance resonances, including a peculiar double-dip signature, are observed. The data are compared to calculations in the “dressed second order” (DSO) framework. In this nonperturbative theory, conductance peak positions and linewidths are affected by charge fluctuations incorporating the properties of the carbon nanotube quantum dot and the ferromagnetic leads. The theory is able to qualitatively reproduce the experimental data.

The influence of the electric field and electric current on the behavior of oxygen vacancies $\left({\mathrm{V}}_{\mathrm{O}}\mathrm{s}\right)$ in ${\mathrm{TiO}}_2$ anatase was investigated with scanning tunneling microscopy (STM). At the anatase (101) surface ${\mathrm{V}}_{\mathrm{O}}\mathrm{s}$ are not stable; they migrate into the bulk at temperatures above 200 K. Scanning a clean anatase (101) surface at a sample bias greater than $\approx +4.3$ V results in surface ${\mathrm{V}}_{\mathrm{O}}\mathrm{s}$ in the scanned area, suggesting that subsurface ${\mathrm{V}}_{\mathrm{O}}\mathrm{s}$ migrate back to the surface. To test this hypothesis, surface ${\mathrm{V}}_{\mathrm{O}}\mathrm{s}$ were first created through bombardment with energetic electrons. The sample was then mildly annealed, which caused the ${\mathrm{V}}_{\mathrm{O}}\mathrm{s}$ to move to the subsurface region, where they formed vacancy clusters. These ${\mathrm{V}}_{\mathrm{O}}$ clusters have various, distinct shapes. Scanning ${\mathrm{V}}_{\mathrm{O}}$ clusters with a high STM bias reproducibly converts them back into groupings of surface ${\mathrm{V}}_{\mathrm{O}}$, with a configuration that is characteristic for each type of cluster. The dependence of the subsurface-to-surface ${\mathrm{V}}_{\mathrm{O}}$ migration on the applied STM bias voltage, tunneling current, and sample temperature was investigated systematically. The results point towards a key role of energetic, “hot” electrons in this process. The findings are closely related to the memristive behavior of oxides and oxygen diffusion in solid-oxide membranes.

We predict that a single-level quantum dot without discernible splitting of its spin states develops a spin-precession resonance in charge transport when embedded into a spin valve. The resonance occurs in the generic situation of Coulomb blockaded transport with ferromagnetic leads whose polarizations deviate from perfect antiparallel alignment. The resonance appears when electrically tuning the interaction-induced exchange field perpendicular to one of the polarizations—a simple condition relying on vectors in contrast to usual resonance conditions associated with energy splittings. The spin resonance can be detected by stationary $dI/dV$ spectroscopy and by oscillations in the time-averaged current using a gate-pulsing scheme. The generic noncollinearity of the ferromagnets and junction asymmetry allow for an all-electric determination of the spin-injection asymmetry, the anisotropy of spin relaxation, and the magnitude of the exchange field. We also investigate the impact of a nearby superconductor on the resonance position. Our simplistic model turns out to be generic for a broad class of coherent few-level quantum systems.

The DFT/vdW-WF2s1 method, recently developed to include the van der Waals interactions in the density functional theory and describe adsorption processes on metal surfaces by taking metal-screening effects into account, is applied to the case of the interaction of Xe and graphene with a transition-metal surface, namely, Ni(111). In general, the adsorption of rare-gas atoms on metal surfaces is important because it is prototypical for physisorption processes. Moreover, the interaction of graphene with Ni(111) is of great interest for practical applications, for instance concerning the efficient and large-scale production of high-quality graphene; from a theoretical point of view, it is particularly challenging, since it can be described by a delicate interplay between chemisorption and physisorption processes. The first-principles simulation of transition metals requires particular care also because they can be viewed as intermediate systems between simple metals and insulating crystals. Even in these cases the method performs well as demonstrated by comparing our results with available experimental data and other theoretical investigations. We confirm that the rare-gas Xe atom is preferentially adsorbed on the top-site configuration on the Ni(111) surface too. Our approach, based on the use of the maximally localized Wannier functions, also allow us to well characterize the bonds between graphene and Ni(111).

We observed a drastic change in electronic domain structures during the metal-insulator transition (MIT) via strong elastic coupling in single crystalline ${\mathrm{VO}}_2$ films. The domain structure drastically changed from one type to another due to increasing elastic coupling with film thickness. This drastic change was caused by energetic competition between elastic coupling and structural disorder during the MIT, suggesting the microscopic stress engineering facilitate the electronic domain control in functional oxides.

The results of experimental study of quantum interference effects in small magnetic fields in narrow HgTe quantum wells of hole-type conductivity with a normal energy spectrum are presented. Interpretation of the data is performed with taking into account the strong spin-orbit splitting of the energy spectrum of the two-dimensional hole subband. It is shown that the phase relaxation time found from the analysis of the shape of magnetoconductivity curves in the case when the Fermi level lies in the monotonic part of the energy spectrum of the valence band behaves itself analogously to that observed in narrow HgTe quantum wells of electron-type conductivity. It increases in magnitude with the increasing conductivity and decreasing temperature following the $1/T$ law. Such a behavior corresponds to the inelasticity of electron-electron interaction as the main mechanism of the phase relaxation and agrees well with the theoretical predictions. For the higher conductivity, despite the fact that the dephasing time remains inversely proportional to the temperature, it strongly decreases with the increasing conductivity. It is presumed that the reason for such a peculiarity could be nonmonotonic character of the hole energy spectrum. An additional channel of inelastic interaction occurs when the Fermi level approaches the secondary maxima in the depth of the valence band.

We investigate a semiconductor $p\text{−}n$ junction in contact with superconducting leads that is operated under forward bias as a light-emitting diode. The presence of superconductivity results in a significant increase of the electroluminescence in a sharp frequency window. We demonstrate that the tunneling of Cooper pairs induces an additional luminescence peak on resonance. There is a transfer of superconducting to photonic coherence that results in the emission of entangled photon pairs and squeezing of the fluctuations in the quadrature amplitudes of the emitted light. We show that the squeezing angle can be electrically manipulated by changing the relative phase of the order parameters in the superconductors. We finally derive the conditions for lasing in the system and show that the laser threshold is reduced due to superconductivity. This reveals how the macroscopic coherence of a superconductor can be used to control the properties of light.