We investigate both theoretically and experimentally the nonlinear propagation of surface-plasmon polaritons (SPP) on a single air/metal interface. Inspired by nonlinear dielectric waveguide theory, we analytically derive a model that describes the nonlinear propagation of SPPs, thus bridging the description of plasmonic and dielectric waveguides. The model, the numerical simulations, and the experiments, which are carried out in the 100 fs regime, reveal that the SPP undergoes strong ultrafast self-action which manifests itself through self-induced absorption. Our observations are consistent with a large, bulk, third-order nonlinear susceptibility $\left({\chi }^{\left(3\right)}\right)$ of gold and provide a self-consistent theory of self-action of SPPs at an air/metal interface. Experimentally, we find $\mathrm{Im}\left\{{\chi }^{-\left(3\right)}\right\}\sim 3\times {10}^{-16}{\mathrm{m}}^2/{\mathrm{V}}^2$. These findings have important implications in the nonlinear physics of plasmonics and metamaterials as they provide evidence that nonlinear absorption has a significant effect on the propagation of SPPs excited by intense optical pulses. This self-action is also expected to affect the anomalous absorption of light near subwavelength structures as well as the strength of desirable nonlinear processes such as third-harmonic generation and four-wave mixing, which will inevitably compete with nonlinear absorption.

We investigate the evolution of both the occupied and unoccupied electronic structure in representative compounds of the infinitely adaptive superlattice series $({\mathrm{Sb}}_2$)${}_m-{\mathrm{Sb}}_2{\mathrm{Te}}_3$ ($m=0–3$) by means of angle-resolved photoemission spectroscopy and time-delayed two-photon photoemission, combined with first-principles band-structure calculations. We discover that the topological nature of the surface states and their spin texture are robust, with dispersions evolving from linear (Dirac-like) to parabolic (Rashba-like) along the series, as the materials evolve from semiconductors to semimetals. Our findings provide a promising strategy for engineering the topological states with the desired flexibility needed for realizing different quantum phenomena and spintronics applications.

We report on first-principles total-energy and phonon calculations that clarify structural stability and electronic properties of freestanding bilayer silicene. By extensive structural exploration, we reach all the stable structures reported before and find four additional dynamically stable structures, including the structure with the largest cohesive energy. We find that atomic protrusion from the layer is the principal relaxation pattern which stabilizes bilayer silicene and determines the lateral periodicity. The hybrid-functional calculation shows that the most stable bilayer silicene is a semiconductor with the energy gap of 1.3 eV.

We study the density of states and the optical conductivity of a Anderson lattice which is immersed in a massless Dirac Fermi sea, as characterized by a linear dispersion relation. As a result of the hybridization $V$ with the $f$-electron levels, the pseudogap in the conduction band becomes duplicated and is shifted both into the upper and lower quasiparticle bands. We find that, due to the linear dispersion of the Dirac fermions, the Kondo-insulator gap is observable in the optical conductivity in contrast to the Kondo lattice system in a conventional conduction band. The reason is that the Kondo-insulator gap becomes a direct gap in the Dirac Fermi sea, while it is a strongly indirect gap in conventional Anderson lattices. The gap ${\Delta }_{\mathrm{gap}}\left(T\right)$ depends on temperature due to Kondo correlations. We find that the optical conductivity attains two peaks. It is vanishing exactly at $2b\left(T\right)V$ where $b\left(T\right)V$ is the effective hybridization between the Dirac electrons and the $f$ electrons, which depends on temperature $T$.

Recent experiments demonstrated that the isovalent doping system gives a similar phase diagram as the heterovalent doped cases. For example, with the phosphorous (P) doping, the magnetic order in ${\mathrm{BaFe}}_2\left({\mathrm{As}}_{1-x}{\mathrm{P}}_x\right){}_2$ compound is first suppressed, then the superconductivity dome emerges to an extended doping region but eventually it disappears at large $x$. With the help of a minimal two-orbital model for both ${\mathrm{BaFe}}_2{\mathrm{As}}_2$ and ${\mathrm{BaFe}}_2{\mathrm{P}}_2$, together with the self-consistent lattice Bogoliubov-de Gennes (BdG) equation, we calculate the phase diagram against the P content $x$ in which the doped isovalent P atoms are treated as impurities. We show that our numerical results can qualitatively compare with the experimental measurements.

We present a theoretical study of optical electron-spin orientation and spin-dependent Shockley-Read-Hall recombination in the longitudinal magnetic field, taking into account the hyperfine coupling between the bound-electron spin and the nuclear spin of a deep paramagnetic center. The master rate equations for the coupled system are extended to describe the nuclear spin relaxation by using two distinct relaxation times, ${\tau }_{n1}$ and ${\tau }_{n2}$, respectively, for defect states with one and two (singlet) bound electrons. The general theory is developed for an arbitrary value of the nuclear spin $I$. The magnetic-field and excitation-power dependencies of the electron and nuclear spin polarizations are calculated for the value of $I=1/2$. In this particular case the nuclear effects can be taken into account by a simple replacement of the bound-electron spin relaxation time by an effective time dependent on free-electron and hole densities and free-electron spin polarization. The role of nuclear spin relaxation is visualized by isolines of the electron spin polarization on a two-dimensional graph with the axes ${\log }_2\left({\tau }_{n1}\right)$ and ${\log }_2\left({\tau }_{n2}\right)$.

Structural properties of amorphous In-based oxides, $\mathrm{In}-X-\mathrm{O}$ with $X=\text{Zn}$, Ga, Sn, or Ge, are investigated using ab initio molecular dynamics liquid-quench simulations. The results reveal that indium retains its average coordination of 5.0 upon 20% $X$ fractional substitution for In, whereas $X$ cations satisfy their natural coordination with oxygen atoms. This finding suggests that the carrier generation is primarily governed by In atoms, in accord with the observed carrier concentration in amorphous In-O and $\mathrm{In}-X-\mathrm{O}$. At the same time, the presence of $X$ affects the number of six-coordinated In atoms as well as the oxygen sharing between the ${\mathrm{InO}}_6$ polyhedra. Based on the obtained interconnectivity and spatial distribution of the ${\mathrm{InO}}_6$ and ${X\mathrm{O}}_x$ polyhedra in amorphous $\mathrm{In}-X-\mathrm{O}$, composition-dependent structural models of the amorphous oxides are derived. The results help explain our Hall mobility measurements in $\mathrm{In}-X-\mathrm{O}$ thin films grown by pulsed-laser deposition and highlight the importance of long-range structural correlations in the formation of amorphous oxides and their transport properties.

We present room temperature electronic and magnetotransport measurements of polycrystalline graphene, grown by chemical vapor deposition, on a $\mathrm{Si}{\mathrm{O}}_2$ dielectric. The measured graphene devices are intentionally spatially inhomogeneous such that the length of the sample is much greater (>1000 times) than the average grain size. At magnetic field $B=0\mathrm{T}$ the electronic transport is well described by a diffusive transport model with contributions from grain boundary scattering significantly larger in the high charge carrier density limit. We find the largest percent change in the magnetoresistivity occurs at the film's Dirac point where the magnetotransport is largely dependent upon charge disorder. Away from the Dirac point we find a modified expression for the charge carrier density dependence of the magnetoresistivity with respect to the case of single-crystal graphene.

We model third-harmonic generation in doped monolayer graphene at terahertz frequencies by employing a nearest-neighbor tight-binding model in the length gauge. We show that for a given incident-field amplitude there is an optimum Fermi level that maximizes the emitted third-harmonic field. The optimum Fermi level depends very strongly on the incident-field amplitude as well as on the scattering time and increasing either enhances the third-harmonic response. We consider the general case of Fermi-level-independent scattering as well as three different scattering mechanisms that are Fermi-level dependent: phonon, long-range impurity, and short-range impurity scattering. For each case, we determine the optimal Fermi level as well as the amplitude of the optimized third-harmonic response for single-cycle incident fields with central frequencies of 1 THz and amplitudes in the range of 25–75 kV/cm. We find that although nonlinear processes beyond third order suppress third-harmonic generation, we still obtain third-harmonic amplitudes as large as 1.6% of the fundamental of the transmitted field.

The electronic structure of $\mathrm{SrTi}{\mathrm{O}}_3\left(001\right)$ surfaces was studied using scanning tunneling spectroscopy and density-functional theory. With high dynamic range measurements, an in-gap transition level was observed on SrO-terminated surfaces, at 2.7 eV above the valence band maximum. The density of centers responsible for this level was found to increase with surface segregation of oxygen vacancies and decrease with exposure to molecular oxygen. Based on these findings, the level is attributed to surface O vacancies. A level at a similar energy is predicted theoretically on SrO-terminated surfaces. For $\mathrm{Ti}{\mathrm{O}}_2$-terminated surfaces, no discrete in-gap state was observed, although one is predicted theoretically. This lack of signal is believed to be due to the nature of the defect wave function involved, as well as the possible influence of transport limitations in the tunneling spectroscopy measurements.

We propose a device consisting of an antidot periodically driven in time by a magnetic field as a fractional quantum Hall counterpart of the celebrated mesoscopic capacitor-based single-electron source. We fully characterize the setup as an ideal emitter of individual quasiparticles and electrons into fractional quantum Hall edge channels of the Laughlin sequence. Our treatment relies on a master equation approach and identifies the optimal regime of operation for both types of sources. The quasiparticle/quasihole emission regime involves in practice only two charge states of the antidot, allowing for an analytic treatment. We show the precise quantization of the emitted charge, we determine its optimal working regime, and we compute the phase-noise/shot-noise crossover as a function of the escape time from the emitter. The emission of electrons, which calls for a larger amplitude of the drive, requires a full numerical treatment of the master equations as more quasiparticle charge states are involved. Nevertheless, in this case the emission of one electron charge followed by one hole per period can also be achieved, and the overall shape of the noise spectrum is similar to that of the quasiparticle source, but the presence of additional quasiparticle processes enhances the noise amplitude.

Optically detected paramagnetic centers in wide-band-gap semiconductors are emerging as a promising platform for nanoscale metrology at room temperature. Of particular interest are applications where the center is used as a probe to interrogate other spins that cannot be observed directly. Using the nitrogen-vacancy (NV) center in diamond as a model system, we propose a strategy to determine the spatial coordinates of weakly coupled nuclear spins. The central idea is to label the target nucleus with a spin polarization that depends on its spatial location, which is subsequently revealed by making this polarization flow back to the NV for readout. Using extensive analytical and numerical modeling, we show that the technique can attain high spatial resolution depending on the NV lifetime and target spin location. No external magnetic field gradient is required, which circumvents complications resulting from changes in the direction of the applied magnetic field, and considerably simplifies the required instrumentation. Extensions of the present technique may be adapted to pinpoint the locations of other paramagnetic centers in the NV vicinity or to yield information on dynamical processes in molecules on the diamond surface.

We report the presence of OH groups on the surfaces of ultrahigh-vacuum-prepared ${\mathrm{Fe}}_3{\mathrm{O}}_4\left(001\right)$ films by means of x-ray photoelectron spectroscopy (XPS). In addition, the variations of Fe electronic states induced by H atoms adsorbed on a Jahn-Teller distorted ${\mathrm{Fe}}_3{\mathrm{O}}_4\left(001\right)$ surface have been studied using in situ scanning tunneling microscopy and scanning tunneling spectroscopy (STS). The obtained XPS spectra of the films have demonstrated the presence of surface OH groups. The STS spectra measured on Fe sites with and without neighboring OH groups have indicated that adsorbed H and their neighboring Fe atoms behave as electron-donor and electron-trapping sites, respectively. This phenomenon changes the Fe atoms from a semiconducting to a metalliclike nature. These local electronic state modifications are related to the occupation of Fe 3d states just below the Fermi level, corresponding to an ${\mathrm{Fe}}^{2+}$-like state. The orbital occupation can be explained by the gain of electrons from the adsorbed H atoms.

Graphene-based materials have long been considered as promising building blocks for a new generation of high-frequency (terahertz) electronic devices, but their use is complicated by the lack of an intrinsic band gap in graphene itself. Here we exploit synthetically controllable incommensuration of twisted graphene bilayers as a scaffold for intercalation of alkali metal ions with the periodicity of the bilayer supercell. Systematic exploration of the energy profiles of the ions as a function of position suggests that the alkali metal ions aggregate commensurately with the symmetry of the twisted bilayer. The intercalated alkali metal ions act as a source of a periodic perturbation on the level of the bilayer supercell, which permits opening and engineering of a band gap between graphene's π bands. The twist angle between the graphene layers determines the structure and disorder of the intercalant sublattice and, consequently, the magnitude of the band gap. Appropriate choices of the intercalant and twist angle thus permit band-gap engineering in graphene. We offer arguments that the impact of intercalation on the all important charge mobility of graphene will be rather small.

We investigated the crystal and electronic structures of ferroelectric $\mathrm{B}{\mathrm{i}}_4\mathrm{T}{\mathrm{i}}_3{\mathrm{O}}_{12}$ single-crystalline thin films site-specifically substituted with $\mathrm{LaCo}{\mathrm{O}}_3$ (LCO). The epitaxial films were grown by pulsed laser epitaxy on $\mathrm{NdGa}{\mathrm{O}}_3$ and $\mathrm{SrTi}{\mathrm{O}}_3$ substrates to vary the degree of strain. With increasing the LCO substitution, we observed a systematic increase in the $c$-axis lattice constant of the Aurivillius phase related with the modification of pseudo-orthorhombic unit cells. These compositional and structural changes resulted in a systematic decrease in the band gap, i.e., the optical transition energy between the oxygen $2p$ and transition-metal $3d$ states, based on a spectroscopic ellipsometry study. In particular, the $\mathrm{Co}3d$ state seems to largely overlap with the Ti $t_{2g}$ state, decreasing the band gap. Interestingly, the applied tensile strain facilitates the band-gap narrowing, demonstrating that epitaxial strain is a useful tool to tune the electronic structure of ferroelectric transition-metal oxides.

Topological magnon insulators host spatially confined edge magnons brought about by the Dzyaloshinskii-Moriya interaction. Bringing two topological magnon insulators into contact results in topologically protected unidirectional interface magnons. These interface modes decay rapidly toward the bulk regions of the sample. As a result, heat and spin currents associated with these magnons are as well unidirectional and strongly confined to a few-nanometer-wide strip along the interface. On top of this, these interface currents follow any geometry owing to the topological nature of the magnons. In this theoretical study, we propose and analyze two recipes for composing magnon waveguides with nanoscale confinement, one from topologically different phases, another from identical phases. We further identify material classes to construct these magnon waveguides and propose an experiment to verify their topological nature.

We present results of measurements obtained from a mesoscopic ring of a highly disordered superconductor. Superimposed on a smooth magnetoresistance background we find periodic oscillations with a period that is independent of the strength of the magnetic field. The period of the oscillations is consistent with charge transport by Cooper pairs. The oscillations persist unabated for more than 90 periods, through the transition to the insulating phase, up to our highest field of 12 T.

Liquid hydrogen is a dense Bose fluid whose equilibrium properties are both calculable from first principles using various theoretical approaches and of interest for the understanding of a wide range of questions in many-body physics. Unfortunately, the pair correlation function $g\left(r\right)$ inferred from neutron scattering measurements of the differential cross section $\frac{d\sigma }{d\Omega }$ from different measurements reported in the literature are inconsistent. We have measured the energy dependence of the total cross section and the scattering cross section for slow neutrons with energies between 0.43 and 16.1 meV on liquid hydrogen at 15.6 K (which is dominated by the parahydrogen component) using neutron transmission measurements on the hydrogen target of the NPDGamma collaboration at the Spallation Neutron Source at Oak Ridge National Laboratory. The relationship between the neutron transmission measurement we perform and the total cross section is unambiguous, and the energy range accesses length scales where the pair correlation function is rapidly varying. At 1 meV our measurement is a factor of 3 below the data from previous work. We present evidence that these previous measurements of the hydrogen cross section, which assumed that the equilibrium value for the ratio of orthohydrogen and parahydrogen has been reached in the target liquid, were in fact contaminated with an extra nonequilibrium component of orthohydrogen. Liquid parahydrogen is also a widely used neutron moderator medium, and an accurate knowledge of its slow neutron cross section is essential for the design and optimization of intense slow neutron sources. We describe our measurements and compare them with previous work.

We have performed powder inelastic neutron scattering measurements on the unconventional superconductor $\beta -\mathrm{FeSe}$ $(T_{\mathrm{c}}\simeq 8\mathrm{K})$. The spectra reveal highly dispersive paramagnetic fluctuations emerging from the square-lattice wave vector $(\pi ,0)$ extending beyond 80 meV in energy. Measurements as a function of temperature at an energy of $\sim 13\mathrm{meV}$ did not show any variation from $T_{\mathrm{c}}$ to 104 K. The results show that FeSe is close to an instability towards $(\pi ,0)$ antiferromagnetism that is characteristic of the parent phases of the high-$T_{\mathrm{c}}$ iron arsenide superconductors, and that the iron paramagnetic moment is neither affected by the orthorhombic-to-tetragonal structural transition at $T_{\mathrm{s}}\simeq 90\mathrm{K}$ nor does it undergo a change in spin state over the temperature range studied.