We consider a Rashba nanowire with a proximity gap which can be brought into the topological phase by tuning external magnetic field or chemical potential. We study spin and charge of the bulk quasiparticle states when passing through the topological transition for open and closed systems. We show, analytically and numerically, that the spin of bulk states around the topological gap reverses its sign when crossing the transition due to band inversion, independent of the presence of Majorana fermions in the system. This spin reversal can be considered as a bulk signature of topological superconductivity that can be accessed experimentally. We find a similar behavior for the charge of the bulk quasiparticle states, also exhibiting a sign reversal at the transition. We show that these signatures are robust against random static disorder.

High-resolution valence band spectra of ultrathin Ag films on Sn/Si(111)$\sqrt{3}\times \sqrt{3}$ show intrinsic splitting of the quantum-well states (QWSs). Especially at low coverages, the QWSs of such a system display delicate coupling characters with the bulk bands from the substrate. The observed QWS splitting agrees well with the result of the theoretical calculation. We found that the splitting originates from an interface with a finite thickness. In addition, the interface also causes a large $\mathit{sp}$ band splitting due to the Umklapp scattering in the $\overline{\mathrm{\Gamma }}-\overline{M}$ direction of the Ag(111) surface Brillouin zone.

We study charge and spin transport along grain boundaries in single layer graphene in the presence of a quantizing perpendicular magnetic field. Transport states in a grain boundary are produced by the hybridization of Landau zero modes with interfacial states. In selected energy regimes quantum Hall edge states can be deflected either fully or partially into grain boundary states. The degree of edge state deflection is studied in the nonlocal conductance and in the shot noise. We also consider the possibility of grain boundaries as gate-switchable spin filters, a functionality enabled by counterpropagating transport channels laterally confined in the grain boundary.

Two-dimensional (2D) electrides are a new concept material in which anionic electrons are confined in the interlayer space between positively charged layers. We have carried out angle-resolved photoemission spectroscopy measurements on ${\mathrm{Y}}_2\mathrm{C}$, which is a possible 2D electride, in order to verify the formation of 2D electride states in ${\mathrm{Y}}_2\mathrm{C}$. We clearly observe the existence of semimetallic “electride bands” near the Fermi level, as predicted by ab initio calculations, which conclusively demonstrates that ${\mathrm{Y}}_2\mathrm{C}$ is a quasi-2D electride with electride bands derived from interlayer anionic electrons.

We use the poor man's scaling approach to study the phase boundaries of a pair of quantum impurity models featuring a power-law density of states $\rho \left(\varepsilon \right)\propto {\left|\varepsilon \right|}^r$, either vanishing (for $r>0)$ or diverging (for $r<0)$ at the Fermi energy $\varepsilon =0$, that gives rise to quantum phase transitions between local-moment and Kondo-screened phases. For the Anderson model with a pseudogap (i.e., $r>0)$, we find the phase boundary for (a) $0<r<1/2$, a range over which the model exhibits interacting quantum critical points both at and away from particle-hole (p-h) symmetry, and (b) $r>1$, where the phases are separated by first-order quantum phase transitions that are accessible only for broken p-h symmetry. For the p-h-symmetric Kondo model with easy-axis or easy-plane anisotropy of the impurity-band spin exchange, the phase boundary and scaling trajectories are obtained for both $r>0$ and $r<0$. Throughout the regime of weak-to-moderate impurity-band coupling in which poor man's scaling is expected to be valid, the approach predicts phase boundaries in excellent qualitative and good quantitative agreement with the nonperturbative numerical renormalization group, while also establishing the functional relations between model parameters along these boundaries.

A quantum hydrodynamic model is used to study the edge modes of chiral Berry plasmons in two-dimensional materials with nonzero Berry flux. A quantum effect of collective electron motions appears in systems with a high electron density. For the considered edge plasmon, the transcendental equation of the dispersion relation is solved nonlinearly and semianalytically. We predict a one-way chiral edge state in the presence of the quantum statistical effect and quantum diffraction effect. Indeed, the plasmon frequencies for counterpropagating edge modes exhibit different long-wavelength limits. The quantum effect can enhance the chirality of edge plasmons and their spatial confinement.

Larmor's theorem holds for magnetic systems that are invariant under spin rotation. In the presence of spin-orbit coupling this invariance is lost and Larmor's theorem is broken: for systems of interacting electrons, this gives rise to a subtle interplay between the spin-orbit coupling acting on individual single-particle states and Coulomb many-body effects. We consider a quasi-two-dimensional, partially spin-polarized electron gas in a semiconductor quantum well in the presence of Rashba and Dresselhaus spin-orbit coupling. Using a linear-response approach based on time-dependent density-functional theory, we calculate the dispersions of spin-flip waves. We obtain analytic results for small wave vectors and up to second order in the Rashba and Dresselhaus coupling strengths $\alpha$ and $\beta$. Comparison with experimental data from inelastic light scattering allows us to extract $\alpha$ and $\beta$ as well as the spin-wave stiffness very accurately. We find significant deviations from the local density approximation for spin-dependent electron systems.

Using a nonequilibrium-Green-function method, we numerically studied the transport properties of a superconductor-graphene-superconductor Josephson junction hybrid system in the quantum Hall regime. Our numerical calculations show that there are two interference patterns of the critical current due to the unique band structure of graphene. One is caused by the usual intraband Andreev retroreflection process, and the other one is caused by the interband specular Andreev reflection process. In the Andreev retroreflection regime, chiral Andreev edge states are formed and a distinct supercurrent can be observed. The critical current displays an AB oscillation behavior and the period is approximately $2{\mathrm{\Phi }}_0=h/e$. As for the specular Andreev refection process, the reflected holes are bent back to the reverse direction of the incident electrons and the supercurrent flows along both edges. It is similar to a superconductor ring Josephson junction and the period is ${\mathrm{\Phi }}_0=h/2e$. However, the critical current for the specular Andreev reflection process is very small and is unlikely to be observable in an experiment. Thus, we conclude that our numerical calculations are inconsistent to the experimental findings by Amet et al. [Science 352, 966 (2016)].

The radiative heat transfer between two dielectrics can be strongly enhanced in the near field in the presence of surface phonon-polariton resonances. Nevertheless, the spectral mismatch between the surface modes supported by two dissimilar materials is responsible for a dramatic reduction of the radiative heat flux they exchange. In the present paper we study how the presence of a graphene sheet, deposited on the material supporting the surface wave of lowest frequency, allows us to widely tune the radiative heat transfer, producing an amplification factor going up to one order of magnitude. By analyzing the Landauer energy transmission coefficients we demonstrate that this amplification results from the interplay between the delocalized plasmon supported by graphene and the surface polaritons of the two dielectrics. We finally show that the effect we highlight is robust with respect to the frequency mismatch, paving the way to an active tuning and amplification of near-field radiative heat transfer in different configurations.

For decades, numerous attempts have been made to produce polar oxynitride perovskites, where some of the oxygen is replaced by nitrogen, but a polar ordered oxynitride has never been demonstrated. Caracas and Cohen [Appl. Phys. Lett. 91, 092902 (2007)] studied possible ordered polar oxynitrides within density-functional theory (DFT) and found a few candidates that were predicted to be insulating and at least metastable. $\mathrm{YSi}{\mathrm{O}}_2\mathrm{N}$ stood out with huge predicted polarization and nonlinear optic coefficients. In this study, we demonstrate the synthesis of perovskite-structured $\mathrm{YSi}{\mathrm{O}}_2\mathrm{N}$ by using a combination of a diamond-anvil cell and in situ laser-heating techniques. Subsequent in situ x-ray diffraction, second-harmonic generation, and Raman-scattering measurements confirm that it is polar and a strong nonlinear optical material, with structure and properties similar to those predicted by DFT.

The effect of Ti and Hf impurities on the $\left(111\right)$ antiphase boundary (APB) energy of ${\mathrm{Ni}}_3\mathrm{Al}$ is investigated via ab initio calculations. Cluster expansion is performed to predict supercell total energies sampled in a Monte Carlo approach that accounts for nondilute point defects at finite temperature, obtaining APB energies as a function of impurity concentration and temperature. Of the two ternary elements, Hf is more effective in increasing the APB energy. While the $\left(111\right)$ APB energy of a pure ${\mathrm{L}1}_2$ material requires at least second-nearest-neighbor interactions, we observe a strong correlation between impurity-induced APB energy enhancement and formation of first-nearest-neighbor Ni-Ni bonds across the APB due to symmetry breaking. Using a linear-chain model and effective bond energies derived from effective cluster interactions, we propose a mechanism that explains why Hf is more effective than Ti.

We report on dynamic effects associated with thermally annealing amorphous indium-oxide films. In this process, the resistance of a given sample may decrease by several orders of magnitude at room temperatures, while its amorphous structure is preserved. The main effect of the process is densification, i.e., increased system density. The study includes the evolution of the system resistivity during and after the thermal treatment, the changes in the conductance noise, and the accompanying changes in the optical properties. The sample resistance is used to monitor the system dynamics during the annealing period as well as the relaxation that ensues after its termination. These reveal slow processes that fit well with a stretched-exponential law, a behavior that is commonly observed in structural glasses. There is an intriguing similarity between these effects and those obtained in high-pressure densification experiments. Both protocols exhibit the “slow spring-back” effect, a familiar response of memory foams. A heuristic picture based on a modified Lennard-Jones potential for the effective interparticle interaction is argued to qualitatively account for these densification-rarefaction phenomena in amorphous materials, whether affected by thermal treatment or by application of high pressure.

Coherent phonon transport is regarded as a promising strategy for controlling thermal properties in solids using the wave nature of phonons. However, no clear distinction between the spatial and temporal phonon coherence has been accounted for and a formalism that quantifies these two effects is still to be found. In this work, we propose a statistical approach for calculating the spatial and temporal coherence spectra using molecular dynamics simulations. We provide a microscopic assessment of these properties and we theoretically demonstrate that, while temporal and spatial coherence can be analytically related under specific conditions, they represent two characteristic lengths that set apart different physical effects. The former is associated with the phonon mean free path while the latter can be regarded as a measure of localization, representing the spatial extension of phonon wave packets. This provides a framework to engineer heat conduction in solids by quantitatively revealing the wave/particle nature of the vibrational modes.

The magnetic phase diagram of a plaquette dimerized and frustrated antiferromagnetic ladder system is studied by using a combination of numerical and analytical techniques. For the strongly frustrated regime, series expansions and bond operator techniques are employed to analyze zero magnetic field, whereas low-energy effective models are used to study the complete magnetization process. The interplay between frustration and dimerization gives rise to a rich plateau structure that is captured by effective models and corroborated by numerical density matrix renormalization group simulations, in particular, the emergence of intermediate plateaus at $M=\frac{1}{4}$ and $\frac{3}{4}$ of saturation in the magnetization curve is clarified.

Multicore magnetic nanoparticles show promising features for biomedical applications. Their magnetic properties, however, are not well understood to date, so that several ad hoc assumptions are often needed to interpret experimental results. Here, we present a comprehensive computer simulation study on the effect of dipolar interactions and magnetic anisotropy on the equilibrium magnetization and magnetization relaxation dynamics of monodisperse multicore magnetic nanoparticles in viscous solvents. We include thermal fluctuations of the internal Néel relaxation via the stochastic Landau-Lifshitz-Gilbert equation coupled to rotational Brownian motion of the cluster. We find that the effective magnetic moment of the cluster is reduced compared to the noninteracting case due to frustrated dipole-dipole interactions. Furthermore, the magnetization relaxation is found to proceed in a two-step fashion with a fast initial decay being followed by a long-time relaxation. For moderate dipolar interaction strengths, the latter can be approximated quite well by an exponential decay with rate given by the sum of the relaxation rates in the immobilized state and the Brownian rotation. These findings can be helpful for a better interpretation of experimental data obtained from magnetization relaxation measurements.

Understanding the structural origins of the properties of amorphous materials remains one of the most important challenges in structural science. In this study, we demonstrate that local “structural simplicity”, embodied by the degree to which atomic environments within a material are similar to each other, is a powerful concept for rationalizing the structure of amorphous silicon ($a$-Si) a canonical amorphous material. We show, by restraining a reverse Monte Carlo refinement against pair distribution function (PDF) data to be simpler, that the simplest model consistent with the PDF is a continuous random network (CRN). A further effect of producing a simple model of $a$-Si is the generation of a (pseudo)gap in the electronic density of states, suggesting that structural homogeneity drives electronic homogeneity. That this method produces models of $a$-Si that approach the state-of-the-art without the need for chemically specific restraints (beyond the assumption of homogeneity) suggests that simplicity-based refinement approaches may allow experiment-driven structural modeling techniques to be developed for the wide variety of amorphous semiconductors with strong local order.

Thermal conductivities of $\mathrm{L}{\mathrm{a}}_3\mathrm{C}{\mathrm{u}}_3{X}_4(X=\mathrm{P},\mathrm{As},\mathrm{Sb},\mathrm{Bi})$ compounds are examined using first-principles density functional theory and Boltzmann transport methods. We observe a trend of increasing lattice thermal conductivity (${\kappa }_l$) with increasing atomic mass, challenging our expectations, as lighter mass systems typically have larger sound speeds and weaker intrinsic scattering. In particular, we find that $\mathrm{L}{\mathrm{a}}_3\mathrm{C}{\mathrm{u}}_3{\mathrm{P}}_4$ has the lowest ${\kappa }_l$, despite having larger sound speed and the most restricted available phase space for phonon-phonon scattering, an important criterion for estimating and comparing ${\kappa }_l$ among like systems. The origin of this unusual behavior lies in the strength of the individual anharmonic phonon scattering matrix elements, which are much larger in $\mathrm{L}{\mathrm{a}}_3\mathrm{C}{\mathrm{u}}_3{\mathrm{P}}_4$ than in the heavier $\mathrm{L}{\mathrm{a}}_3\mathrm{C}{\mathrm{u}}_3\mathrm{B}{\mathrm{i}}_4$ system. Our finding provides insights into the interplay of harmonic and anharmonic properties of complex, low-thermal-conductivity compounds, of potential use for thermoelectric and thermal barrier coating applications.