We present high-frequency ultrasonic measurements on the filled skutterudite ${\mathrm{SmOs}}_4{\mathrm{Sb}}_{12}$ under hydrostatic pressure. The results clarify that the $4f$ electrons in this compound transform from delocalized at ambient pressure to localized at high pressures with a crossover pressure of approximately 0.7 GPa. This drastic change in the $4f$ electrons under pressure is apparently related to the non-Fermi-liquid state, which appears in an intermediate-pressure range of 0.5–1.5 GPa. The results of our analysis strongly suggest that the ferro-octupolar interaction becomes dominant at high pressure. Moreover, we report the pressure dependence of the ultrasonic dispersion, which is due to rattling, over a wide range of ultrasonic frequencies up to 323 MHz. The drastic change in the ultrasonic dispersions and the frequency-dependent elastic anomaly in the $C_{11}$ mode at lower temperatures imply a possible coupling between rattling phonons and $4f$ electrons.

A first-principles investigation of cubic ${\mathrm{BaRuO}}_3$, by combining density functional theory with dynamical mean-field theory and a hybridization expansion continuous time quantum Monte Carlo solver, has been carried out. Nonmagnetic calculations with appropriately chosen on-site Coulomb repulsion $U$ and Hund's exchange $J$ for single-particle dynamics and static susceptibility show that cubic ${\mathrm{BaRuO}}_3$ is in a spin-frozen state at temperatures above the ferromagnetic transition point. A strong redshift with increasing $J$ of the peak in the real frequency dynamical susceptibility indicates a dramatic suppression of the Fermi liquid coherence scale as compared to the bare parameters in cubic ${\mathrm{BaRuO}}_3$. The self-energy also shows clear deviation from Fermi liquid behavior that manifests in the single-particle spectrum. Such a clean separation of energy scales in this system provides scope for an incoherent spin-frozen (SF) phase that extends over a wide temperature range, to manifest in non-Fermi liquid behavior and to be the precursor for the magnetically ordered ground state.

We theoretically investigate the effects of exchange splitting, electric field and strain on the supercurrent in silicene-based Josephson junctions. It is observed that a $0-\pi$ transition is induced by the exchange splitting, and the critical supercurrent at the transition point is nonvanishing. Owing to the unique buckling structure of silicene, the supercurrent can be effectively controlled by the electric field. For the strain dressed supercurrent, it is revealed that the current-phase relation is profoundly influenced by the strain, and the supercurrent exhibits a switch effect with increasing the strength of strain. These findings will potentially provide some intriguing insights into the correlated transport in silicene-based Josephson junctions.

We investigate the interaction between the electrons of a two-dimensional metal and the acoustic phonons of an underlying piezoelectric substrate. Fundamental inequalities can be obtained from general energy arguments. As a result, phonon mediated attraction can be proven to never overcome electron Coulomb repulsion, at least for long phonon wavelengths. We study the influence of these phonons on the possible pairing instabilities of a two-dimensional electron gas such as graphene.

We consider a graphene sheet suspended above a conducting surface. Treating graphene as an elastic membrane subjected to Casimir force, we study its stability against sagging towards the conductor. There exists a critical elevation at the edges below which the central part of the suspended graphene nucleates a trunk that sinks under the action of the Casimir force. The dependence of the critical elevation on temperature, dimensions, and the elastic stress applied to the graphene sheet is computed.

The confinement of a superconductor in a thin film changes its Fermi-level density of states and is expected to change its critical temperature $T_c$. Previous calculations have reported large discontinuities of $T_c$ when the chemical potential coincides with a subband edge. By solving the BCS gap equation exactly, we show that such discontinuities are artifacts and that $T_c$ is a continuous function of the film thickness. We also find that $T_c$ is reduced in thin films compared with the bulk if the confinement potential is lower than a critical value, while for stronger confinement $T_c$ increases with decreasing film thickness, reaches a maximum, and eventually drops to zero. Our numerical results are supported by several exact solutions. We finally interpret experimental data for ultrathin lead thin films in terms of a thickness-dependent effective mass.

We use the random Green's matrix model to study the scaling properties of the localization transition for scalar waves in a three-dimensional (3D) ensemble of resonant point scatterers. We show that the probability density $p\left(g\right)$ of normalized decay rates of quasimodes $g$ is very broad at the transition and in the localized regime and that it does not obey a single-parameter scaling law for finite system sizes that we can access. The single-parameter scaling law holds, however, for the small-$g$ part of $p\left(g\right)$ which we exploit to estimate the critical exponent $\nu$ of the localization transition. Finite-size scaling analysis of small-$q$ percentiles $g_q$ of $p\left(g\right)$ yields an estimate $\nu \simeq 1.55\pm 0.07$. This value is consistent with previous results for the Anderson transition in the 3D orthogonal universality class and suggests that the localization transition under study belongs to the same class.

Magnetization hysteresis loops of a three-dimensional nanoscale analog of spin ice based on the nickel inverse opal-like structure (IOLS) have been studied at room temperature. The samples are produced by filling nickel into the voids of artificial opal-like films. The spin ice behavior is induced by tetrahedral elements within the IOLS, which have the same arrangement of magnetic moments as a spin ice. The thickness of the films vary from a two-dimensional, i.e., single-layered, antidot array to a three-dimensional, i.e., multilayered, structure. The coercive force, the saturation, and the irreversibility field have been measured in dependence of the thickness of the IOLS for in-plane and out-of-plane applied fields. The irreversibility and saturation fields change abruptly from the antidot array to the three-dimensional IOLS and remain constant upon further increase of the number of layers $n$. The coercive force $H_c$ seems to increase logarithmically with increasing $n$ as $H_c=H_{c0}+\alpha ln(n+1)$. The logarithmic law implies the avalanchelike remagnetization of anisotropic structural elements connecting tetrahedral and cubic nodes in the IOLS. We conclude that the “ice rule” is the base of mechanism regulating this process.

We present a muon spin relaxation study on the Ising pyrochlore ${\mathrm{Nd}}_2{\mathrm{Zr}}_2{\mathrm{O}}_7$ which develops an “all-in-all-out” magnetic order below 0.4 K. At 20 mK, far below the ordering transition temperature, the zero-field muon spin relaxation spectra show no static features and can be well described by a dynamical Gaussian-broadened Gaussian Kubo-Toyabe function indicating strong fluctuations of the ordered state. The spectra of the paramagnetic state (below 4.2 K) reveal anomalously slow paramagnetic spin dynamics and show only a small difference with the spectra of the ordered state. We find that the fluctuation rate decreases with decreasing temperature and becomes nearly temperature independent below the transition temperature, indicating persistent slow spin dynamics in the ground state. The field distribution width shows a small but sudden increase at the transition temperature and then becomes almost constant. The spectra in applied longitudinal fields are well fitted by the conventional dynamical Gaussian Kubo-Toyabe function, which further supports the dynamical nature of the ground state. The fluctuation rate shows a peak as a function of external field which is associated with a field-induced spin-flip transition. The strong dynamics in the ordered state is attributed to the transverse coupling of the Ising spins introduced by the multipole interactions.

We consider decay of supercurrent via phase slips in a discrete one-dimensional superconductor (a chain of nodes connected by superconducting links), aiming to explain the experimentally observed power-3/2 scaling of the activation barrier in nanowires at currents close to the critical. We find that, in this discrete model, the power-3/2 scaling holds for both long and short wires even in the presence of bulk superconducting leads, despite the suppression of thermal fluctuations at the ends. We also consider decay via tunneling (quantum phase slips), which becomes important at low temperatures. We find numerically the relevant Euclidean solutions (periodic instantons) and determine the scaling of the tunneling exponent near the critical current. The scaling law is power 5/4, different from that of the thermal activation exponent.

Based on ab initio calculations, we determine the features and relative stability of different models proposed to describe the $(2\times 2)$-FeO(111) reconstruction. Our results suggest that both wurtzite and spinel-like environments are possible, and their coexistence explains phenomena of biphase ordering. The surface phase diagram reflects a competition of charge and magnetic compensation effects, and reveals the important influence of the substrate on the final surface structure. Though antiferromagnetic couplings are dominant, frustration and the delicate balance of surface and bulk exchange interactions lead to a net surface magnetization that accounts for the large measured values.

A pseudomagnetic field kink can be realized along a graphene nanoribbon using strain engineering. Electron transport along this kink is governed by snake states that are characterized by a single propagation direction. Those pseudomagnetic fields point towards opposite directions in the $K$ and $K^{\prime }$ valleys, leading to valley polarized snake states. In a graphene nanoribbon with armchair edges this effect results in a valley filter that is based only on strain engineering. We discuss how to maximize this valley filtering by adjusting the parameters that define the stress distribution along the graphene ribbon.

We reported the collimation and splitting effects of the diffraction of valley electrons in graphene. When the incident energy increases from the neutral point, the diffraction tends to be collimated for one valley and split for the other valley. The difference in the diffraction between valleys results in valley-dependent transport. We investigated the left-right conductance of a four-terminal graphene device. The conductance ratio between the two valleys was derived to be $1-(8/3)E$, where $E$ is the incident energy in units of the atom-atom hopping. The ratio is independent of the device dimensions and reflects the intrinsic properties of the electronic structure of graphene.

The single-band current-dipole Kubo formula for the dynamical conductivity of heavily doped graphene from Kupčić [Phys. Rev. B 91, 205428 (2015)] is extended to a two-band model for conduction $\pi$ electrons in lightly doped graphene. Using a posteriori relaxation-time approximation in the two-band quantum transport equations, with two different relaxation rates and one quasiparticle lifetime, we explain a seemingly inconsistent dependence of the dc conductivity of ultraclean and dirty lightly doped graphene samples on electron doping, in a way consistent with the charge continuity equation. It is also shown that the intraband contribution to the effective number of conduction electrons in the dc conductivity vanishes at $T=0$ K in the ultraclean regime, but it remains finite in the dirty regime. The present model is shown to be consistent with a picture in which the intraband and interband contributions to the dc conductivity are characterized by two different mobilities of conduction electrons, the values of which are well below the widely accepted value of mobility in ultraclean graphene. The dispersions of Dirac and $\pi$ plasmon resonances are reexamined to show that the present, relatively simple expression for the dynamical conductivity tensor can be used to study simultaneously single-particle excitations in the dc and optical conductivity and collective excitations in energy loss spectroscopy experiments.

The particle-particle interaction potential of an $N\text{-atom}$ cluster is expanded in $n\text{-body}$ contributions. The expansion allows us to determine the magnitude of each one of the $n\text{-body}$ terms, and consequently quantifies how $n\text{-body}$ a potential really is. This way we obtain bounds for the relative error due to truncation, a feature that could be applicable in several contexts like the search of minimal energy cluster conformations, to obtain adequate seeds for further ab initio refinement, or to speed up molecular dynamics computations. We develop the formalism, and test the procedure numerically for the Lennard-Jones, Murrel-Motram, Gupta, and Sutton-Chen potentials. The contributions of the $n\text{-body}$ terms for Ag, Al, Au, Co, Cu, Fe, Ir, Ni, Pb, Pd, Pt, and Rh clusters are computed up to $n=9$; they show that the importance and magnitude of the $n>2$ interaction terms depend on the particular element. The relevance of the $n\text{-body}$ corrections as a function of cluster size is also explored for $N\le 50$, and for a linear chain of $N\le 1000$ atoms.

The Seebeck coefficient plays a fundamental role in identifying the efficiency of a thermoelectric device. Its theoretical evaluation for atomistic models is routinely based on density functional theory calculations combined with the Landauer-Büttiker approach to quantum transport. This combination, however, suffers from serious drawbacks for devices in the Coulomb blockade regime. We show how to cure the theory through a simple correction in terms of the temperature derivative of the exchange correlation potential. Our results compare well with both rate equations and experimental findings on carbon nanotubes.

The Anderson impurity model is studied by means of the self-consistent hybridization expansions in its noncrossing (NCA) and one-crossing (OCA) approximations. We have found that for the one-channel spin-$1/2$ particle-hole symmetric Anderson model, the NCA results are qualitatively wrong for any temperature, even when the approximation gives the exact threshold exponents of the ionic states. Actually, the NCA solution describes an overscreened Kondo effect, because it is the same as for the two-channel infinite-$U$ single-level Anderson model. We explicitly show that the NCA is unable to distinguish between these two very different physical systems, independently of temperature. Using the impurity entropy as an example, we show that the low-temperature values of the NCA entropy for the symmetric case yield the limit $S_{\mathrm{imp}}(T=0)\to ln\sqrt{2},$ which corresponds to the zero temperature entropy of the overscreened Kondo model. Similar pathologies are predicted for any other thermodynamic property. On the other hand, we have found that the OCA approach lifts the artificial mapping between the models and restores correct properties of the ground state, for instance, a vanishing entropy at low enough temperatures $S_{\mathrm{imp}}(T=0)\to 0$. Our results indicate that the very well known NCA should be used with caution close to the symmetric point of the Anderson model.