We perform a theoretical study of nonequilibrium effects in charge transport through a hybrid single-electron transistor based on a small normal metal (N) island with the gate-controlled number of electrons, tunnel-coupled to voltage-biased superconducting (S) electrodes (SINIS). Focusing on the turnstile mode of the transistor operation with the gate voltage driven periodically, and electrons on the island being out of equilibrium, we find that the current quantization accuracy is a nonmonotonic function of the relaxation rate ${\mathrm{\Gamma }}_{\mathcal{F}}$ of the distribution function $\mathcal{F}\left(\epsilon \right)$ on the island due to tunneling, as compared to the drive frequency $f$, electron-electron $1/{\tau }_{ee}$, and electron-phonon $1/{\tau }_{eph}$ relaxation rates. Surprisingly, in the strongly nonequilibrium regime, $f\gg {\mathrm{\Gamma }}_{\mathcal{F}}\gg {\tau }_{ee}^{-1},{\tau }_{eph}^{-1}$, the turnstile current plateau is recovered, similarly to the ideal equilibrium regime, ${\tau }_{eph}^{-1}\gg {\mathrm{\Gamma }}_{\mathcal{F}}$. The plateau is destroyed in the quasiequilibrium regime when the electron-electron relaxation is faster than tunneling.

All single-valent oxide spinels are insulators. The relatively small activation energy in the temperature dependence of resistivity in vanadate spinels led to the speculation that the spinels are near the crossover from localized to itinerant electronic behavior, and the crossover could be achieved under pressure. We have performed a number of experiments and calculations aimed at obtaining information regarding structural changes under high pressure for the whole series of vanadate spinels, as well as transport and magnetic properties under pressure for $\mathrm{Mg}{\mathrm{V}}_2{\mathrm{O}}_4$. We have also studied the crystal structure under pressure of wide-gap insulators $A\mathrm{C}{\mathrm{r}}_2{\mathrm{O}}_4$ ($A=\mathrm{Mg}$, Mn, Fe, Zn) for comparison. Moreover, the relationship between the bulk modulus and the cell volume of $A{\mathrm{V}}_2{\mathrm{O}}_4$ ($A=\mathrm{Mg}$, Mn, Fe, Co, Zn) has been simulated by a density functional theory calculation. The proximity of $A{\mathrm{V}}_2{\mathrm{O}}_4$ spinels to the electronic state crossover under high pressure has been tested by three criteria: (1) a predicted critical V-V bond length, (2) the observation of a sign change in the pressure dependence of Néel temperature, and (3) measurement of a reduced bulk modulus. The obtained results indicate that, although the crossover from localized to itinerant π bonding $\text{V-}3d$ electrons in the $A{\mathrm{V}}_2{\mathrm{O}}_4$ spinels is approached by reducing under pressure the V-V separation $R$, the critical separation $R_{\mathrm{c}}$ is not reached by 20 GPa in $\mathrm{Co}{\mathrm{V}}_2{\mathrm{O}}_4$, which has the smallest V-V separation in the $A{\mathrm{V}}_2{\mathrm{O}}_4$ ($A=\mathrm{Mg}$, Mn, Fe, Co, Zn) spinels.

Memristive switching devices, candidates for resistive random access memory technology, have been shown to switch off through a progression of states with quantized conductance and subsequent noninteger conductance (in terms of conductance quantum $G_0)$. We have performed calculations based on density functional theory to model the switching process for a $\mathrm{Pt}\text{−}{\mathrm{HfO}}_2$-Pt structure, involving the movement of one or two oxygen atoms. Oxygen atoms moving within a conductive oxygen vacancy filament act as tunneling barriers, and partition the filament into weakly coupled quantum wells. We show that the low-bias conductance decreases exponentially when one oxygen atom moves away from interface. Our results demonstrate the high sensitivity of the device conductance to the position of oxygen atoms.

We report quantum transport and Dirac fermions in ${\mathrm{YbMnBi}}_2$ single crystals. ${\mathrm{YbMnBi}}_2$ is a layered material with anisotropic conductivity and magnetic order below 290 K. Magnetotransport properties, nonzero Berry phase, and small cyclotron mass indicate the presence of Dirac fermions. Angular-dependent magnetoresistance indicates a possible quasi-two-dimensional Fermi surface, whereas the deviation from the nontrivial Berry phase expected for Dirac states suggests the contribution of parabolic bands at the Fermi level or spin-orbit coupling.

Semiconductor quantum-dot cavity systems are promising sources for solid-state-based on-demand generation of single photons for quantum communication. Commonly, the spectral characteristics of the emitted single photon are fixed by system properties such as electronic transition energies and spectral properties of the cavity. In the present work we study cavity-enhanced single-photon generation from the quantum-dot biexciton through a partly stimulated nondegenerate two-photon emission. We show that frequency and linewidth of the single photon can be fully controlled by the stimulating laser pulse, ultimately allowing for efficient all-optical spectral shaping of the single photon.

As an alternative to atomistic calculations of long-wavelength acoustic modes of atomically thin layers, which are known to converge very slowly, we propose a quantitatively predictive and physically intuitive approach based on continuum elasticity theory. We describe a layer, independent of its thickness, by a membrane and characterize its elastic behavior by a $(3\times 3)$ elastic matrix as well as the flexural rigidity. We present simple quantitative expressions for frequencies of long-wavelength acoustic modes, which we determine using two-dimensional elastic constants calculated by ab initio density functional theory. The calculated spectra accurately reproduce observed and calculated long-wavelength phonon spectra of graphene and phosphorene, the monolayer of black phosphorus. Our approach also correctly describes the observed dependence of the radial breathing mode frequency on the diameters of carbon fullerenes and nanotubes.

Based on first-principles calculations using density functional theory, we study the vibrational properties and thermal expansion of monoatomic two-dimensional honeycomb lattices: graphene, silicene, germanene, and blue phosphorene. We focus on the similarities and differences of their properties, and try to understand them from their lattice structures. We illustrate that, from graphene to blue phosphorene, a phonon band gap develops due to large buckling-induced mixing of the in-plane and out-of-plane phonon modes. This mixing also influences their thermal properties. Using quasiharmonic approximation, we find that all of them show negative thermal expansion at room temperature.

Wave effects of phonons can give rise to controllability of heat conduction in nanostructures beyond that by particle scattering at surfaces and interfaces. In this paper, we propose a new class of three-dimensional nanostructures: a silicon-nanowire-cage (SiNWC) structure consisting of silicon nanowires (SiNWs) connected by nano-cross-junctions. We perform equilibrium molecular dynamics simulations and find an ultralow value of thermal conductivity of SiNWC, $0.173\mathrm{W}{\mathrm{m}}^{-1}{\mathrm{K}}^{-1}$, which is one order lower than that of SiNWs. By further modal analysis and atomistic Green's function calculations, we identify that the large reduction is due to significant phonon localization induced by the phonon local resonance and hybridization at the junction part in a wide range of phonon modes. This localization effect does not require the cage to be periodic, unlike the phononic crystals, and can be realized in structures that are easier to synthesize, for instance in a form of randomly oriented SiNW network.

The consistency with Onsager's theorem is examined for commonly used perturbative approaches, such as the Redfield and second-order von Neumann master equations, for thermoelectric transport through nanostructures. We study a double quantum dot, which requires coherences between states for a correct description, and we find that these perturbative approaches violate Onsager's theorem. We show that the deviations from the theorem scale with the lead-coupling strength in an order beyond the one considered systematically in the respective approach.

We study the temperature dependence of nuclear magnetic relaxation (NMR) rates to detect unconventional superconductivity in doped topological insulators, such as $M(=\mathrm{Cu},\text{Nb},{\mathrm{Sr})}_x{\mathrm{Bi}}_2{\mathrm{Se}}_3$ and ${\mathrm{Sn}}_{1-x}{\mathrm{In}}_x\mathrm{Te}$. The Hebel-Slichter coherence effect below a critical temperature $T_{\mathrm{c}}$ depends on the superconducting states predicted by a minimal model of doped topological insulators. In a nodal anisotropic topological state similar to the ABM phase in ${}^3\mathrm{He}$, the NMR rate has a conventional $s\text{-wave-like}$ coherence peak below $T_{\mathrm{c}}$. In contrast, in a fully-gapped isotropic topological superconducting state, this rate below $T_{\mathrm{c}}$ exhibits an antipeak profile. Moreover, in a twofold in-plane anisotropic topological superconducting state, there is no coherence effect, which is similar to that in a chiral $p\text{-wave}$ state. We also claim in a model of ${\text{Cu}}_x{\text{Bi}}_2{\text{Se}}_3$ that a signal of the fully-gapped odd-parity state is attainable from the change of the antipeak behavior depending on doping level. Thus, we reveal that the NMR rates shed light on unconventional superconductivity in doped topological insulators.

The strength of a material can be altered by temperature, pressure, grain size, and orientation distributions. At the microscale, neighboring grains often play important roles in the elastic and plastic deformation process. By applying high pressure to a mixture of germanium and gold powder in the vicinity of the germanium phase transition pressure, we found that the deformation behavior of gold largely correlates with that of the surrounding germanium. The deviatoric strain and compressibility of Au behaves anomalously when Ge undergoes a diamond to β-tin structure transition, accompanying a large volume and strength drop. The results demonstrate that the intrinsic strength of a mixed phase could be largely controlled by the other surrounding phase, which is fundamentally important in understanding the deformation mechanism of multiphase materials, especially when one phase undergoes dramatic changes in strength under high pressure conditions.

We present ab initio results for linker decorations in Mg-MOF74, i.e., attaching various metals $\mathcal{M}=\text{Li}$, Na, K, Sc, Cr, Mn, Fe, Ni, Cu, Zn, Rb, Pd, Ag, and Pt near the ring of the linker, creating new strong adsorption sites and thus maximizing small-molecule uptake. We find that in most cases these decorations influence the overall form and structure of Mg-MOF74 only marginally. After the initial screening, we chose metals that bind favorably to the linker and further investigated adsorption of ${\mathrm{H}}_2,{\mathrm{CO}}_2$, and ${\mathrm{H}}_2\mathrm{O}$ for $\mathcal{M}=\text{Li}$, Na, K, and Sc. For the case of ${\mathrm{H}}_2$ we show that up to 24 additional guest molecules can be adsorbed in the metal-organic framework (MOF) unit cell, with binding energies comparable to the original open-metal sites at the six corners of the channel. This leads to a fivefold increase of the molecule uptake in Mg-MOF74, with tremendous impact on many applications in general and hydrogen storage in particular, where the gravimetric hydrogen density increases from 1.63 to 7.28 mass % and the volumetric density increases from 15.10 to 75.50 g ${\mathrm{H}}_2{\mathrm{L}}^{-1}$.

Spin Hall effect in PdPt alloys in contact with ferromagnetic ${\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}$ alloys has been studied by spin torque ferromagnetic resonance technique. The spin torque spin Hall angle (ST-SHA) proves to be contributed by the interfacial and bulk SHAs. The bulk SHA is dominated by the skew scattering and reaches a maximal value for the largest randomization of Pt and Pd atoms at the intermediate alloy composition. In particular, the interfacial SHA becomes prominent for Pt-rich alloys. This phenomenon indicates the enhanced Rashba spin-orbit coupling at the interface, as Pt has a stronger spin-orbit coupling and larger $z$-potential gradient compared to Pd. The present work highlights the interfacial SHA and provides a pathway to improve the functionality and performance of the next generation spintronic devices.

We study the dynamics of topological defects in the magnetic texture of rectangular permalloy thin-film elements during relaxation from random magnetization initial states. Our full micromagnetic simulations reveal complex defect dynamics during relaxation towards the stable Landau closure domain pattern, manifested as temporal power-law decay, with a system-size-dependent cutoff time, of various quantities. These include the energy density of the system and the number densities of the different kinds of topological defects present in the system. The related power-law exponents assume nontrivial values and are found to be different for the different defect types. The exponents are robust against a moderate increase in the Gilbert damping constant and introduction of quenched structural disorder. We discuss details of the processes allowed by conservation of the winding number of the defects, underlying their complex coarsening dynamics.

Magnetocrystalline anisotropy (MCA) in doped ${\mathrm{Ce}}_2{\mathrm{Co}}_{17}$ and other competing structures was investigated using density functional theory. We confirmed that the MCA contribution from dumbbell Co sites is very negative. Replacing Co dumbbell atoms with a pair of Fe or Mn atoms greatly enhance the uniaxial anisotropy, which agrees quantitatively with experiment, and this enhancement arises from electronic-structure features near the Fermi level, mostly associated with dumbbell sites. With Co dumbbell atoms replaced by other elements, the variation of anisotropy is generally a collective effect and contributions from other sublattices may change significantly. Moreover, we found that Zr doping promotes the formation of 1-5 structure that exhibits a large uniaxial anisotropy, such that Zr is the most effective element to enhance MCA in this system.

Combining the recent scanning tunneling microscopy (STM) and angle-resolved photoemission spectroscopy (ARPES) measurements, we construct a tight-binding model suitable for describing the band structure of monolayer FeSe grown on ${\mathrm{SrTiO}}_3$. Then we propose a possible pairing function, which can well describe the gap anisotropy observed by ARPES and has a hidden sign-changing characteristic. Finally, as a test of this pairing function, we further study the nonmagnetic impurity-induced bound states, to be verified by future STM experiments.

We present a detailed study of the combined effects of Sr substitution and hydrostatic pressure on the ground-state properties of ${\mathrm{CaFe}}_2{\mathrm{As}}_2$. Measurements of the electrical resistance and magnetic susceptibility, both at ambient and finite pressure $P\le 2$ GPa, were performed on ${\mathrm{Ca}}_{1-x}{\mathrm{Sr}}_x{\mathrm{Fe}}_2{\mathrm{As}}_2$ single crystals grown out of Sn flux. We find that by Sr substitution the transition temperature to the magnetic/structural phase is enhanced and therefore a higher pressure is needed to suppress the transition to lowest temperature. In addition, the transition to the collapsed tetragonal phase is found at a pressure, which is distinctly higher than in the pure compound. This implies that the stability ranges of both phases shift on the pressure-axis upon doping, but the latter one with a higher rate. These observations suggest the possibility of separating the two phase lines, which intersect already at elevated temperatures for $x=0$ and low Sr concentration levels. For $x=0.177$, we find strong evidence that both phases remain separated down to the lowest temperature and that a zero-resistance state emerges in this intermediate pressure window. This observation indicates that Sr substitution combined with hydrostatic pressure provides another route for stabilizing superconductivity in ${\mathrm{CaFe}}_2{\mathrm{As}}_2$. Our results are consistent with the notion that (i) preserving the fluctuations associated with the structural-magnetic transition to low temperatures is vital for superconductivity to form in this material and that (ii) the nonmagnetic collapsed tetragonal phase is detrimental for superconductivity.

Whether $\mathrm{CaCr}{\mathrm{O}}_3$ is a Mott insulator or a correlated metal is still controversial. We have performed measurements of magnetization, specific heat, and thermal conductivity on $\mathrm{CaCr}{\mathrm{O}}_3$ samples selected from many batches of high-pressure synthesis. The single-crystal $\mathrm{CaCr}{\mathrm{O}}_3$ sample exhibits an unprecedentedly sharp transition at a Néel temperature $T_N\approx 90\mathrm{K}$. The critical behavior of specific heat cannot be rationalized by the renormalization group theory for a second-order magnetic transition. More surprisingly, the thermal conductivity $\kappa$ exhibits an anomalous drop on cooling through $T_N$, which is opposite to all known influence on $\kappa$ from either spin or orbital ordering. We have argued, on the basis of anomalies found in all three measurements and structural data, for the coexistence of itinerant π-bonding electrons in a $c$-axis band and localized $xy$ electrons in $xy$ orbitals responsible for type-C antiferromagnetic order below $T_N$ and the occupation of a pure, localized $xy$ orbital undergoing a Bose-Einstein condensate at $T_N$.