Using a linear-scaling Kubo simulation approach, we have quantitatively investigated the effects of confinement and surface roughness on phonon transport in silicon nanowires (SiNWs) as thick as 55 nm in diameter $R$. The confinement effect leads to significant reduction of phonon group velocity $v$ in SiNWs compared to bulk silicon except at extremely low phonon frequencies $f$, which very likely persists in SiNWs several hundreds of nanometers thick, suggesting the inapplicability of bulk properties, including anharmonic phonon scattering, to SiNWs. For instance, the velocity can be reduced by more than 30% for phonons with $f>4.5$ THz in 55-nm-thick nanowires. In rough SiNWs Casimir's limit, which is valid in confined macroscopic systems, can underestimate the surface scattering by more than one order of magnitude. For a roughness profile with Lorentzian correlation characterized by root-mean-square roughness $\sigma$ and correlation length $L_r$, the frequency-dependent phonon diffusivity $D$ follows power-law dependences $D\propto R^{\alpha }{\sigma }^{-\beta }{L}_r^{\gamma }$, where $\alpha \sim 2$ and $\beta \sim 1$. On average, $\gamma$ increases from 0 to 0.5 as $R/\sigma$ increases. The mean free path and the phonon lifetime essentially follow the same power-law dependences. These dependences are in striking contrast to Casimir's limit, i.e., $D\sim vR/3$, and manifest the dominant role of the change in the number of atoms due to roughness. The thermal conductivity $\kappa$ can vary by one order of magnitude with varying $\sigma$ and $L_r$ in SiNWs, and increasing $\sigma$ and shortening $L_r$ can efficiently lower $\kappa$ below Casimir's limit by one order of magnitude. Our work provides different insights to understand the ultralow thermal conductivity of SiNWs reported experimentally and guidance to manipulate $\kappa$ via surface roughness engineering.

We address the importance of the modern theory of orbital magnetization for spintronics. Based on an all-electron first-principles approach, we demonstrate that the predictive power of the routinely employed “atom-centered” approximation is limited to materials like elemental bulk ferromagnets, while the application of the modern theory of orbital magnetization is crucial in chemically or structurally inhomogeneous systems such as magnetic thin films, and materials exhibiting nontrivial topology in reciprocal and real space, e.g., Chern insulators or noncollinear systems. We find that the modern theory is particularly crucial for describing magnetism in a class of materials that we suggest here—topological orbital ferromagnets.

Structural, magnetic, and electrical-transport properties of $\alpha \text{−}\mathrm{LiFe}{\mathrm{O}}_2$, crystallizing in the rocksalt structure with random distribution of Li and Fe ions, have been studied by synchrotron x-ray diffraction, ${}^{57}\mathrm{Fe}$ Mössbauer spectroscopy, and electrical resistance measurements at pressures up to 100 GPa using diamond anvil cells. It was found that the crystal structure is stable at least to 82 GPa, though a significant change in compressibility has been observed above 50 GPa. The changes in the structural properties are found to be on a par with a sluggish $\mathrm{F}{\mathrm{e}}^{3+}$ high- to low-spin (HS-LS) transition $\left(S=5/2\to S=1/2\right)$ starting at 50 GPa and not completed even at ∼100 GPa. The HS-LS transition is accompanied by an appreciable resistance decrease remaining a semiconductor up to 115 GPa and is not expected to be metallic even at about 200 GPa. The observed feature of the pressure-induced HS-LS transition is not an ordinary behavior of ferric oxides at high pressures. The effect of $\mathrm{F}{\mathrm{e}}^{3+}$ nearest and next-nearest neighbors on the features of the spin crossover is discussed.

We numerically investigate the near-field radiative heat transfer (NFRHT) between graphene and anisotropic magneto-dielectric hyperbolic metamaterials (AMDHMs) according to the fluctuational dissipation theorem. In this configuration, multiple modes, including the $p$- and $s$-polarized surface phonon polaritons (SPhPs) and hyperbolic modes supported by AMDHMs as well as the high-frequency antisymmetric modes supported by graphene for $p$ polarization, can be observed. These extraordinary propagating modes enable the total NFRHT flux between graphene and AMDHMs to exceed that between graphene and SiC nanowires by several times. Numerical results suggest that the hyperbolic modes and SPhPs for both polarizations effectively impact the NFRHT flux via tuning the geometry of AMDHMs and the conductivity of graphene. This study paves the way toward studying the NFRHT involving graphene and metamaterials and facilitates in-depth study of the $s$-polarized NFRHT.

Rashba spin-orbit coupling and a magnetic field perpendicular to the Rashba axis have been predicted to open a partial gap (“helical gap”) in the energy spectrum of noninteracting or weakly interacting one-dimensional quantum wires. By comparing kinetic energy and Coulomb energy we show that this gap opening typically occurs at low electron densities where the Coulomb energy dominates. To address this strongly correlated limit, we investigate Rashba wires using Wigner crystal theory. We find that the helical gap exists even in the limit of strong interactions but its dependence on electron density differs significantly from the weakly interacting case. In particular, we find that the critical magnetic field for opening the gap becomes an oscillatory function of electron density. This changes strongly the expected signature of the helical gap in conductance measurements.

In this paper, we investigate the properties of Fe monolayers deposited on the (111) surface of the topological insulator $\mathrm{S}{\mathrm{b}}_2\mathrm{T}{\mathrm{e}}_3$ by density functional theory simulations. We consider high-coverage monolayers and assume ferromagnetic configurations. We show that upon relaxation, the Fe atoms partly penetrate into the surface and that their magnetic moments are reduced due to the chemical interaction with the Te and Sb atoms of the two topmost layers. We compute the magnetic anisotropy energies and show that the easy axis is in-plane. We investigate hexagonal warping effects and find that the clean $\mathrm{S}{\mathrm{b}}_2\mathrm{T}{\mathrm{e}}_3$(111) displays a large warping term. In spite of this, the surface-state gap for in-plane magnetization is below 1 meV. For very high coverages corresponding to three Fe atoms in the unit cell of $\mathrm{S}{\mathrm{b}}_2\mathrm{T}{\mathrm{e}}_3$(111), no surface Dirac cones are observed except in the case of a metastable model consisting of a hexagonal Fe monolayer lying on top of the substrate, which also exhibits an out-of-plane easy axis. Finally, we discuss the relevance of our paper to recent experiments about magnetic impurities deposited on topological insulators.

Deformations in graphene systems are central elements in the novel field of straintronics. Various strain geometries have been proposed to produce specific properties, but their experimental realization has been limited. Because strained folds can be engineered on graphene samples on appropriate substrates, we study their effects on graphene transport properties. We show the existence of an enhanced local density of states (LDOS) along the direction of the strained fold that originates from localization of higher energy states and provides extra conductance channels at lower energies. In addition to exhibiting sublattice symmetry breaking, these states are valley polarized, with quasiballistic properties in smooth disorder potentials. We confirmed that these results persist in the presence of strong edge disorder, making these geometries viable electronic waveguides. These findings could be tested in properly engineered experimental settings.

We present an experimental and theoretical study of the optically detected magnetic resonance signals for ensembles of negatively charged nitrogen-vacancy (NV) centers in a ${}^{13}\mathrm{C}$ isotopically enriched single-crystal diamond. We observe four broad transition peaks with superimposed sharp features at zero magnetic field and study their dependence on an applied magnetic field. A theoretical model that reproduces all qualitative features of these spectra is developed. Understanding the magnetic-resonance spectra of NV centers in an isotopically enriched diamond is important for emerging applications in nuclear magnetic resonance.

We explore the response of many-body localized (MBL) systems to periodic driving of arbitrary amplitude, focusing on the rate at which they exchange energy with the drive. To this end, we introduce an infinite-temperature generalization of the effective “heating rate” in terms of the spread of a random walk in energy space. We compute this heating rate numerically and estimate it analytically in various regimes. When the drive amplitude is much smaller than the frequency, this effective heating rate is given by linear response theory with a coefficient that is proportional to the optical conductivity; in the opposite limit, the response is nonlinear and the heating rate is a nontrivial power law of time. We discuss the mechanisms underlying this crossover in the MBL phase. We comment on implications for the subdiffusive thermal phase near the MBL transition, and for response in imperfectly isolated MBL systems.

We study the influence of narrowed distributions of the nuclear Overhauser field on the decoherence of a central electron spin in quantum dots. We describe the spin dynamics in quantum dots by the central spin model. We use analytic solutions for uniform couplings and the time dependent density-matrix renormalization group (tDMRG) for nonuniform couplings. With these tools we calculate the dynamics of the central spin for large baths of nuclear spins with or without external magnetic field applied to the central spin. The focus of our study is the influence of initial mixtures with narrowed distributions of the Overhauser field and of applied magnetic fields on the decoherence of the central spin.

To reveal the relative importance of charge doping and defect scattering in substitutionally modified 122 iron pnictides, we perform a systematic first-principles study on selected bands at the Fermi level. Disorder effects are induced by various substitutions using an orbital-based coherent potential approximation. Pronounced level shifts of individual bands suggest that transition-metal substitutions introduce mobile charge carriers into the system. However, important deviations from such a rigid-band scenario as well as spectral broadenings due to impurity scattering correlate with the band character. Finally, a $T$-matrix analysis exhibits a larger intraband than interband scattering consistent with an $s^{+-}$ pairing state. Comparing different substitutions reveals an increase of pair breaking along the transition-metal series.

Accelerated molecular dynamics reveals a mechanism transition and strong temperature dependence of dislocation nucleation from grain boundaries (GBs) in Cu. At stress levels up to $\sim 90\%$ of the ideal dislocation-nucleation stress, atomic shuffling at the $E$ structural unit in a GB acts as a precursor to dislocation nucleation, and eventually a single dislocation is nucleated. At very high stress levels near the ideal dislocation-nucleation stress, a multiple dislocation is collectively nucleated. In these processes, the activation free energy and activation volume depend strongly on temperature. The strain-rate dependence of the critical nucleation stress is studied and the result shows that the mechanism transition from the shuffling-assisted dislocation-nucleation mechanism to the collective dislocation-nucleation mechanism occurs during the strain rate increasing from ${10}^{-4}{\mathrm{s}}^{-1}\mathrm{to}{10}^{10}{\mathrm{s}}^{-1}$.

High-temperature atomic configurations of fcc Fe-Cr-Ni alloys with alloy composition close to austenitic steel are studied in statistical thermodynamic simulations with effective interactions obtained in ab initio calculations. The latter are done taking longitudinal spin fluctuations (LSF) into consideration within a quasiclassical phenomenological model. It is demonstrated that the magnetic state affects greatly the alloy properties, and in particular, it is shown that the LSF substantially modify the bonding and interatomic interactions of fcc Fe-Cr-Ni alloys even at ambient conditions. The calculated atomic short-range order is in reasonable agreement with existing experimental data for ${\mathrm{Fe}}_{0.56}{\mathrm{Cr}}_{0.21}{\mathrm{Ni}}_{0.23}$, which has strong preference for the (001)-type ordering between Ni and Cr atoms. A similar ordering tendency is found for the ${\mathrm{Fe}}_{0.75}{\mathrm{Cr}}_{0.17}{\mathrm{Ni}}_{0.08}$ alloy composition, which approximately corresponds to the widely used 304 and 316 austenitic steel grades.

Chemical doping at the Sr and Ti sites is a feasible way to alter the quantum paraelectric state of $\mathrm{SrTi}{\mathrm{O}}_3$ perovskite. Doping with Pr is known to induce relaxor ferroelectricity at room temperature in the $\mathrm{S}{\mathrm{r}}_{1-x}\mathrm{P}{\mathrm{r}}_x\mathrm{Ti}{\mathrm{O}}_3$ solid solution. The relationship between its dielectric properties and structural phase transition has been debated, but no definitive structural argument has been proposed. Here we present a systematic structural study of $\mathrm{S}{\mathrm{r}}_{1-x}\mathrm{P}{\mathrm{r}}_x\mathrm{Ti}{\mathrm{O}}_3$ $\left(0.020\le x\le 0.150\right)$. We establish the structural phase diagram using high-resolution x-ray powder diffraction by finding the antiferrodistortive structural phase transitions for all the compositions studied. By using pair distribution function analysis, we show the mismatch between local and long-range structures in terms of increased local order parameters. Finally, we propose a correlation between the local structural order parameters and the emergence of hard polar modes as found by Raman spectroscopy. Our results are quantitatively consistent with recent theoretical calculations showing that the increase of local tetragonality and local octahedral tilting above a critical value in fact underlie the polar instability. This confirms that structural orders involving both polar and antiferrodistortive characters compete and cooperate at different levels, promoting ferroelectricity in $\mathrm{S}{\mathrm{r}}_{1-x}\mathrm{P}{\mathrm{r}}_x\mathrm{Ti}{\mathrm{O}}_3$.

The ternary intermetallic compounds ${\mathrm{Gd}}_2{\mathrm{NiSi}}_3$ and ${\mathrm{Er}}_2{\mathrm{NiSi}}_3$ are synthesized in chemically single phase, which are characterized using dc magnetization, ac magnetic susceptibility, heat capacity, and neutron diffraction studies. Neutron diffraction and heat capacity studies confirm that long-range magnetic ordering coexists with the frustrated glassy magnetic components for both compounds. The static and dynamical features of dc magnetization and frequency-dependent ac susceptibility data reveal that ${\mathrm{Gd}}_2{\mathrm{NiSi}}_3$ is a canonical spin-glass system, while ${\mathrm{Er}}_2{\mathrm{NiSi}}_3$ is a reentrant spin cluster-glass system. The spin freezing temperature merges with the long-range antiferromagnetic ordering temperature at 16.4 K for ${\mathrm{Gd}}_2{\mathrm{NiSi}}_3$. ${\mathrm{Er}}_2{\mathrm{NiSi}}_3$ undergoes antiferromagnetic ordering at 5.4 K, which is slightly above the spin freezing temperature at 3 K. The detailed studies of nonequilibrium dynamical behavior, viz., the memory effect and relaxation behavior using different protocols, suggest that both compounds favor the hierarchical model over the droplet model. A large magnetocaloric effect is observed for both compounds. Maximum values of isothermal entropy change $\left(-\mathrm{\Delta }{S}_M\right)$ and relative cooling power (RCP) are found to be 18.4 J/kg K and 525 J/kg for ${\mathrm{Gd}}_2{\mathrm{NiSi}}_3$ and 22.6 J/kg K and 540 J/kg for ${\mathrm{Er}}_2{\mathrm{NiSi}}_3$, respectively, for a change in field from 0 to 70 kOe. The values of RCP are comparable to those of the promising refrigerant materials. A correlation between large RCP and magnetic frustration is discussed for developing new magnetic refrigerant materials.

Hybrid Co/Pd-Permalloy multilayer structures exhibit multiple domain phases resulting from mutual spin imprinting, dependent on the precise layer composition and thickness. While such hybrid structures can support either a pure Landau closure-domain pattern or a perpendicular exchange-spring magnetization structure, numerical analysis also revealed an anomalous mixed Landau-maze domain state. Mutual imprinting of the spin configuration between the layers was strongest for the latter multidomain state, which we propose is a consequence of exchange energy dissipation over the two lateral dimensions. An analytical derivation of domain phase boundaries was consistent with the micromagnetic simulations and gave important insight into their origin.

In this paper we present material specific calculations of superconductor–normal metal heterostructures using density functional theory combined with a semiphenomenological parametrization of the electron-phonon coupling. In particular, we calculate the quasiparticle spectrum of different normal metal overlayers on a Nb(100) host. We find that the Andreev reflection leads to the formation of momentum dependent quasiparticle bands in the normal metal. As a consequence, the spectrum has a strongly momentum dependent induced gap. We develop a model to calculate the superconducting critical temperature from the thickness dependence of the induced gap. In the case of Au/Nb(100) heterostructures we find very good agreement with experiments. Moreover, predictions are made for similar heterostructures of other compounds.

Spin splitting bands that arise in the conduction band minimum (CBM) of the ${\text{WS}}_2$ monolayer (ML) play an important role in the spin-orbit phenomena such as spin-valley coupled electronics. However, application of strain strongly modifies electronic properties of the ${\text{WS}}_2$ ML, which is expected to significantly affect the properties of the spin splitting bands. Here, by using fully-relativistic first-principles calculations based on density-functional theory, we show that a substantial spin splitting band observed in the CBM is effectively controlled and tuned by applying the biaxial strain. We also find that these spin splitting bands induce spin textures exhibiting fully out-of-plane spin polarization in the opposite direction between the $K$ and $Q$ points and their time reversals in the first Brillouin zone. Our study clarifies that the strain plays a significant role in the spin-orbit coupling of the ${\text{WS}}_2$ ML, which has very important implications in designing future spintronics devices.

Cerium dioxide $\left({\mathrm{CeO}}_2\right)$ is considered as a model material for the experimental study of radiation damage in the standard nuclear fuel uranium dioxide $\left({\mathrm{UO}}_2\right)$. In this paper, we present a first-principles study in the framework of the DFT+$U$ approach to investigate the charged point defects and the incorporation of the fission gases Xe and Kr in ${\mathrm{CeO}}_2$ and compare it with published data in ${\mathrm{UO}}_2$. All intrinsic charge states are considered for point defects in contrast to previous published studies. Our calculations prove that ${\mathrm{CeO}}_2$ shows similar behavior to ${\mathrm{UO}}_2$ in the formation of point defects with the same charge states under stoichiometric and nonstoichiometric conditions. The charge states of vacancies have an important effect on the incorporation of fission gas atoms in ${\mathrm{CeO}}_2$. The bound Schottky defect with the two oxygen vacancies along the (100) direction is found to be energetically preferable to trap Xe and Kr atoms both in ${\mathrm{CeO}}_2$ and ${\mathrm{UO}}_2$. Xe and Kr atoms in the cation vacancy sites under nonformal charge states (different from $4-$) in ${\mathrm{CeO}}_2$, unlike in ${\mathrm{UO}}_2$, lose electrons to their neighboring atoms, which is traced back to the absence of the +5 valence state for Ce in contrast to its existence for U.

We demonstrate that current-current correlations (in particular the shot noise), can be used to study the intrinsic superconductivity in a slightly doped Weyl semimetal. The system studied is an N-WS tunneling junction where the left electrode is a normal metal while the right electrode is a Weyl superconductor (WS). The superconductivity supports surface state with crossed flat bands thereby impact the low-energy spectrum. This spectrum displays a modified density of states in the gap region that strongly affects transport characteristics of the N-WS junction. The Fano factor is calculated as a function of the applied bias and is shown to be dependent essentially on the orientation of the surface of WS relative to the tunneling direction. If this orientation supports the occurrence of low-energy state, then the shot noise power decreases with decreasing voltage, a property similar to that prevailing in a junction with Majorana bound state.