We report calculations of the electronic structure, vibrational properties, and transport for the $p$-type semiconductors, $\mathrm{SrAg}Ch\mathrm{F}$ ($Ch = \mathrm{S}$, Se, and Te). We find soft phonons with low frequency optical branches intersecting the acoustic modes below $50 {\text{cm}}^{-1}$, indicative of a material with low thermal conductivity. The bands at and near the valence-band maxima are highly two-dimensional, which leads to high thermopowers even at high carrier concentrations, which is a combination that suggests good thermoelectric performance. These materials may be regarded as bulk realizations of superlattice thermoelectrics.

Ultrafast lattice dynamics of a few quintuple layers (QLs) of the topological insulator (TI) ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ is studied with time-resolved optical pump-probe spectroscopy. Both optical and acoustic phonons are photogenerated and detected. Here, in order to get new insights into out-of-equilibrium electron-phonon coupling and phonons dynamics in confined TI, different nanostructures are investigated (single or polycrystalline QL assemblies and nanocrystallized islands). Contrary to previous literature claims, we show that even for nanostructures containing only 10 QLs, the symmetric A1g(I) coherent optical phonon is efficiently photogenerated and no restriction due to structural confinement appears. We also observe that whatever the arrangement of the nanostructures, the A1g(I) optical phonon features are similar (lifetime). We also report the observation of confined coherent acoustic phonons propagating from QLs to QLs whose spectrum is, this time, very sensitive to the atomic arrangement. In the case of the single-crystalline ultrathin film, time-of-flight analysis of these acoustic phonons provides direct estimates of the elastic properties of these nanostructures as well as some estimates of van der Waals interactions between QLs.

We employed in situ pulsed laser deposition (PLD) and angle-resolved photoemission spectroscopy (ARPES) to investigate the mechanism of the metal-insulator transition (MIT) in ${\mathrm{NdNiO}}_3$ (NNO) thin films, grown on ${\mathrm{NdGaO}}_3$(110) and ${\mathrm{LaAlO}}_3$(100) substrates. In the metallic phase, we observe three-dimensional hole and electron Fermi surface (FS) pockets formed from strongly renormalized bands with well-defined quasiparticles. Upon cooling across the MIT in NNO/NGO sample, the quasiparticles lose coherence via a spectral weight transfer from near the Fermi level to localized states forming at higher binding energies. In the case of NNO/LAO, the bands are apparently shifted upward with an additional holelike pocket forming at the corner of the Brillouin zone. We find that the renormalization effects are strongly anisotropic and are stronger in NNO/NGO than NNO/LAO. Our study reveals that substrate-induced strain tunes the crystal field splitting, which changes the FS properties, nesting conditions, and spin-fluctuation strength, and thereby controls the MIT via the formation of an electronic order parameter with ${\mathrm{Q}}_{\mathrm{AF}}\sim (1/4,1/4,1/4\pm \delta )$.

The process of resonance energy transfer (RET) in a nanostructure influenced by a vicinal, nonabsorbing third body is studied within the framework of molecular quantum electrodynamics. Direct RET and the influence of neighboring matter have been studied previously, mainly for molecules. However, a complete study or unified understanding of direct and indirect RET in nanostructures with different dimensionalities is still lacking. Therefore, there is a strong need for a complete theory that models RET for the cases of quantum wells, nanowires, and quantum dots. We construct a detailed picture of excitation energy transfer in nanostructures and how it is affected by another quantum object, which includes the derivation of quantum amplitudes based on second- and fourth-order time-dependent perturbation theories, and the derivation of transfer rates and distance dependencies, providing a complete picture and understanding of RET in nanostructures. The results of the derivations indicate that the dimensionality of the nanostructure determines the controllability of the RET rate. Furthermore, third-body mediation leads to a nonvanishing RET in the coupling of nanowire to nanowire and quantum dot to quantum dot.

We investigate magnetic and superconducting properties of layered $f$-electron superlattices within the fluctuation exchange approximation (FLEX). We show that spin fluctuations, which are characterized by the maximum value of the spin susceptibility in the three-dimensional (3D) Brillouin zone, are strongly suppressed in $f$-electron superlattices. However, effective 2D spin fluctuations can be increased due to the spatial confinement of the $f$ electrons. Therefore the tendency towards $d_{x^2-y^2}$-wave superconductivity, mediated by these spin fluctuations, can be strongly increased in $f$-electron superlattices. This is in sharp contrast to superlattices composed of conventional $s$-wave superconductors, where superconductivity is generally suppressed.

We investigate the molecular beam epitaxy (MBE) growth of $\mathrm{GaA}{\mathrm{s}}_{1-x}\mathrm{B}{\mathrm{i}}_x$ film using density functional theory with spin-orbit coupling to understand the growth of this film, especially the mechanisms of Bi incorporation. We study the stable adsorption structures and kinetics of the incident molecules ($\mathrm{A}{\mathrm{s}}_2$ molecule, Ga atom, Bi atom, and $\mathrm{B}{\mathrm{i}}_2$ molecule) on the $\left(2\times 1\right)\text{-G}{\mathrm{a}}_{\mathrm{sub}}\left|\right|\mathrm{Bi}$ surface and a proposed $q\left(1\times 1\right)\text{-G}{\mathrm{a}}_{\mathrm{sub}}\left|\right|\mathrm{AsAs}$ surface, where $\mathrm{G}{\mathrm{a}}_{\mathrm{sub}}\left|\right|XY$ refers to a Ga-terminated $\mathrm{GaAs}\left(001\right)$ substrate with surface layers of $X$ and $Y$. The $q\left(1\times 1\right)\text{-G}{\mathrm{a}}_{\mathrm{sub}}\left|\right|\mathrm{AsAs}$ surface has a quasi-$(1\times 1)$ As layer above the Ga-terminated GaAs substrate and a randomly oriented As dimer layer on top. We obtain the desorption and diffusion barriers of the adsorbed molecules and also the reaction barriers of three key processes related to Bi evolution, namely, Bi incorporation, $\mathrm{As}/\mathrm{Bi}$ exchange, and Bi clustering. The results help explain the experimentally observed dependence of Bi incorporation on the $\mathrm{As}/\mathrm{Ga}$ ratio and growth temperature. Furthermore, we find that $\mathrm{A}{\mathrm{s}}_2$ exchange with Bi of the $\left(2\times 1\right)\text{-G}{\mathrm{a}}_{\mathrm{sub}}\left|\right|\mathrm{Bi}$ surface is a key step controlling the kinetics of the Bi incorporation. Finally, we explore two possible methods to enhance the Bi incorporation, namely, replacing the MBE growth mode from codeposition of all fluxes with a sequential deposition of fluxes and applying asymmetric in-plane strain to the substrate.

We demonstrate theoretically that a dissipationless spin current can flow a long distance through a diffusive normal metal by using superconductors interfaced with magnetic insulators. The magnitude of this spin supercurrent is controlled via the magnetization orientation of the magnetic insulators. The spin supercurrent obtained in this way is conserved in the normal metal just like the charge current and interestingly has a term that is independent of the superconducting phase difference. The quantum state of the system can be switched between 0 and $\pi$ by reversing the insulators from a parallel to antiparallel configuration with an external field. We show that the spin current is carried through the normal metal by superconducting triplet (odd-frequency) correlations and that the superconducting phase difference can be used to enhance these, leaving clear spectroscopic fingerprints in the density of states.

We examine the phenomenon of flexoelectric switching of polarization in ultrathin films of barium titanate induced by a tip of an atomic force microscope (AFM). The spatial distribution of the tip-induced flexoelectricity is computationally modeled both for perpendicular mechanical load (point measurements) and for sliding load (scanning measurements), and compared with experiments. We find that (i) perpendicular load does not lead to stable ferroelectric switching in contrast to the load applied in the sliding contact load regime, due to nontrivial differences between the strain distributions in both regimes: ferroelectric switching for the perpendicular load mode is impaired by a strain gradient inversion layer immediately underneath the AFM tip; while for the sliding load regime, domain inversion is unimpaired within a greater material volume subjected to larger values of the mechanically induced electric field that includes the region behind the sliding tip; (ii) beyond a relatively small value of an applied force, increasing mechanical pressure does not increase the flexoelectric field inside the film, but results instead in a growing volume of the region subjected to such field that aids domain nucleation processes; and (iii) the flexoelectric coefficients of the films are of the order of few nC/m, which is much smaller than for bulk $\mathrm{BaTi}{\mathrm{O}}_3$ ceramics, indicating that there is a “flexoelectric size effect” that mirrors the ferroelectric one.

We have investigated the impact of Ru substitution for Co on the behavior of the ferromagnetic superconductor UCoGe by performing x-ray diffraction, magnetization, specific heat, and electrical resistivity measurements on polycrystalline samples of the ${\mathrm{UCo}}_{1-x}{\mathrm{Ru}}_x\mathrm{Ge}$ series $(0\ge x\le 0.9)$. The initial Ru substitution up to $x\approx 0.1$ leads to a simultaneous sharp increase of the Curie temperature and spontaneous magnetization up to maximum values of $T_{\mathrm{C}}=8.6\text{K}$ and $M_{\mathrm{S}}=0.1{\mu }_{\mathrm{B}}$ per formula unit, respectively, whereas superconductivity vanishes already for $x\approx 0.03$. Further increase of the Ru content beyond $x\approx 0.1$ leads to a precipitous decrease of both $T_{\mathrm{C}}$ and $M_{\mathrm{S}}$ towards a ferromagnetic quantum critical point (QCP) at $x_{\mathrm{cr}}=0.31$. Consequently, the $T-x$ magnetic phase diagram consists of a well-developed ferromagnetic dome. We discuss the evolution of ferromagnetism with $x$ on the basis of band structure changes due to varying $5f$-ligand hybridization. This scenario is supported by the results of electronic structure calculations and consideration of the simplified periodic Anderson model. The analysis of the temperature dependencies of the electrical resistivity and heat capacity at low temperatures of the samples in the vicinity of the QCP reveals a non-Fermi-liquid behavior and assigns the ferromagnetic quantum phase transition to be most likely of a continuous Hertz-Millis type.

While point defects in elemental (Si) and compound (GaAs, GaN, AlN) semiconductors have been extensively studied both experimentally and theoretically, only limited theoretical studies of these defects exist for technologically important binary $\left(\mathrm{S}{\mathrm{i}}_x\mathrm{G}{\mathrm{e}}_{1\text{−}x}\right)$ and pseudobinary $(\mathrm{I}{\mathrm{n}}_x\mathrm{G}{\mathrm{a}}_{1\text{−}x}\mathrm{As}$, $\mathrm{I}{\mathrm{n}}_{\mathrm{x}}\mathrm{G}{\mathrm{a}}_{1\text{−}x}\mathrm{N}$, $\mathrm{A}{\mathrm{l}}_x\mathrm{G}{\mathrm{a}}_{1\text{−}x}\mathrm{N})$ semiconductor alloys. Here, we use density-functional theory and a recently developed bounds-analysis approach to survey the atomic structures, formation energies, and charge-state transition levels of the stable and metastable states of As interstitials in the pseudobinary alloy $\mathrm{I}{\mathrm{n}}_{0.5}\mathrm{G}{\mathrm{a}}_{0.5}\mathrm{As}$. Our studies consider seven different candidate defect structures for the As interstitial, with calculations performed for selected defect charge states in the range $q=-2$ to $+3$. In each case, the mean and standard deviations of the defect-formation energy are determined using statistical sampling methods that place the defect into a wide variety of differing local-alloy environments. When examined from the point of view of the mean formation energy of the defect, the stable configurations of the As interstitial in $\mathrm{I}{\mathrm{n}}_{0.5}\mathrm{G}{\mathrm{a}}_{0.5}\mathrm{As}$ are found to resemble previous findings for GaAs, with a ${\mathrm{C}}_{1\mathrm{h}}\text{−}\mathrm{p}{001}_{\mathrm{III}}$ interstitial structure in a $q=+1$ charge state favored near midgap and below, and a ${\mathrm{C}}_{2\mathrm{v}}\text{−}110_{\mathrm{a}}$ split-interstitial structure in a $q=-1$ charge state favored above midgap (the named point-group symmetries refer to the underlying symmetry that the alloy defect would possess if within GaAs). The statistical sampling reveals a strong dependence of the defect-formation energy on the local-alloy environment, with the standard deviation $\sigma$ of the formation energy approaching 0.21 eV for the most stable As-interstitial structures. Because the range of ground-state energies encountered by an As-interstitial defect when moving through the alloy is found to be quite large, approaching $\sim 1.2\mathrm{eV}(\pm 3\sigma )$, defect-diffusion pathways in $\mathrm{I}{\mathrm{n}}_{0.5}\mathrm{G}{\mathrm{a}}_{0.5}\mathrm{As}$ will have spatial and temporal complexities not found in GaAs.

The spin-orbit interaction was found to split the cyclotron resonance of heavy holes confined in high-mobility, compressively strained germanium quantum wells. The interference between coherent spin-split cyclotron resonances was tracked on picosecond time scales using terahertz time-domain spectroscopy. Analysis in the time domain, or using a time-frequency decomposition based on the Gabor-Morlet wavelet, was necessary when the difference between cyclotron frequencies was comparable to the linewidth. The cubic Rashba spin-orbit coefficient $\beta$ was determined via two methods: (i) the magnetic-field dependence of the cyclotron frequencies, and (ii) the spin-resolved subband densities. An enhanced $\beta$ and spin polarization was created by tailoring the strain to enhance the spin-orbit interaction. The amplitude modulation of the narrow, interfering cyclotron resonances is a signature of spin coherences persisting for more than 10 ps.

We investigate theoretically the effect of nearby As (arsenic) vacancies on the magnetic properties of substitutional Mn (manganese) impurities on the GaAs (110) surface, using a microscopic tight-binding model which captures the salient features of the electronic structure of both types of defects in GaAs. The calculations show that the binding energy of the Mn acceptor is essentially unaffected by the presence of a neutral As vacancy, even at the shortest possible ${\mathrm{V}}_{\mathrm{As}}$-Mn separation. On the other hand, in contrast to a simple tip-induced-band-bending theory and in agreement with experiment, for a positively charged As vacancy the Mn-acceptor binding energy is significantly reduced as the As vacancy is brought closer to the Mn impurity. For two Mn impurities aligned ferromagnetically, we find that nearby charged As vacancies enhance the energy level splitting of the associated coupled acceptor levels, leading to an increase of the effective exchange interaction. Neutral vacancies leave the exchange splitting unchanged. Since it is experimentally possible to switch reversibly between the two charge states of the vacancy, such a local electric manipulation of the magnetic dopants could result in an efficient real-time control of their exchange interaction.

We theoretically show that buckled two-dimensional graphenelike materials (silicene and germanene) subjected to a femtosecond strong optical pulse can be controlled by the optical field component normal to their plane. In such strong fields, these materials are predicted to exhibit nonreciprocal reflection, optical rectification, and generation of electric currents both parallel and normal to the in-plane field direction. Reversibility of the conduction band population is also field- and carrier-envelope phase controllable. There is a net charge transfer along the material plane that is also dependent on the normal field component. Thus a graphenelike buckled material behaves analogously to a field-effect transistor controlled and driven by the electric field of light with subcycle (femtosecond) speed.

Shock-induced freezing in liquids has long been a subject of interest as well as mystery. With large-scale molecular dynamics simulations, we demonstrate that homogeneous crystal nucleation in liquid Cu can be realized under effective supercooling $\left(\theta \right)$, via quasi-isentropic compression or ramp wave loading with a particle velocity $u_{\mathrm{p}}$ achieved within a ramp time $\tau$. The simulations yield the $\tau \text{−}{u}_{\mathrm{p}}\text{−}\theta$ relations for homogeneous crystallization at the spatial and temporal scales of molecular dynamics; a ramp wave loading with $\tau \sim 100$ ps is essentially isentropic for liquid Cu. Based on classical nucleation theory, we predict $\theta$, and thus $u_{\mathrm{p}}$, as required for homogenous nucleation in experiments with larger length and time scales. Homogeneous nucleation can also be achieved with shock loading in initially supercooled liquids.

Complex metallic alloys having isolated transition-metal elements in the surface layer have been reported to work well as selective hydrogenation catalysts. We report an experimental determination of the surface structure of one such compound $\mathrm{A}{\mathrm{l}}_{13}\mathrm{F}{\mathrm{e}}_4\left(010\right)$. The structure was determined using low-energy electron diffraction. The best-fit structure terminates in a layer similar to the puckered bulk layer but lacking some of the Al and Fe atoms. Protruding Fe atoms are located in the middle of adjacent pentagonal Al formations, connected to each other by Al “glue” atoms. The top interlayer spacing is compressed relative to the bulk with oscillating relaxations observed for subsequent layers at the surface.

We present a comprehensive set of first-principles electronic structure calculations of the properties of substitutional transition metal solutes and point defects in austenite (face-centered cubic, paramagnetic Fe). Clear trends were observed in these quantities across the transition metal series, with solute-defect interactions strongly related to atomic size, and only weakly related to more subtle details of magnetic or electronic structure. Oversized solutes act as strong traps for both vacancy and self-interstitial defects and as nucleation sites for the development of protovoids and small self-interstitial loops. The consequent reduction in defect mobility and net defect concentrations in the matrix explains the observation of reduced swelling and radiation-induced segregation. Our analysis of vacancy-mediated solute diffusion demonstrates that below about 400 K Ni and Co will be dragged by vacancies and their concentrations should be enhanced at defect sinks. Cr and Cu show opposite behavior and are depleted at defect sinks. The stable configuration of some oversized solutes is neither interstitial nor substitutional; rather they occupy two adjacent lattice sites. The diffusion of these solutes proceeds by a novel mechanism, which has important implications for the nucleation and growth of complex oxide nanoparticles contained in oxide dispersion strengthened steels. Interstitial-mediated solute diffusion is negligible for all except the magnetic solutes (Cr, Mn, Co, and Ni). Our results are consistent across several antiferromagnetic states and surprising qualitative similarities with ferromagnetic (body-centered cubic) Fe were observed; this implies that our conclusions will be valid for paramagnetic iron.

The thermodynamic properties of vector $\left[O\right(2)$ and complex spherical] models with four-body interactions are analyzed. When defined in dense topologies, these are effective models for the nonlinear interaction of scalar fields in the presence of a stochastic noise, as has been well established for the case of the mode-locking laser formation in a closed cavity. With the help of an efficient Monte Carlo algorithm we show how, beyond the fully connected case, rich phenomenology emerges. Below a certain dilution threshold, the spherical model condenses in a nonequipartite way, while in the $XY$ model the transition becomes continuous and the $O\left(2\right)$ symmetry remains unbroken. We attribute this fact to the invariance under gauge transformations. The introduction of topological inhomogeneities in the network of quadruplets induces some features: again symmetry conservation; the vanishing of two-point correlators; and a dynamical correlation function presenting two time scales, the large one being related to the transition between different degenerated configurations, connected by particular gauge transformations. We discuss possible experimental implications of these results in the context of nonlinear optics.

Using high-resolution sound velocity measurements we have obtained a very precise magnetic phase diagram of ${\mathrm{Ba}}_3{\mathrm{CoSb}}_2{\mathrm{O}}_9$, a material that is considered to be an archetype of the spin-$\frac{1}{2}$ triangular-lattice antiferromagnet. Results obtained for the field parallel to the basal plane (up to 18 T) show three phase transitions, consistent with predictions based on simple two-dimensional isotropic Heisenberg models and previous experimental investigations. The phase diagram obtained for the field perpendicular to the basal plane clearly reveals an easy-plane character of this compound and, in particular, our measurements show a single first-order phase transition at $H_{c1}=12.0$ T which can be attributed to a spin flop between an umbrella-type configuration and a coplanar $V$-type order where spins lie in a plane perpendicular to the $ab$ plane. At low temperatures, softening of the lattice within some of the ordered phases is also observed and may be a result of residual spin fluctuations.