N-doped Co${}_x$Zn${}_{1-x}$O epitaxial films were grown by reactive magnetron sputtering using a range of different N${}_2$ and O${}_2$ concentrations. They were studied by x-ray absorption near edge spectroscopy (XANES) and x-ray linear dichroism (XLD). The incorporation of N into the ZnO host lattice could be determined via the XLD at the N $K$-edge in comparison with the O $K$-edge and respective simulations of XANES and XLD of the various dopant configurations. The addition of nitrogen predominantly leads to the formation of molecular nitrogen on O sites (split interstitial). Only for samples grown with very low O${}_2$ partial pressure was a small fraction of the N found to substitute for O in its atomic form.

We investigate the nucleation and growth of anisotropic and strained quantum dots in heteroepitaxy by means of kinetic Monte Carlo simulations. Surface energy anisotropy is introduced in order to depict Ge-like dots with (105) facets growing on a Si (100) substrate. Three dimensional islands, mainly in the form of square-base pyramids, are reported and their coarsening is found to be interrupted during annealing. The resulting island density follows the scaling law $\rho \sim {(D/F)}^{-\alpha }$ with $\alpha \simeq 0.6$ as a function of the diffusion $D$ to flux $F$ ratio. The island size distribution follows the scaling law resulting from the assumption of a single length scale.

We investigate the effect of strain and isotopic disorder on thermal transport in suspended graphene by equilibrium molecular dynamics simulations. We show that the thermal conductivity of unstrained graphene, calculated from the fluctuations of the heat current at equilibrium, is finite and converges with size at finite temperature. In contrast, the thermal conductivity of strained graphene diverges logarithmically with the size of the models, when strain exceeds a relatively large threshold value of 2%. An analysis of phonon populations and lifetimes explains the divergence of the thermal conductivity as a consequence of changes in the occupation of low-frequency out-of-plane phonons and an increase in their lifetimes due to strain.

We investigate optical transitions of noninteracting electron systems consisting of two symmetric energy bands touching each other at the Fermi energy (e.g., graphene at half-filling). Optical conductivity is obtained using Kubo formula at zero temperature. We show that for particles whose pseudospin direction is determined solely by the direction of their momentum, the optical conductivity has power-law frequency dependence with the exponent $(d-2)/z$ where $d$ is the dimension of the system and $z$ is the dynamical exponent. According to our result two-dimensional systems with the above pseudospin characteristics always exhibit frequency-independent optical conductivity.

Most standard models for the thermal boundary conductance (TBC) assume isotropic properties and thus are inappropriate for layered and chainlike materials such as graphite, Bi${}_2$Te${}_3$, and high-density polyethylene (HDPE). To model such anisotropic materials, here a framework is introduced whereby the first Brillouin zone and the isoenergy surfaces of the Debye dispersion relation are both generalized from spherical to ellipsoidal. This model is checked by comparison with the experimental specific heat capacity of graphite and HDPE, as well as the phonon irradiation of graphite calculated from lattice dynamics. The anisotropic TBC model performs at least six times better than the standard isotropic diffuse mismatch model at explaining the measured TBC between graphite and various metals reported by Schmidt et al. [J. Appl. Phys. 107, 104907 (2010)]. The model further reveals an unexpected guideline to engineer the TBC: due to phonon focusing effects, in many cases the TBC across an interface can be increased by reducing a phonon velocity component parallel to the plane of the interface.

One of the main sources of error associated with the calculation of defect formation energies using plane-wave density functional theory (DFT) is finite size error resulting from the use of relatively small simulation cells and periodic boundary conditions. Most widely used methods for correcting this error, such as that of Makov and Payne, assume that the dielectric response of the material is isotropic and can be described using a scalar dielectric constant $\epsilon$. However, this is strictly only valid for cubic crystals, and cannot work in highly anisotropic cases. Here we introduce a variation of the technique of extrapolation based on the Madelung potential that allows the calculation of well-converged dilute limit defect formation energies in noncubic systems with highly anisotropic dielectric properties. As an example of the implementation of this technique we study a selection of defects in the ceramic oxide Li${}_2$TiO${}_3$ which is currently being considered as a lithium battery material and a breeder material for fusion reactors.

We report the results of extensive neutron powder diffraction experiments on a ternary compound CeRuSn in a temperature range from 2 to 520 K. At room temperature CeRuSn orders in a monoclinic structure that is related to the CeCoAl type by the doubling of the $c$-axis parameter caused by the splitting of the original single sites for each element into two sites. Upon cooling a very broad hysteretic structural transition has been observed in CeRuSn leading to the presence of higher-ordered variants having multiple $c$-axis parameters. Depending on the temperatures, one or the other variant seems to be more stable, but the sample as a whole becomes multiphase. We find also evidence for further symmetry lowering at low temperatures. CeRuSn is found to exhibit an antiferromagnetic order below $T_N=2.8\left(1\right)$ K. The order is described by two incommensurate propagation vectors $q_1=(0,0,\approx 0.30)$ and $q_2=(0,0,\approx 0.43)$ with respect to the doubled crystal structure variant. Although the details of the magnetic structure remain elusive, the best agreement has been found for a complicated distorted cycloidal magnetic structure with Ce moments of $\approx$0.7 ${\mu }_B$ being confined to the $ac$ plane.

We show that the topological Majorana modes in nanowires much longer than the superconducting coherence length are adiabatically connected with discrete zero-energy states generically occurring in short nanowires. We demonstrate that these zero-energy crossings can be tuned by an external magnetic field and are protected by the particle-hole symmetry. We study the evolution of the low-energy spectrum and the splitting oscillations as a function of magnetic field, wire length, and chemical potential, manifestly establishing that the low-energy physics of short wires is related to that occurring in long wires. This physics, which represents a hallmark of spinless $p$-wave superconductivity, can be observed in tunneling conductance measurements.

We present the evolution of the initial (up to $\sim$10 kbar) hydrostatic pressure dependencies of $T_c$ and of the ambient pressure jump in the heat capacity associated with the superconducting transition as a function of K doping in the Ba${}_{1-x}$K${}_x$Fe${}_2$As${}_2$ family of iron-based superconductors. The pressure derivatives show a weak but distinct anomaly near $x\sim 0.7$. In the same concentration region, $\Delta C_p{|}_{T_c}$ deviates from the $\Delta C_p\propto T^3$ scaling found for most BaFe${}_2$As${}_2$-based superconductors. These results are consistent with a possible significant modification of the superconducting state, occurring near $x\sim 0.7$.

A frozen short-range, strain-ordered state has been observed in several doped ferroelastic/martensitic alloys. The reported strain glass behavior has been attributed to atomic-scale point defects such as dopant atoms. We report here how nanoscale precipitates can also lead to such glassy behavior. Nanosized, randomly distributed Ti${}_3$Ni${}_4$-like precipitates, produced by aging/annealing at 473 K for 3 h, prohibit the B2 → B19′ martensitic transition that occurs in a precipitate-free state. The strain glass transition is characterized by a mechanical susceptibility/modulus anomaly with Vogel-Fulcher type frequency-dependence, ergodicity-breaking, invariance in average structure and nanosized strain domains. Our work emphasizes that heterogeneities or in general disordering effects in ferroelastics will also give rise to signatures characteristic of strain glass behavior.

The diffusion barriers of protonic defects in Gd-doped BaCeO${}_3$, a compound candidate as electrolyte for protonic ceramic fuel cells, have been investigated by density functional theory calculations, starting from a previously computed energy landscape consisting of 16 kinds of stable sites (eight close to dopants and eight far from them). The simplified string method has been used to determine accurately the minimum energy paths between those sites that might imply either proton reorientations, intraoctahedral, or interoctahedral hopping mechanisms. At contrast with simple cubic perovskites such as barium stannate or barium zirconate, very different values for energy barriers (from 0.02 to 0.58 eV) are found in this highly distorted orthorhombic perovskite, and no specific process appears to be clearly rate limiting. Some interoctahedral hoppings (when possible) are found to be more favorable than the intraoctahedral ones, while reorientations exhibit a wide range of energy barriers.

The longitudinal spin Seebeck effect (LSSE) is investigated in various garnet ferrites Y${}_{3-x}{R}_x$Fe${}_{5-y}{M}_y$O${}_{12}$ ($R=\text{Gd}$, Ca; $M=\text{Al}$, Mn, V, In, Zr) by means of the inverse spin Hall effect in Pt films. The magnitude of the LSSE voltage in the Pt/Y${}_{3-x}{R}_x$Fe${}_{5-y}{M}_y$O${}_{12}$ samples is found to be enhanced with increasing concentration of Fe in the garnet ferrites, which can be explained by a change in the spin-mixing conductance at the Pt/Y${}_{3-x}{R}_x$Fe${}_{5-y}{M}_y$O${}_{12}$ interfaces. We also investigate the dependence of the LSSE voltage on macroscopic magnetic parameters of Y${}_{3-x}{R}_x$Fe${}_{5-y}{M}_y$O${}_{12}$. The experimental results show that the LSSE voltage in the Pt/Y${}_{3-x}{R}_x$Fe${}_{5-y}{M}_y$O${}_{12}$ samples has a positive correlation with the Curie temperature and the saturation magnetization, but no clear correlation with the gyromagnetic ratio and the Gilbert damping constant of the samples.

The spin and layer (pseudospin) degrees of freedom are entangled coherently in the canted antiferromagnetic phase of the bilayer quantum Hall system at the filling factor $\nu =2$. A complex Goldstone mode emerges describing such a combined degree of freedom. In the zero tunneling-interaction limit (${\Delta }_{\text{SAS}}\to 0$), its phase field provokes a supercurrent carrying both spin and charge within each layer. The Hall resistance is predicted to become anomalous precisely as in the $\nu =1$ bilayer system in the counterflow and drag experiments. Furthermore, it is shown that the total current flowing in the bilayer system is a supercurrent carrying solely spins in the counterflow geometry. It is intriguing that all these phenomena occur only in imbalanced bilayer systems.

The density functional theory (DFT) plus Gutzwiller variational method is used to investigate the ground-state physical properties of the newly discovered superconducting KNi${}_2$S${}_2$. Five Ni-3$d$ Wannier-orbital bases are constructed with DFT, to combine with local Coulomb interaction to describe the normal-state electronic structure of the Ni-based superconductor. The band structure and the mass enhancement are studied based on a multiorbital Hubbard model by using the Gutzwiller approximation method. Our results indicate that the correlation effects lead to the mass enhancement of KNi${}_2$S${}_2$. Different from the band structure calculated from the DFT results, there are three energy bands across the Fermi level along the $X-Z$ line due to the existence of the correlation effects, which induces a very complicated Fermi surface along the $X-Z$ line. We have also investigated the variation of the quasiparticle weight factor with hole or electron doping and find that the mass enhancement character has been maintained with the doping.

The electronic and optical properties of fully fluorinated graphene i.e. fluorographene (CF), and partially fluorinated graphene (C${}_4$F) are studied by means of the first-principles many-body Green's function method, i.e. $GW$–Bethe–Salpeter equation. In conformity with experimental observations, fluorination of graphene causes relatively wide quasiparticle band gaps, 7.01 eV for CF and 5.52 eV for C${}_4$F. The optical absorption properties of CF and C${}_4$F are of excitonic nature, leading to the formation of bound excitons with significantly high binding energies of 1.96 and 1.31 eV assigned to the first bright exciton of CF and C${}_4$F, respectively. The disagreement between the ab initio calculated electronic band gap and the experimentally optical gap seems to be mainly due to the binding energy of excitons rather than the presence of defects. Contrary to previous theoretical predictions, this paper demonstrates that CF seems to be less likely to realize the excitonic Bose–Einstein condensation because the exciton wave functions are relatively delocalized and not well separated. In the case of C${}_4$F, however, it indicates a charge-transfer excitation.

We theoretically study various aspects of the electron-surface optical phonon interaction effects in graphene on a substrate made of polar materials. We calculate the electron self energy in the presence of the surface phonon-mediated electron-electron interaction focusing on how the linear chiral graphene dispersion is renormalized by the surface phonons. The electron self energy as well as the quasiparticle spectral function in graphene are calculated, taking into account electron-polar optical phonon interaction by using a many-body perturbative formalism. The scattering rate of free electrons due to polar interaction with surface optical phonons in a dielectric substrate is calculated as a function of the electron energy, temperatures, and carrier density. Effects of screening on the self energy and scattering rate are discussed. Our theory provides a comprehensive quantitative (and qualitative) picture for surface phonon interaction induced many-body effects and hot-electron relaxation in Dirac materials.

We report on the theoretical study of laser-assisted attosecond photoemission from metals. The full time-dependent quantum approach reveals the role of the resonant interband and nonresonant surface emission processes in formation of final atto-streaking spectra. The present results explain recent experimental data on magnesium and show that the valence band streaking essentially reflects the respective weight of surface and resonant bulk electron ejection.

Using angle-resolved photoelectron spectroscopy and ab initio $GW$ calculations, we unambiguously show that the widely investigated three-dimensional topological insulator Bi${}_2$Se${}_3$ has a direct band gap at the $\Gamma$ point. Experimentally, this is shown by a three-dimensional band mapping in large fractions of the Brillouin zone. Theoretically, we demonstrate that the valence-band maximum is located at the $\Gamma$ point only if many-body effects are included in the calculation. Otherwise, it is found in a high-symmetry mirror plane away from the zone center.