Far-infrared absorption measurements have been performed in nominally undoped ${\mathrm{GaAs}}_{1-x}{\mathrm{Bi}}_x$ (0.6% ⩽ $x$ ⩽ 10.6%) for magnetic field up to 30 T. For 0.6% ⩽ $x$ ⩽ 4.5%, the Lyman series of an acceptor has been observed. An exceedingly high value of the ground-state $g$ factor provides strong evidence of Bi-related acceptor states. For $x$ ⩾ 5.6%, however, these acceptors suddenly disappear. Such anomalous dependence on Bi concentration parallels those recently reported in ${\mathrm{GaAs}}_{1-x}{\mathrm{Bi}}_x$ for other electronic and structural properties.

Muon spin rotation and relaxation experiments in the pyrochlore iridate Eu${}_2$Ir${}_2$O${}_7$ yield a well-defined muon spin precession frequency below the metal-insulator/antiferromagnetic transition temperature $T_M=120$ K, indicative of long-range commensurate magnetic order and thus ruling out quantum spin liquid and spin-glass-like ground states. The dynamic muon spin relaxation rate is temperature-independent between 2 K and $~T_M$ and yields an anomalously long Ir${}^{4+}$ spin correlation time, suggesting a singular density of low-lying spin excitations. Similar behavior is found in other pyrochlores and geometrically frustrated systems, but also in the unfrustrated iridates BaIrO${}_3$ and Sr${}_2$Ir${}_2$O${}_4$. Eu${}_2$Ir${}_2$O${}_7$ may be only weakly frustrated; if so, the singularity might be associated with the small-gap insulating state rather than frustration.

We investigate novel phases that emerge from the interplay of electron correlations and strong spin-orbit interactions. We focus on describing the topological semimetal, a three-dimensional phase of a magnetic solid, and argue that it may be realized in a class of pyrochlore iridates (such as ${\mathrm{Y}}_2$Ir${}_2$O${}_7$) based on calculations using the $\text{LDA}+U$ method. This state is a three-dimensional analog of graphene with linearly dispersing excitations and provides a condensed-matter realization of Weyl fermions that obeys a two-component Dirac equation. It also exhibits remarkable topological properties manifested by surface states in the form of Fermi arcs, which are impossible to realize in purely two-dimensional band structures. For intermediate correlation strengths, we find this to be the ground state of the pyrochlore iridates, coexisting with noncollinear magnetic order. A narrow window of magnetic “axion” insulator may also be present. An applied magnetic field is found to induce a metallic ground state.

The phases with spontaneously broken symmetries corresponding to antiferromagnetic and $d$-wave superconducting order in the two-dimensional $t-t^{\text{'}}$-Hubbard model are investigated by means of the functional renormalization group. The introduction of composite boson fields in the magnetic, charge density, and superconducting channels allows an efficient parametrization of the four-fermion vertex and the study of regimes where either the antiferromagnetic or superconducting order parameter, or both, are nonzero. We compute the phase diagram and the temperature dependence of the order parameter below the critical temperature, where antiferromagnetic and superconducting order show a tendency of mutual exclusion.

The nature of the magnetism in the simplest iron arsenide is of fundamental importance in understanding the interplay between localized and itinerant magnetism and superconductivity. We present the magnetic structure of the itinerant monoarsenide FeAs with the B31 structure. Powder neutron diffraction confirms incommensurate modulated magnetism with wave vector $\mathbf{q}=(0.395\pm 0.001){\mathbf{c}}^{*}$ at 4 K, but can not distinguish between a simple spiral and a collinear spin-density-wave structure. Polarized single-crystal diffraction confirms that the structure is best described as a noncollinear spin-density wave arising from a combination of itinerant and localized behavior with spin amplitude along the $b$-axis direction being ($15\pm 5$)$\%$ larger than in the $a$ direction. Furthermore, the propagation vector is temperature dependent, and the magnetization near the critical point indicates a two-dimensional Heisenberg system. The magnetic structures of closely related systems are discussed and compared to that of FeAs.

The Landau level spectra and the quantum Hall effect of ABA-stacked multilayer graphenes are studied in the effective-mass approximation. The low-energy effective-mass Hamiltonian may be partially diagonalized into an approximate block-diagonal form, with each diagonal block contributing parabolic bands except for an additional block describing Dirac-like bands with a linear dispersion in a multilayer with an odd number of layers. We fully include the band parameters and, taking into account the symmetry of the lattice, we analyze their effect on the block-diagonal Hamiltonian. Next-nearest-layer couplings are shown to be particularly important in determining the low-energy spectrum and the phase diagram of the quantum Hall conductivity by causing energy shifts, level anticrossings, and valley splitting of the low-lying Landau levels.

By means of an electron diffraction technique, crystal-lattice modulations were investigated for a hexaferrite, Ba${}_{0.5}$Sr${}_{1.5}$Zn${}_2$Fe${}_{12}$O${}_{22}$, showing a helimagnetic order below $T_{\mathrm{N}}=320$ K. We observed a lattice modulation with a wave vector $\text{Q}=(0,0,3\delta )$ below $T_{\mathrm{N}}$. The value of $\delta$ varies between $~0.23$ and 0.5 as a function of temperature and corresponds well to that of the magnetic modulation vector. By applying magnetic fields, Ba${}_{0.5}$Sr${}_{1.5}$Zn${}_2$Fe${}_{12}$O${}_{22}$ was found to exhibit successive changes in the lattice modulation, accompanied by the modifications of the helical magnetic order. We concluded that the observed lattice modulation is induced by the helical magnetic order via exchange magnetostriction.

We determine the thermodynamic phase diagram of the epitaxial SrTiO${}_3$/Si heterostructure from first-principles density functional theory calculations. We demonstrate that the system is not thermodynamically stable with respect to formation of an interfacial SiO${}_2$ region under any experimentally attainable oxygen partial pressures. To understand the experimental observation of an atomically abrupt interface without a SiO${}_2$ layer, we construct a kinetic model of the growth process. We show that the observed stability of the nonequilibrium interface under both growth and ambient conditions is directly related to the interface chemistry, which gives rise to a large (2.2 eV) barrier for oxygen diffusion to the interface layer and kinetically traps an atomically abrupt interface structure. Our model leads to the prediction of an interface structure in good agreement with experiment, demonstrating that a combination of first-principles computations, thermodynamics, and simple kinetic modeling can be used to determine non $+$ -thermodynamic equilibrium atomic structures in complex heterosystems.

We present an angle-resolved photoelectron spectroscopy study of YBa${}_2$Cu${}_3$O${}_{7-\delta }$ films in situ grown by pulsed laser deposition. We have successfully produced underdoped surfaces with ordered oxygen vacancies within the CuO chains resulting in a clear ortho-II band folding of the Fermi surface. This indicates that order within the CuO chains affects the electronic properties of the CuO${}_2$ planes. Our results highlight the importance of having not only the correct surface carrier concentration, but also a very well ordered and clean surface in order that photoemission data on this compound be representative of the bulk.

We study the superconducting state in recently discovered high-$T_{\mathrm{c}}$ superconductor ${\mathrm{K}}_x$Fe${}_2$Se${}_2$ based on the ten-orbital Hubbard-Holstein model without hole pockets. When the Coulomb interaction is large, a spin-fluctuation-mediated $d$-wave state appears due to the nesting between electron pockets. Interestingly, the symmetry of the body-centered tetragonal structure in ${\mathrm{K}}_x$Fe${}_2$Se${}_2$ requires the existence of nodes in the $d$-wave gap, although a fully gapped $d$-wave state is realized in the case of a simple tetragonal structure. In the presence of moderate electron-phonon interaction due to Fe-ion optical modes, however, orbital fluctuations give rise to the fully gapped $s_{++}$-wave state without sign reversal. Therefore, both superconducting states are distinguishable by careful measurements of the gap structure or the impurity effect on $T_{\mathrm{c}}$.

Spin-orbit (SO) coupling is an important ingredient in many recently discovered phenomena such as the spin-Hall effect and topological insulators. Of particular interest is topological superconductivity, with its potential application in topological quantum computation. The absence of disorder in ultracold atomic systems makes them ideal for quantum computation applications; however, the SO coupling schemes proposed thus far are experimentally impractical owing to large spontaneous emission rates in the alkali fermions. In this paper, we develop a scheme to generate Rashba SO coupling with a low spontaneous emission extension to a recent experiment. We show that this scheme generates a Fermi surface spin texture for ${}^{40}\mathrm{K}$ atoms, which is observable in time-of-flight measurements. The chiral spin texture, together with conventional $s$-wave interactions, leads to topological superconductivity and non-Abelian Majorana quasiparticles.

We investigate the Nernst-Ettingshausen and thermal Hall effects on the surface of a topological insulator on which a ferromagnet is attached. We present general expressions of the Peltier and the thermal Hall conductivities, which are reduced to simple forms at low temperatures. It is shown that the Peltier and the thermal Hall conductivities show nonmonotonic dependence on temperature. At low temperatures, they have a linear dependence on temperature. From the behavior of the Peltier conductivity at low temperatures, one can estimate the magnitude of the gap induced by time-reversal symmetry breaking. Moreover, we find that the Peltier conductivity can be used to map the Berry phase structure. These results open up the possibility of controlling transverse heat transport magnetically.

A layered iridate Ba${}_2$IrO${}_4$ was synthesized using a high-pressure synthesis technique. Its electronic state was studied through electrical resistivity, magnetic susceptibility, and μSR experiments. It was found that Ba${}_2$IrO${}_4$ crystallizes in a ${\mathrm{K}}_2$NiF${}_4$-type structure (space group $I$4/mmm) with lattice parameters $a$ $=$4.030(1) Å and $c$ $=$13.333(4) Å. The structure includes flat IrO${}_2$ square planar lattices with straight Ir-O-Ir bonds. Ba${}_2$IrO${}_4$ is a Mott insulator (activation energy $\Delta E_a$ ∼ 0.07 eV) driven by a spin-orbit interaction. The magnetic susceptibility and μSR studies revealed that the magnetic ground state is antiferromagnetic long-range order ($T_N$ ∼ 240 K) in which the magnetic moment (∼0.34${\mu }_B$/Ir atom) is significantly reduced by a low-dimensional quantum spin fluctuation with a large intraplane correlation |$J$|. The behavior is similar to that in parent materials of high-$T_C$ cuprate superconductors such as La${}_2$CuO${}_4$.

Mesoscale experiment and simulation permit harvesting information about both geometric features and texture in polycrystals. The grain boundary character distribution (GBCD) is an empirical distribution of the relative length [in two dimensions (2D)] or area (in 3D) of an interface with a given lattice misorientation and normal. During the growth process, an initially random distribution of boundary types reaches a steady state that is strongly correlated to the interfacial energy density. In simulation, it is found that if the given energy density depends only on lattice misorientation, then the steady-state GBCD and the energy are related by a Boltzmann distribution. This is among the simplest nonrandom distributions, corresponding to independent trials with respect to the energy. In this paper, we derive an entropy-based theory that suggests that the evolution of the GBCD satisfies a Fokker-Planck equation, an equation whose stationary state is a Boltzmann distribution. Cellular structures coarsen according to a local evolution law, curvature-driven growth, and are limited by space-filling constraints. The interaction between the evolution law and the constraints is governed primarily by the force balance at triple junctions, the natural boundary condition associated with curvature-driven growth, and determines a dissipation relation. A simplified coarsening model is introduced that is driven by the boundary conditions and reflects the network level dissipation relation of the grain growth system. It resembles an ensemble of inertia-free spring-mass dashpots. Application is made of the recent characterization of Fokker-Planck kinetics as a gradient flow for a free energy in deriving the theory. The theory predicts the results of large-scale two-dimensional simulations and is consistent with experiment.

Mesoscopic systems provide us a unique experimental stage to address nonequilibrium quantum statistical physics. By using a simple tunneling model, we describe the electron exchange process via a quantum coherent conductor between two reservoirs, which yields the fluctuation theorem (FT) in mesoscopic transport. We experimentally show that such a treatment is semiquantitatively validated in the current and noise measurement in an Aharonov-Bohm ring. The experimental proof of the microreversibility assumed in the derivation of FT is presented.

Neutron and x-ray powder and single crystal synchrotron diffraction of ${\mathrm{Cs}}_{\text{y}}{\mathrm{Fe}}_{2-\text{x}}{\mathrm{Se}}_2$ show the presence of superstructure reflections with propagation vector $k=[\frac{2}{5},\frac{1}{5},1]$ with respect to the average crystal structure $I4/\mathit{mmm}$ ($a~4,c~15$ Å). The propagation vector star corresponds to the 5 times bigger unit cell given by transformation $\mathbf{A}=2\mathbf{a}+\mathbf{b}$, $\mathbf{B}=-\mathbf{a}+2\mathbf{b}$, $\mathbf{C}=\mathbf{c}$. A solution for the atomic structure is found in the space group $I4/m$ with an ordered pattern of iron vacancies corresponding to the iron deficiency $x=0.29$ and Cs stoichiometry $y=0.83$. The superstructure satellites are more pronounced in the neutron diffraction patterns suggesting that they can have some magnetic contribution. We have sorted out all possible symmetry adapted magnetic configurations and found that the presence of antiferromagnetic ordering with the ordered magnetic moment of Fe with $\simeq$$2{\mu }_B$ does not contradict the experimental data. However, the solutions space is highly degenerate and we cannot choose a specific solution. Instead we propose possible magnetic configurations with the Fe magnetic moments in $(\mathit{ab})$ plane or along $c$ axis. The superstructure is destroyed above $T_s\simeq 500$ K by a first-order-like transition.

For exploring group-IV semiconductor spintronics with ferromagnetic Heusler compounds, we study the local structural ordering of the stoichiometric Fe${}_3$Si and off-stoichiometric Fe${}_{3.2}$Si${}_{0.8}$ films epitaxially grown on Ge(111) at a very low temperature of 130 ${}^{ˆ}$C. Analyzing their ${}^{57}\mathrm{Fe}$ Mössbauer spectra, we can discuss the site occupation of Fe atoms in the films grown directly on a semiconductor substrate, where the influence of the interfacial reactions between Fe${}_3$Si or Fe${}_{3.2}$Si${}_{0.8}$ and Ge on the Mössbauer spectra is minimized. As a result, we can quantitatively obtain the local degree of the D0${}_3$ ordering ($~$67%) for the as-grown stoichiometric films, whereas we can not see the structural ordering for the as-grown off-stoichiometric films. Comparing the analytic data between as-grown and annealed films, we find that the postannealing can act effectively on the improvement of the structural ordering only for the off-stoichiometric films.

We present Density Functional Theory (DFT) investigations in the General Gradient Approximation (GGA) of the structural and electronic properties of the Ag${}_2$V${}_4$O${}_{11}$ (SVO) compound. We carried out a detailed study of the different structures of SVO proposed in the literature, by comparing the results obtained using DFT and the DFT+U approach. We found that two of the proposed structures are equally probable, the third one being unstable. We have obtained detailed information concerning the structural and electronic properties of SVO, including previously non-existent information on one of the SVO structures, considered hypothetical yet probable in the light of experimental facts from analogous compounds. From the analysis of the electronic density of states and of the Highest Occupied Molecular Orbital (HOMO) and Lowest Unoccupied Molecular Orbital (LUMO) states, we propose that, during the earlier stage of the reaction of lithium insertion and de-insertion in Li/SVO primary batteries, the reduction of ${\mathrm{V}}^{5+}$ takes place before that of Ag. In addition, our results allow to predict that only one kind of vanadium atom would be firstly reduced.

Structural investigations by means of transmission electron microscopy (TEM) on ${\mathrm{K}}_{0.8}$Fe${}_x$Se${}_2$ and KFe${}_x$Se${}_2$, with 1.5 ⩽ $x$ ⩽ 1.8, have revealed a rich variety of microstructure phenomena. Materials with 1.5 ⩽ $x$ ⩽ 1.6 often show a superstructure modulation along the [310] zone-axis direction, and this modulation can be well interpreted by the Fe-vacancy order, which likely yields a superstructure phase of ${\mathrm{K}}_2$Fe${}_4$Se${}_5$. The superconducting ${\mathrm{K}}_{0.8}$Fe${}_x$Se${}_2$ and KFe${}_x$Se${}_2$ (1.7 ⩽ $x$ ⩽ 1.8) materials contain clear phase separation, in particular, along the $c$-axis direction, recognizable as visible parallel lamellae in the crystals; this fact suggests that the superconducting phase could have the Fe-vacancy disordered state.

We investigate the electronic reconstruction in (LaNiO${}_3$)${}_n$/(LaAlO${}_3$)${}_3$ ($n=3$, 5, and 10) superlattices due to the quantum confinement (QC) by dc transport and resonant soft x-ray absorption spectroscopy. In proximity to the QC limit, a Mott-type transition from an itinerant electron behavior to a localized state is observed. The system exhibits tendency toward charge-order during the transition. $\mathit{Ab}$ $\mathrm{initio}$ cluster calculations are in good agreement with the absorption spectra, indicating that the apical ligand hole density is highly suppressed, resulting in a strong modification of the electronic structure. At the dimensional crossover cellular dynamical-mean-field calculations support the emergence of a Mott insulator ground state in the heterostructured ultrathin slab of LaNiO${}_3$.