The search for oxide-based room-temperature ferromagnetism has been one of the holy grails in condensed matter physics. Room-temperature ferromagnetism observed in Nb-doped SrTiO${}_3$ single crystals is reported in this Rapid Communication. The ferromagnetism can be eliminated by air annealing (making the samples predominantly diamagnetic) and can be recovered by subsequent vacuum annealing. The temperature dependence of magnetic moment resembles the temperature dependence of carrier density, indicating that the magnetism is closely related to the free carriers. Our results suggest that the ferromagnetism is induced by oxygen vacancies. In addition, hysteretic magnetoresistance was observed for magnetic field parallel to the current, indicating that the magnetic moments are in the plane of the samples. The x-ray photoemission spectroscopy, the static time-of-flight and the dynamic secondary ion mass spectroscopy and proton induced x-ray emission measurements were performed to examine the magnetic impurities, showing that the observed ferromagnetism is unlikely due to any magnetic contaminant.

Recent density functional theory (DFT) calculations by Först et al. [Phys. Rev. Lett. 96, 175501 (2006)] have predicted that vacancies in both low and high carbon steels have a carbon dimer bound to them. This is likely to change the thinking of metallurgists in the kinetics of the development of microstructures. While the notion of a C${}_2$ molecule bound to a vacancy in Fe will potentially assume a central importance in the atomistic modeling of steels, neither a recent tight binding (TB) model nor existing classical interatomic potentials can account for it. Here we present a TB model for C in Fe, based on our earlier work for H in Fe, which correctly predicts the structure and energetics of the C${}_2$ dimer at a vacancy in Fe. Moreover the model is capable of dealing with both concentrated and dilute limits of carbon in both $\alpha$-Fe and $\gamma$-Fe as comparisons with DFT show. We use both DFT and TB to make a detailed analysis of the dimer and to come to an understanding as to what governs the choice of its curious orientation within the vacancy.

We demonstrate that the thermopower ($S$) can be used to probe the spin fluctuations (SFs) in proximity to the quantum critical point (QCP) in Fe-based superconductors. The sensitivity of $S$ to the entropy of charge carriers allows us to observe an increase of $S/T$ in Ba(Fe${}_{1-x}$Co${}_x$)${}_2$As${}_2$ close to the spin-density-wave (SDW) QCP. This behavior is due to the coupling of low-energy conduction electrons to two-dimensional SFs, similar to heavy-fermion systems. The low-temperature enhancement of $S/T$ in the Co substitution range $0.02<x<0.1$ is bordered by two Lifshitz transitions, and it corresponds to the superconducting region, where a similarity between the electron and nonreconstructed hole pockets exists. The maximal $S/T$ is observed in proximity to the commensurate-to-incommensurate SDW transition, for critical $x_c\approx 0.05$, close to the highest superconducting $T_c$. This analysis indicates that low-$T$ thermopower is influenced by critical spin fluctuations which are important for the superconducting mechanism.

The zero-frequency conductivity ($D_c$), the criterion to distinguish between conductors and insulators, is expressed in terms of a geometric phase. $D_c$ is also expressed using the formalism of the modern theory of polarization. The tenet of Kohn [Phys. Rev. 133, A171 (1964)], namely that insulation is due to localization in the many-body space, is refined as follows. Wave functions, which are eigenfunctions of the total current operator, give rise to a finite $D_c$ and are therefore metallic. They are also delocalized. Based on the value of $D_c$ it is also possible to distinguish purely metallic states from states in which the metallic and insulating phases coexist. Several examples which corroborate the results are presented, as well as a numerical implementation. The formalism is also applied to the Hall conductance, and the quantization condition for zero Hall conductance is derived to be $\frac{e{\Phi }_B}{Nhc}=\frac{Q}{M}$, with $Q$ and $M$ as integers.

We study the optically pumped nuclear spin polarization in a single GaAs/AlGaAs quantum well in the quantum Hall system. We apply resistive detection via the contact hyperfine interaction, which provides high sensitivity and selectivity, to probe a small amount of polarized nuclear spins in a single well. The properties of the optical nuclear spin polarization are clearly observed. We theoretically discuss the nuclear spin dynamics accompanied with doped electrons to analyze the experimental data. The optical nuclear polarization spectra exhibit electron-spin-resolved lowest Landau level interband transitions. We find that the phonon emission process, which normally assists the optical pumping process, influences the optical nuclear spin polarization. We also discuss that the electron-electron interaction can play an important role in the optical nuclear spin polarization.

An asymmetric double quantum well (QW) structure with resonant tunneling is suggested to achieve highly efficient four-wave mixing (FWM). We analytically demonstrate that resonant tunneling can induce a highly efficient mixing wave in such a semiconductor structure with a low-light pump wave. In particular, the FWM conversion efficiency can be enhanced dramatically in the vicinity of the center frequency with small propagation distance. This interesting scheme may be used to generate coherent long-wavelength radiation in solid-state systems.

Transport in InAs/GaSb heterostructures with different InAs layer thicknesses is studied using a six-terminal Hall bar geometry with a 2-$\mu$m edge channel length. For a sample with a 12-nm-thick InAs layer, nonlocal resistance measurements with various current/voltage contact configurations reveal that the transport is dominated by edge channels with a negligible bulk contribution. Systematic nonlocal measurements allow us to extract the resistance of individual edge channels, revealing sharp resistance fluctuations indicative of inelastic scattering. Our results show that the InAs/GaSb system can be tailored to have conducting edge channels while keeping a gap in the bulk region and provide a way of studying two-dimensional topological insulators even when quantized transport is absent.

Using first-principles and Wannier function methods, we systematically calculate the electronic band structure and topological edge states of single bilayer Bi (111) film (BL-Bi) as a function of strain and perpendicular electric field to investigate the effects induced by lattice mismatch and interfacial charge transfer when BL-Bi is epitaxially grown on a substrate. We found that the BL-Bi remains with a finite band gap and a nontrivial band topology for strains up to ±6$\%$ and electric fields up to 0.8 eV/Å. This indicates that the BL-Bi is a robust two-dimensional topological insulator against strain and electrical field on a substrate.

We describe an algorithm for studying the entanglement entropy and spectrum of two-dimensional (2D) systems, as a coupled array of $N$ one-dimensional chains in their continuum limit. Using the algorithm to study the quantum Ising model in 2D (both in its disordered phase and near criticality), we confirm the existence of an area law for the entanglement entropy and show that near criticality there is an additive piece scaling as $c_{\mathrm{eff}}\log \left(N\right)/6$ with $c_{\mathrm{eff}}\approx 1$. Studying the entanglement spectrum, we show that entanglement gap scaling can be used to detect the critical point of the 2D model. When short-range (area law) entanglement dominates we find (numerically and perturbatively) that this spectrum reflects the energy spectrum of a single quantum Ising chain.

We present an investigation of the microscopic interplay between excitons and charge carriers by means of combined photoluminescence (PL) and charge carrier transport measurements on organic thin film transistors (OTFTs). For this purpose, the prototypical organic semiconductor diindenoperylene was utilized as the active material. The OTFT accumulation layer provides a spatially defined interaction zone for charges and photogenerated excitons leading to a PL intensity reduction of up to 4.5$\%$. This effect correlates with the accumulated hole carrier density and provides a lower estimate of about $1.1\times {10}^{-10}$ cm${}^3$/s for the recombination rate of nonradiative exciton-hole processes. It is rationalized that these processes are preferentially mediated by trapped holes.

We study theoretically polydiacetylene chains diluted in their monomer matrix. We employ the density matrix renormalization group method on finite chains to calculate the ground state and low-lying excitations of the corresponding Peierls–Hubbard-Ohno Hamiltonian which is characterized by the electron transfer amplitude $t_0$ between nearest neighbors, by the electron-phonon coupling constant $\alpha$, by the Hubbard interaction $U$, and by the long-range interaction $V$. We treat the lattice relaxation in the adiabatic limit, i.e., we calculate the polaronic lattice distortions for each excited state. Using chains with up to 102 lattice sites, we can safely perform the extrapolation to the thermodynamic limit for the ground-state energy and conformation, the single-particle gap, and the energies of the singlet exciton, the triplet ground state, and the optical excitation of the triplet ground state. The corresponding gaps are known with high precision from experiments. We determine a coherent parameter set $(t_0^{*}=2.4\mathrm{eV},{\alpha }^{*}=3.4\mathrm{eV}/\text{Å},U^{*}=6\mathrm{eV},V^{*}=3\mathrm{eV})$ from a fit of the experimental gap energies to the theoretical values which we obtain for 81 parameter points in the four-dimensional search space $(t_0,\alpha ,U,V)$. We identify dark in-gap states in the singlet and triplet sectors as seen in experiments. Using a fairly stiff spring constant, the length of our unit cell is about 1$\%$ larger than its experimental value.

Thin film oxide heterostructures with a bound charge at the interface require electrical compensation, which can involve redistribution of mobile charge carriers. We explore a model LaGaO${}_3$(001)//MgAl${}_2$O${}_4$(001) heterostructure with nominally negatively charged interfaces using first-principles methods and a Poisson-Boltzmann equation. We find that charge compensation by oxygen vacancies with quadratically decaying concentration away from the interface is more favorable than electronic redistribution. These vacancies have a potential to enhance ionic conductivity along the interfaces.

We report the evidence of two different polymorphs for polymeric CO${}_2$-V in tridymitelike (V${}_{\mathrm{TD}}$ in $P$2${}_1$2${}_1$2${}_1$) and β-cristobalitelike (V${}_{\mathrm{CR}}$ in $I$-42$d$) structures. The V${}_{\mathrm{TD}}$ phase is produced by laser-heating phase III (Cmca) above 40 GPa, whereas the V${}_{\mathrm{CR}}$ phase by laser-heating highly compressed phase II ($P$4${}_2$/mnm—iso-space group to stishovite) and IV ($P$4${}_1$2${}_1$2—iso-space group to α-cristobalite) above 35 GPa. The density of the V${}_{\mathrm{CR}}$ (3.988 g/cm${}^3$) is ∼12$\%$ larger than that of the V${}_{\mathrm{TD}}$ (3.559 g/cm${}^3$) at 50 GPa, while the density difference reduces to ∼4$\%$ at ambient pressure. This results in a substantially smaller bulk modulus ($B_o$ $=$ 127 GPa, $B$′ $=$ 5.6) for the V${}_{\mathrm{CR}}$ phase than that of the V${}_{\mathrm{TD}}$ ($B_o$ $=$ 270 GPa, $B$′ $=$ 1.9). The smaller density of the V${}_{\mathrm{TD}}$ is due to the open structure of corner shared CO${}_4$ tetrahedra and a great level of distortion in C-O-C angles resulting in a highly buckled layer structure; yet, the structural relationship gives rise to the specific transition to occur depending on the initial phase, either displacively from phase IV to phase V${}_{\mathrm{CR}}$ or diffusively from phase III to phase V${}_{\mathrm{TD}}$. The results also provide new constraints for the phase/chemical transformation diagram of carbon dioxide.

We present an efficient optimization method to determine the structure of disordered systems in agreement with available experimental data. Our approach permits the application of accurate electronic structure calculations within the structure optimization. This technique is demonstrated within density functional theory by the calculation of a model of amorphous carbon.

We present results of density functional theory calculations on the phonon dispersion and elastic constants of bulk ceria (CeO${}_2$) as a function of positive and negative isotropic strain, which could be induced thermally or by cationic doping. We find that, as the lattice is expanded, there is a significant softening of the $B_{1u}$ mode at the $X$ point. This mode consists of motions of oxygens in the $\left[001\right]$ direction. At a strain of $1.6$$\%$, corresponding to a temperature of 1600 K, the $B_{1u}$ and $E_u$ modes at the $X$ point cross, with an associated high, narrow peak in the phonon density of states appearing. We infer that this crossing indicates a coupling of the modes, leading to a transition to a superionic phase, where conductivity occurs in the $\left[001\right]$ direction, mediated by anion interstitial site occupation. As the lattice is expanded further, the $B_{1u}$ mode continues to soften, becoming imaginary at a strain of $3.4$$\%$, corresponding to a temperature of 2500 K. Following the imaginary mode would result in a cubic to tetragonal phase transition, similar to those known to occur with reducing temperature in zirconia (ZrO${}_2$) and hafnia (HfO${}_2$). Our calculated elastic constants, however, indicate that the structure remains mechanically stable, even at this level of expansion. As confirmed by our semiclassical free energy calculations, the cubic phase of ceria remains the most stable, while the imaginary mode indicates a change to a thermally disordered cubic phase, with the majority of disorder occurring on the anion sublattice. Our results explain the high temperature ionic conductivity in ceria and other fluorite-structured materials in terms of the intrinsic lattice dynamics, and give insight to the stability and anionic disorder at elevated temperatures.

Magnetic order and low-energy spin dynamics in the zero field ground state of Ni${}_3$V${}_2$O${}_8$ are revealed in elastic and inelastic neutron scattering experiments. Neutron diffraction shows that below $T=2.3$ K the Ni${}^{2+}$ moments (spin $S=1$) order in a cycloid pattern with incommensurate wave vector ${\mathit{k}}_{\text{ICM}}=(0,1,\tau )$, where $\tau =0.4030\pm 0.0004$, which is superimposed on a commensurate antiferromagnetic spin arrangement with ${\mathit{k}}_{\text{CM}}=(0,0,0)$. Three spin wave modes are discerned below $E\sim 3$ meV in inelastic measurements and qualitatively described by a model Hamiltonian that involves near neighbor exchange, local anisotropy, and a small biquadratic coupling between the spine and cross-tie sites. Results from both elastic and inelastic scattering experiments suggest that the two sublattices on spine and cross-tie sites are largely decoupled.

Recently spin textures called skyrmions have been discovered in certain chiral magnetic materials without spatial inversion symmetry, and they have attracted enormous attention due to their promising application in spintronics since only a low applied current is necessary to drive their motion. When a conduction electron moves around the skyrmion, its spin is fully polarized by the spin texture and acquires a quantized phase; thus, the skyrmion yields an emergent electrodynamics that in turn determines skyrmion motion and gives rise to a finite Hall angle. As topological excitations, skyrmions behave as particles. In this paper we derive the equation of motion for skyrmions as rigid point particles from a microscopic continuum model and obtain the short-range interaction between skyrmions and the interaction between skyrmions and defects. Skyrmions also experience a Magnus force perpendicular to their velocity due to the underlying emergent electromagnetic field. We validate the equation of motion by studying the depinning transition using both the particle and the continuum models. By using the particle description, we explain the recent experimental observations of the rotation of a skyrmion lattice in the presence of a temperature gradient. We also predict quantum and thermal creep motion of skyrmions in the pinning potential.

Reflection and transmission as a function of temperature (7–300 K and 5–300 K respectively) have been measured on single crystals of the multiferroic compound FeTe${}_2$O${}_5$Br utilizing light spanning from the far infrared to the visible. The complex dielectric function and other optical properties were obtained via Kramers-Kronig analysis and by fits to a Drude-Lortentz model. Analysis of the anisotropic excitation spectra via Drude-Lorentz fitting and lattice dynamical calculations have led to the observation of 43 of the 53 modes predicted along the $b$ axis of the monoclinic cell. The phonon response parallel to the $a$ and $c$ axes are also presented. Assignments to groups (clusters) of phonons have been made and trends within them are discussed in light of our calculated displacement patterns.

We report the results of a high-resolution powder neutron-diffraction study of multiferroic BiFe${}_{0.7}$Mn${}_{0.3}$O${}_3$. We have confirmed the previous assignment of the $\alpha$ phase and $\beta$ phase as having $R3c$ and $Pbnm$ symmetry, respectively. The $\gamma$ phase, however, has been shown unequivocally not to be cubic, as previously reported, but rather to retain octahedral tilting distortions leading to a lower symmetry, most likely rhombohedral with space group $R\overline{3}c$. The $\gamma$ phase of BiFe${}_{0.7}$Mn${}_{0.3}$O${}_3$ is therefore different than that of the parent compound BiFeO${}_3$, which retains an orthorhombic structure up to its decomposition temperature. The lattice parameters of the $\gamma$ phase of BiFe${}_{0.7}$Mn${}_{0.3}$O${}_3$ are pseudocubic throughout its entire stability range, despite the fact that there are still significant distortions from the cubic aristotype structure even at the onset of decomposition.

We study a model of interacting bosons that occupy the first excited $p$-band states of a two-dimensional optical lattice. In contrast to the much studied single band Bose-Hubbard Hamiltonian, this more complex model allows for nontrivial superfluid phases associated with condensation at nonzero momentum and staggered order of the orbital angular momentum in addition to the superfluid-Mott insulator transition. More specifically, we observe staggered orbital angular momentum order in the Mott phase at commensurate filling and superfluidity at all densities. We also observe a transition between the staggered angular momentum superfluid phase and a striped superfluid, with an alternation of the phase of the superfluid along one direction. The transition between these two phases was observed in a recent experiment, which is then qualitatively well described by our model.