We investigate the bulk orbital angular momentum in a two-dimensional time-reversal broken topological superconductor with the Rashba spin-orbit interaction, the Zeeman interaction, and the $s$-wave pairing potential. Prior to the topological phase transition, we find the crossover from $s$ wave to $p$ wave. For the large spin-orbit interaction, even in the topological phase, $L_z/N$ does not reach $-1/2$, which is the intrinsic value in chiral $p$-wave superconductors. Here $L_z$ and $N$ are the bulk orbital angular momentum and the total number of electrons at zero temperature, respectively. Finally, we discuss the effects of nonmagnetic impurities.

We study the ground-state properties of an extended periodic Anderson model to understand the role of Hund's coupling between localized and itinerant electrons using the density-matrix renormalization group algorithm. By calculating the von Neumann entropies we show that two phase transitions occur and two new phases appear as the hybridization is increased in the symmetric half-filled case due to the competition between Kondo effect and Hund's coupling. In the intermediate phase, which is bounded by two critical points, we found a dimerized ground state, while in the other spatially homogeneous phases the ground state is Haldane-like and Kondo-singlet-like, respectively. We also determine the entanglement spectrum and the entanglement diagram of the system by calculating the mutual information thereby clarifying the structure of each phase.

We compute the $K$-edge indirect resonant inelastic x-ray scattering (RIXS) spectrum of a triangular-lattice antiferromagnet in its ordered coplanar three-sublattice ${120}^{\circ }$ magnetic state. By considering the self-energy corrections to the spin-wave spectrum, magnon decay rates, bimagnon interactions within the ladder approximation Bethe-Salpeter scheme, and the effect of three-magnon contributions up to $1/S$ order, we find that the RIXS spectra are nontrivially affected by magnon damping and anisotropy. For a purely isotropic model, the appearance of a multipeak RIXS structure is primarily dictated by the damping of magnon modes. A unique feature of the triangular-lattice indirect RIXS spectra is the nonvanishing intensity at both the zone center $\mathrm{\Gamma }$ point and the antiferromagnetic wave-vector $K$ point. Additionally, we find that a single-peak RIXS spectrum forms at the roton momentum which can be utilized as an experimental signature to detect the presence of rotonlike excitations. However, including $XXZ$ anisotropy leads to additional peak splitting, including at the roton wave vector. The observed splitting is consistent with our earlier theoretical prediction on the effects of spatial anisotropy on the RIXS spectra of a frustrated quantum magnet [C. Luo et al., Phys. Rev. B 89, 165103 (2014)].

We investigate the x-ray photoelectron spectroscopy (XPS) of nominally $d^1$ and $n$-doped $d^0$ transition-metal oxides including ${\mathrm{NbO}}_2,{\mathrm{SrVO}}_3$, and ${\mathrm{LaTiO}}_3$ (nominally $d^1$), as well as $n$-doped ${\mathrm{SrTiO}}_3$ (nominally $d^0$). In the case of single phase $d^1$ oxides, we find that the XPS spectra (specifically photoelectrons from Nb $3d$, V $2p$, Ti $2p$ core levels) all display at least two, and sometimes three distinct components, which can be consistently identified as $d^0,d^1$, and $d^2$ oxidation states (with decreasing order in binding energy). Electron doping increases the $d^2$ component but decreases the $d^0$ component, whereas hole doping reverses this trend; a single $d^1$ peak is never observed, and the $d^0$ peak is always present even in phase-pure samples. In the case of $n$-doped ${\mathrm{SrTiO}}_3$, the $d^1$ component appears as a weak shoulder with respect to the main $d^0$ peak. We argue that these multiple peaks should be understood as being due to the final-state effect and are intrinsic to the materials. Their presence does not necessarily imply the existence of spatially localized ions of different oxidation states nor of separate phases. A simple model is provided to illustrate this interpretation, and several experiments are discussed accordingly. The key parameter to determine the relative importance between the initial-state and final-state effects is also pointed out.

An extensive investigation of oxidation in uranium has been pursued. This includes the utilization of soft x-ray absorption spectroscopy, hard x-ray absorption near-edge structure, resonant (hard) x-ray emission spectroscopy, cluster calculations, and a branching ratio analysis founded on atomic theory. The samples utilized were uranium dioxide $\left(\mathrm{U}{\mathrm{O}}_2\right)$, uranium trioxide $\left(\mathrm{U}{\mathrm{O}}_3\right)$, and uranium tetrafluoride $\left(\mathrm{U}{\mathrm{F}}_4\right)$. A discussion of the role of nonspherical perturbations, i.e., crystal or ligand field effects, will be presented.

We calculate the conductivity $\sigma$ and the Seebeck coefficient $S$ for the phonon-assisted hopping transport in conducting polymers poly(3,4-ethylenedioxythiophene) or PEDOT, experimentally studied by Bubnova et al. [J. Am. Chem. Soc. 134, 16456 (2012)]. We use the Monte Carlo technique as well as the semianalytical approach based on the transport energy concept. We demonstrate that both approaches show a good qualitative agreement for the concentration dependence of $\sigma$ and $S$. At the same time, we find that the semianalytical approach is not in a position to describe the temperature dependence of the conductivity. We find that both Gaussian and exponential density of states (DOS) reproduce rather well the experimental data for the concentration dependence of $\sigma$ and $S$ giving similar fitting parameters of the theory. The obtained parameters correspond to a hopping model of localized quasiparticles extending over 2–3 monomer units with typical jumps over a distance of 3–4 units. The energetic disorder (broadening of the DOS) is estimated to be 0.1 eV. Using the Monte Carlo calculation we reproduce the activation behavior of the conductivity with the calculated activation energy close to the experimentally observed one. We find that for a low carrier concentration a number of free carriers contributing to the transport deviates strongly from the measured oxidation level. Possible reasons for this behavior are discussed. We also study the effect of the dimensionality on the charge transport by calculating the Seebeck coefficient and the conductivity for the cases of three-, two-, and one-dimensional motion.

We analyze the effect known as “spin current swapping” (SCS) due to electron-impurity scattering in a uniform spin-polarized two-dimensional electron gas. In this effect a primary spin current $J_i^a$ (lower index for spatial direction, upper index for spin direction) generates a secondary spin current $J_a^i$ if $i\ne a$, or $J_j^j$, with $j\ne i$, if $i=a$. Contrary to naive expectation, the homogeneous spin current associated with the uniform drift of the spin polarization in the electron gas does not generate a swapped spin current by the SCS mechanism. Nevertheless, a swapped spin current will be generated, if a magnetic field is present, by a completely different mechanism, namely, the precession of the spin Hall spin current in the magnetic field. We refer to this second mechanism as Hanle spin Hall effect, and we notice that it can be observed in an experiment in which a homogeneous drift current is passed through a uniformly magnetized electron gas. In contrast to this, we show that an unambiguous observation of SCS requires inhomogeneous spin currents, such as those that are associated with spin diffusion in a metal, and no magnetic field. An experimental setup for the observation of the SCS is therefore proposed.

We investigate theoretically the nonequilibrium transport through a molecular quantum dot as a function of gate and bias voltage, taking into account the typical situation in molecular electronics. In this respect, our study includes asymmetries both in the capacitances and tunneling rates to the source and drain electrodes, as well as an infinitely large charging energy on the molecule. Our calculations are based on the out-of-equilibrium noncrossing approximation (NCA), which is a reliable technique in the regime under consideration. We find that Coulomb blockade edges and Kondo peaks display strong renormalization in their width and intensity as a function of these asymmetries, and that basic expectations from Coulomb blockade theory must be taken with care in general, especially when Kondo physics is at play. In order to help comparison of theory to experiments, we also propose a simple phenomenological model which reproduces semiquantitatively the Coulomb blockade edges that were numerically computed from the NCA in all regimes of parameters.

Combined in situ x-ray photoemission spectroscopy, scanning tunneling spectroscopy, and angle resolved photoemission spectroscopy of molecular beam epitaxy grown ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ on lattice mismatched substrates reveal high quality stoichiometric thin films with topological surface states without a contribution from the bulk bands at the Fermi energy. The absence of bulk states at the Fermi energy is achieved without counterdoping. We observe that the surface morphology and electronic band structure of ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ are not affected by in vacuo storage and exposure to oxygen, whereas major changes are observed when exposed to ambient conditions. These films help define a pathway towards intrinsic topological devices.

We report an exact map into one-dimensional effective chains of the tight-binding Hamiltonian for electrons in armchair and zigzag graphene nanoribbons with any uniaxial ripple. This mapping is used for studying the effect of uniaxial periodic ripples, taking into account the relative orientation changes between $\pi$ orbitals. Such effects are important for short-wavelength ripples, while for long-wave ones, the system behaves nearly as strained graphene. The spectrum has a complex nature, akin to the Hofstadter butterfly with a rich localization behavior. Gaps at the Fermi level and dispersionless bands were observed, as well. The complex features of the spectrum arise as a consequence of the quasiperiodic or periodic nature of the effective one-dimensional system. Some features of these systems can be understood by considering weakly coupled dimers. The eigenenergies of such dimers are highly degenerate, and the net effect of the ripple can be seen as a perturbation potential that splits the energy spectrum. Several particular cases were analytically solved to understand this feature.

Despite many single-layer materials being reported in the past decade, few of them exhibit magnetism. Here we perform first-principles calculations using accurate hybrid density functional methods (HSE06) to predict that single-layer ${\mathrm{CrSnTe}}_3$ (CST) is a ferromagnetic semiconductor, with band gaps of 0.9 and 1.2 eV for the majority and minority spin channels, respectively. We determine the Curie temperature as 170 K, significantly higher than that of single-layer ${\mathrm{CrSiTe}}_3$ (90 K) and ${\mathrm{CrGeTe}}_3$ (130 K). This is due to the enhanced ionicity of the Sn-Te bond, which in turn increases the superexchange coupling between the magnetic Cr atoms. We further explore the mechanical and dynamical stability and strain response of this single-layer material for possible epitaxial growth. Our study provides an intuitive approach to understand and design single-layer magnetic semiconductors for a wide range of spintronics and energy applications.

Two independent and direct measurements of exciton transport in the fullerene ${\mathrm{C}}_{60}$ unambiguously indicate that singlets are responsible for energy transport and ultimately charge generation in organic photovoltaic cells. The singlet exciton diffusion length, $L_D$ was measured using fits to the external quantum efficiency of planar heterojunction photovoltaics, and via ${\mathrm{C}}_{60}$ fluorescence, giving a mean value of $34\pm 3\mathrm{nm}$. Direct measurement of the ${\mathrm{C}}_{60}$ singlet state transient fluorescence decay gives a lifetime of $590\pm 10\mathrm{ps}$, from which we infer a diffusivity of $0.020\pm 0.004\mathrm{c}{\mathrm{m}}^2/\mathrm{s}$. This is at least ten times that typical for organics, which we attribute to the spherical symmetry of the ${\mathrm{C}}_{60}$ molecule that promotes highly efficient exciton transfer. Further, $L_D$ is used to determine that the ${\mathrm{C}}_{60}$ energy gap is $18\pm 5\mathrm{meV}$ larger than the analogous fullerene ${\mathrm{C}}_{70}$.

The term “surface polarization” is introduced to describe the in-plane polarization existing at the surface of an insulating crystal when the in-plane surface inversion symmetry is broken. Here, the surface polarization is formulated in terms of a Berry phase, with the hybrid Wannier representation providing a natural basis for study of this effect. Tight-binding models are used to demonstrate how the surface polarization reveals itself via the accumulation of charges at the corners and edges for a two dimensional rectangular lattice and for GaAs respectively.

We calculate the momentum dependent spectral function of the Bose-Hubbard model on a simple cubic lattice in three dimensions within the bosonic dynamical mean-field theory (B-DMFT). The continuous-time quantum Monte Carlo method is used to solve the self-consistent B-DMFT equations together with the maximum entropy method for the analytic continuation to real frequencies. Results for weak, intermediate, and strong interactions are presented. In the limit of weak and strong interactions very good agreement with results obtained by perturbation theory is found. By contrast, at intermediate interactions the results differ significantly, indicating that in this regime perturbative methods fail to describe the dynamics of interacting bosons.

${}^7\mathrm{Li}$ and ${}^{51}\mathrm{V}$ NMR measurements up to 9.8 GPa have been made to elucidate local magnetic properties of a heavy-fermion spinel oxide ${\mathrm{LiV}}_2{\mathrm{O}}_4$ which undergoes a metal-insulator transition above $\sim 7$ GPa. The temperature $T$ and pressure $P$ dependences of the ${}^7\mathrm{Li}$ and ${}^{51}\mathrm{V}$ Knight shifts and the nuclear spin-lattice relaxation rates $1/T_1$ show that in the metallic phase, there is a crossover from a high-$T$ region with weak ferromagnetic fluctuations to a low-$T$ one with antiferromagnetic (AFM) fluctuations. The AFM fluctuations are enhanced below 20 K and 1.5 GPa, where a heavy Fermi-liquid state with the modified Korringa relation is formed. The evolution of the magnetic fluctuations is discussed from the aspect of the competition among several magnetic interactions. Above $P_{\mathrm{MI}}\sim 6.7$ GPa, we find the coexistence of metallic and insulating phases due to the first-order metal-insulator transition. The ${}^7\mathrm{Li}$ and ${}^{51}\mathrm{V}$ NMR spectra coming from the insulating phase have $T$-independent small Knight shifts and ${}^7\left(1/T_1\right)$ with the thermally activated $T$ dependence, indicating the formation of a spin-singlet cluster. We propose a model of a spin-singlet tetramer as discussed in geometrically frustrated materials.