A combined representation method for use in band structure calculations. 1: Method

A representation was described whose basis levels combine the important physical aspects of a finite set of plane waves with those of a set of Bloch tight-binding levels. The chosen combination has a particularly simple dependence on the wave vector within the Brillouin Zone, and its use in reducing the standard one-electron band structure problem to the usual secular equation has the advantage that the lattice sums involved in the calculation of the matrix elements are actually independent of the wave vector. For systems with complicated crystal structures, for which the Korringa-Kohn-Rostoker (KKR), Augmented-Plane Wave (APW) and Orthogonalized-Plane Wave (OPW) methods are difficult to apply, the present method leads to results with satisfactory accuracy and convergence

Rich variety of defects in ZnO via an attractive interaction between O-vacancies and Zn-interstitials

As the concentration of intrinsic defects becomes sufficiently high in O-deficient ZnO, interactions between defects lead to a significant reduction in their formation energies. We show that the formation of both O-vacancies and Zn-interstitials becomes significantly enhanced by a strong attractive interaction between them, making these defects an important source of n-type conductivity in ZnO.Comment: 12 pages, 4 figure

The topological system with a twisting edge band: position-dependent Hall resistance

We study a $\nu=1$ topological system with one twisting edge-state band and one normal edge-state band. For the twisting edge-state band, Fermi energy goes through the band three times, thus, having three edge states on one side of the sample; while the normal edge band contributes only one edge state on the other side of the sample. In such a system, we show that it consists of both topologically protected and unprotected edge states, and as a consequence, its Hall resistance depends on the location where the Hall measurement is done even for a translationally invariant system. This unique property is absent in a normal topological insulator

Effects of spin vacancies on magnetic properties of the Kitaev-Heisenberg model

We study the ground state properties of the Kitaev-Heisenberg model in a magnetic field and explore the evolution of spin correlations in the presence of non-magnetic vacancies. By means of exact diagonalizations, the phase diagram without vacancies is determined as a function of the magnetic field and the ratio between Kitaev and Heisenberg interactions. We show that in the (antiferromagnetic) stripe ordered phase the static susceptibility and its anisotropy can be described by a spin canting mechanism. This accounts as well for the transition to the polarized phase when including quantum fluctuations perturbatively. Effects of spin vacancies depend sensitively on the type of the ground state. In the liquid phase, the magnetization pattern around a single vacancy in a small field is determined, and its spatial anisotropy is related to that of non-zero further neighbor correlations induced by the field and/or Heisenberg interactions. In the stripe phase, the joint effect of a vacancy and a small field breaks the six-fold symmetry of the model and stabilizes a particular stripe pattern. Similar symmetry-breaking effects occur even at zero field due to effective interactions between vacancies. This selection mechanism and intrinsic randomness of vacancy positions may lead to spin-glass behavior.Comment: 13 pages, 10 figure

Fast-field cycling NMR is sensitive to the method of cross-linking in BSA gels

This work was supported by ARUK (grant number 19689).Non peer reviewedPublisher PD

Tunable Charge and Spin Seebeck Effects in Magnetic Molecular Junctions

We study the charge and spin Seebeck effects in a spin-1 molecular junction as a function of temperature (T), applied magnetic field (H), and magnetic anisotropy (D) using Wilson's numerical renormalization group. A hard-axis magnetic anisotropy produces a large enhancement of the charge Seebeck coefficient Sc (\sim k_B/|e|) whose value only depends on the residual interaction between quasiparticles in the low temperature Fermi-liquid regime. In the underscreened spin-1 Kondo regime, the high sensitivity of the system to magnetic fields makes it possible to observe a sizable value for the spin Seebeck coefficient even for magnetic fields much smaller than the Kondo temperature. Similar effects can be obtain in C60 junctions where the control parameter is the gap between a singlet and a triplet molecular state.Comment: 5 pages, 4 figure

Steering Magnetic Skyrmions with Nonequilibrium Green's Functions

Magnetic skyrmions, topologically protected vortex-like configurations in spin textures, are of wide conceptual and practical appeal for quantum information technologies, notably in relation to the making of so-called race-track memory devices. Skyrmions can be created, steered and destroyed with magnetic fields and/or (spin) currents. Here we focus on the latter mechanism, analyzed via a microscopic treatment of the skyrmion-current interaction. The system we consider is an isolated skyrmion in a square-lattice cluster, interacting with electrons spins in a current-carrying quantum wire. For the theoretical description, we employ a quantum formulation of spin-dependent currents via nonequilibrium Green's functions (NEGF) within the generalized Kadanoff-Baym ansatz (GKBA). This is combined with a treatment of skyrmions based on classical localized spins, with the skyrmion motion described via Ehrenfest dynamics. With our mixed quantum-classical scheme, we assess how time-dependent currents can affect the skyrmion dynamics, and how this in turn depends on electron-electron and spin-orbit interactions in the wire. Our study shows the usefulness of a quantum-classical treatment of skyrmion steering via currents, as a way for example to validate/extract an effective, classical-only, description of skyrmion dynamics from a microscopic quantum modeling of the skyrmion-current interaction.Comment: 10 pages, 8 figures, contribution to the proceedings of "Progress in Nonequilibrium Green's Functions VII

A comprehensive study of electric, thermoelectric and thermal conductivities of Graphene with short range unitary and charged impurities

Motivated by the experimental measurement of electrical and hall conductivity, thermopower and Nernst effect, we calculate the longitudinal and transverse electrical and heat transport in graphene in the presence of unitary scatterers as well as charged impurities. The temperature and carrier density dependence in this system display a number of anomalous features that arise due to the relativistic nature of the low energy fermionic degrees of freedom. We derive the properties in detail including the effect of unitary and charged impurities self-consistently, and present tables giving the analytic expressions for all the transport properties in the limit of small and large temperature compared to the chemical potential and the scattering rates. We compare our results with the available experimental data. While the qualitative variations with temperature and density of carriers or chemical potential of all transport properties can be reproduced, we find that a given set of parameters of the impurities fits the Hall conductivity, Thermopower and the Nernst effect quantitatively but cannot fit the conductivity quantitatively. On the other hand a single set of parameters for scattering from Coulomb impurities fits conductivity, hall resistance and thermopower but not Nernst

Electron-ion and ion-ion potentials for modeling warm-dense-matter: applications to laser-heated or shock-compressed Al and Si

The pair-interactions U_{ij}(r) determine the thermodynamics and linear transport properties of matter via the pair-distribution functions (PDFs), i.e., g_{ij}(r). Great simplicity is achieved if U_{ij}(r) could be directly used to predict material properties via classical simulations, avoiding many-body wavefunctions. Warm dense matter (WDM) is encountered in quasi-equilibria where the electron temperature $T_e$ differs from the ion temperature T_i, as in laser-heated or in shock-compressed matter. The electron PDFs g_{ee}(r) as perturbed by the ions are used to evaluate fully non-local exchange-correlation corrections to the free energy, using Hydrogen as an example. Electron-ion potentials for ions with a bound core are discussed with Al and Si as examples, for WDM with T_e \ne T_i, and valid for times shorter than the electron-ion relaxation time. In some cases the potentials develop attractive regions, and then become repulsive and `Yukawa-like' for higher $T_e$. These results clarify the origin of initial phonon-hardening and rapid release. Pair-potentials for shock-heated WDM show that phonon hardening would not occur in most such systems. Defining meaningful quasi-equilibrium static transport coefficients consistent with the dynamic values is addressed. There seems to be no meaningful `static conductivity' obtainable by extrapolating experimental or theoretical \sigma(\omega, T_i, T_e) to \omega \to 0, unless T_i \to T_e as well. Illustrative calculations of quasi-static resistivities R(T_i,T_e) of laser-heated as well as shock-heated Aluminum and Silicon are presented using our pseudopotentials, pair-potentials and classical integral equations. The quasi-static resistivities display clear differences in their temperature evolutions, but are not the strict \omega \to 0 limits of the dynamic values.Comment: 12 pages, 6 figues, Latex file