4,646 research outputs found
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 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
Calculated NMR T_2 relaxation due to vortex vibrations in cuprate superconductors
We calculate the rate of transverse relaxation arising from vortex motion in
the mixed state of YBa_2Cu_3O_7 with the static field applied along the c axis.
The vortex dynamics are described by an overdamped Langevin equation with a
harmonic elastic free energy. We find that the variation of the relaxation with
temperature, average magnetic field, and local field is consistent with
experiments; however, the calculated time dependence is different from what has
been measured and the value of the rates calculated is roughly two orders of
magnitude slower than what is observed. Combined with the strong experimental
evidence pointing to vortex motion as the dominant mechanism for T_2
relaxation, these results call into question a prior conclusion that vortex
motion is not significant in T_1 measurements in the vortex state.Comment: 6 pages, 5 figures, to be published in Phys. Rev.
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