5,145 research outputs found
Spin Hall effect in infinitely large and finite-size diffusive Rashba two-dimensional electron systems: A helicity-basis nonequilibrium Green's function approach
A nonequilibrium Green's function approach is employed to investigate the
spin-Hall effect in diffusive two-dimensional electron systems with Rashba
spin-orbit interaction. Considering a long-range electron-impurity scattering
potential in the self-consistent Born approximation, we find that the spin-Hall
effect arises from two distinct interband polarizations in helicity basis: a
disorder-unrelated polarization directly induced by the electric field and a
polarization mediated by electron-impurity scattering. The disorder-unrelated
polarization is associated with all electron states below the Fermi surface and
produces the original intrinsic spin-Hall current, while the disorder-mediated
polarization emerges with contribution from the electron states near the Fermi
surface and gives rise to an additional contribution to the spin-Hall current.
Within the diffusive regime, the total spin-Hall conductivity vanishes in {\it
infinitely large} samples, independently of temperature, of the spin-orbit
coupling constant, of the impurity density, and of the specific form of the
electron-impurity scattering potential. However, in a {\it finite-size} Rashba
two-dimensional semiconductor, the spin-Hall conductivity no longer always
vanishes. Depending on the sample size in the micrometer range, it can be
positive, zero or negative with a maximum absolute value reaching as large as
order of magnitude at low temperatures. As the sample size increases,
the total spin-Hall conductivity oscillates with a decreasing amplitude. We
also discuss the temperature dependence of the spin-Hall conductivity for
different sample sizes.Comment: 9 pages, 3 figures, extended version of cond-mat/041162
Efficient implementation of the nonequilibrium Green function method for electronic transport calculations
An efficient implementation of the nonequilibrium Green function (NEGF)
method combined with the density functional theory (DFT) using localized
pseudo-atomic orbitals (PAOs) is presented for electronic transport
calculations of a system connected with two leads under a finite bias voltage.
In the implementation, accurate and efficient methods are developed especially
for evaluation of the density matrix and treatment of boundaries between the
scattering region and the leads. Equilibrium and nonequilibrium contributions
in the density matrix are evaluated with very high precision by a contour
integration with a continued fraction representation of the Fermi-Dirac
function and by a simple quadratureon the real axis with a small imaginary
part, respectively. The Hartree potential is computed efficiently by a
combination of the two dimensional fast Fourier transform (FFT) and a finite
difference method, and the charge density near the boundaries is constructed
with a careful treatment to avoid the spurious scattering at the boundaries.
The efficiency of the implementation is demonstrated by rapid convergence
properties of the density matrix. In addition, as an illustration, our method
is applied for zigzag graphene nanoribbons, a Fe/MgO/Fe tunneling junction, and
a LaMnOSrMnO superlattice, demonstrating its applicability to a wide
variety of systems.Comment: 20 pages, 11 figure
Quantum master equation scheme of time-dependent density functional theory to time-dependent transport in nano-electronic devices
In this work a practical scheme is developed for the first-principles study
of time-dependent quantum transport. The basic idea is to combine the transport
master-equation with the well-known time-dependent density functional theory.
The key ingredients of this paper include: (i) the partitioning-free initial
condition and the consideration of the time-dependent bias voltages which base
our treatment on the Runge-Gross existence theorem; (ii) the non-Markovian
master equation for the reduced (many-body) central system (i.e. the device);
and (iii) the construction of Kohn-Sham master equation for the reduced
single-particle density matrix, where a number of auxiliary functions are
introduced and their equations of motion (EOM) are established based on the
technique of spectral decomposition. As a result, starting with a well-defined
initial state, the time-dependent transport current can be calculated
simultaneously along the propagation of the Kohn-Sham master equation and the
EOM of the auxiliary functions.Comment: 9 pages, no figure
Inelastic resonant tunneling through single molecules and quantum dots: spectrum modification due to nonequilibrium effects
Resonant electron transport through a mesoscopic region (quantum dot or
single molecule) with electron-phonon interaction is considered at finite
voltage. In this case the standard Landauer-B\"uttiker approach cannot be
applied. Using the nonequilibrium Green function method we show that due to a
nonequilibrium distribution function of electrons in the mesoscopic region, the
inelastic scattering rate and spectral function of the dot become functions of
the voltage and have to be calculated self-consistently.Comment: 4 pages, 3 figure
Causal vs. Noncausal Description of Nonlinear Wave Mixing; Resolving the Damping-Sign Controversy
Frequency-domain nonlinear wave mixing processes may be described either
using response functions whereby the signal is generated after all interactions
with the incoming fields, or in terms of scattering amplitudes where all fields
are treated symetrically with no specific time ordering. Closed Green's
function expressions derived for the two types of signals have different
analytical properties. The recent controversy regarding the sign of radiative
damping in the linear (Kramers Heisenberg) formula is put in a broader context
Excitonic Dynamical Franz-Keldysh Effect
The Dynamical Franz-Keldysh Effect is exposed by exploring near-bandgap
absorption in the presence of intense THz electric fields. It bridges the gap
between the DC Franz- Keldysh effect and multi-photon absorption and competes
with the THz AC Stark Effect in shifting the energy of the excitonic resonance.
A theoretical model which includes the strong THz field non-perturbatively via
a non-equilibrium Green Functions technique is able to describe the Dynamical
Franz-Keldysh Effect in the presence of excitonic absorption.Comment: 4 pages in revtex with 5 figures included using epsf. Submitted to
Physical Review Letter
Phonon Hall Effect in Four-Terminal Junctions
Using an exact nonequilibrium Green's function formulism, the phonon Hall
effect for paramagnetic dielectrics is studied in a four-terminal device
setting. The temperature difference in the transverse direction of the heat
current is calculated for two-dimensional models with the magnetic field
perpendicular to the plane. We find a surprising result that the square lattice
does not have the phonon Hall effect while a honeycomb lattice has. This can be
explained by symmetry. The temperature difference changes sign if the magnetic
field is sufficiently large.Comment: 4 pages, 5 figure
Two mini-band model for self-sustained oscillations of the current through resonant tunneling semiconductor superlattices
A two miniband model for electron transport in semiconductor superlattices
that includes scattering and interminiband tunnelling is proposed. The model is
formulated in terms of Wigner functions in a basis spanned by Pauli matrices,
includes electron-electron scattering in the Hartree approximation and modified
Bhatnagar-Gross-Krook collision tems. For strong applied fields, balance
equations for the electric field and the miniband populations are derived using
a Chapman-Enskog perturbation technique. These equations are then solved
numerically for a dc voltage biased superlattice. Results include
self-sustained current oscillations due to repeated nucleation of electric
field pulses at the injecting contact region and their motion towards the
collector. Numerical reconstruction of the Wigner functions shows that the
miniband with higher energy is empty during most of the oscillation period: it
becomes populated only when the local electric field (corresponding to the
passing pulse) is sufficiently large to trigger resonant tunneling.Comment: 26 pages, 3 figures, to appear in Phys. Rev.
A pertubative approach to the Kondo effect in magnetic atoms on nonmagnetic substrates
Recent experimental advances in scanning tunneling microscopy make the
measurement of the conductance spectra of isolated and magnetically coupled
atoms on nonmagnetic substrates possible. Notably these spectra are
characterized by a competition between the Kondo effect and spin-flip inelastic
electron tunneling. In particular they include Kondo resonances and a
logarithmic enhancement of the conductance at voltages corresponding to
magnetic excitations, two features that cannot be captured by second order
perturbation theory in the electron-spin coupling. We have now derived a third
order analytic expression for the electron-spin self-energy, which can be
readily used in combination with the non-equilibrium Green's function scheme
for electron transport at finite bias. We demonstrate that our method is
capable of quantitative description the competition between Kondo resonances
and spin-flip inelastic electron tunneling at a computational cost
significantly lower than that of other approaches. The examples of Co and Fe on
CuN are discussed in detail
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