1,285 research outputs found
General transport properties of superconducting quantum point contacts: a Green functions approach
We discuss the general transport properties of superconducting quantum point
contacts. We show how these properties can be obtained from a microscopic model
using nonequilibrium Green function techniques. For the case of a one-channel
contact we analyze the response under different biasing conditions: constant
applied voltage, current bias and microwave-induced transport. Current
fluctuations are also analyzed with particular emphasis on thermal and
shot-noise. Finally, the case of superconducting transport through a resonant
level is discussed. The calculated properties show a remarkable agreement with
the available experimental data from atomic-size contacts measurements. We
suggest the possibility of extending this comparison to several other
predictions of the theory.Comment: 10 pages, revtex, 8 figures, submitted to a special issue of
Superlattices and Microstructure
A self-consistent treatment of non-equilibrium spin torques in magnetic multilayers
It is known that the transfer of spin angular momenta between current
carriers and local moments occurs near the interface of magnetic layers when
their moments are non-collinear. However, to determine the magnitude of the
transfer, one should calculate the spin transport properties far beyond the
interface regions. Based on the spin diffusion equation, we present a
self-consistent approach to evaluate the spin torque for a number of layered
structures. One of the salient features is that the longitudinal and transverse
components of spin accumulations are inter-twined from one layer to the next,
and thus, the spin torque could be significantly amplified with respect to
treatments which concentrate solely on the transport at the interface due to
the presence of the much longer longitudinal spin diffusion length. We conclude
that bare spin currents do not properly estimate the spin angular momentum
transferred between to the magnetic background; the spin transfer that occurs
at interfaces should be self-consistently determined by embedding it in our
globally diffuse transport calculations.Comment: 21 pages, 6 figure
Transport through superconductor/magnetic dot/superconductor structures
The coupling of two s-wave superconductors through a small magnetic dot is
discussed. Assuming that the dot charging energy is small compared to the
superconducting gap, , and that the moment of the dot is
classical, we develop a simple theory of transport through the dot. The
presence of the magnetic dot will position Andreev bound states within the
superconducting gap at energies tunable with the magnetic properties of the
dot. Studying the Josephson coupling it is shown that the constructed junction
can be tuned from a "0" to a ""-junction via a degenerate two-level state
either by changing the magnetic moment of the dot or by changing temperature.
Furthermore, it is shown that details of the magnetic dot can be extracted from
the sub-harmonic structure in the current-voltage characteristics of the
junction.Comment: 5 pages, 4 figures, paper presented at the conference SDP 2001 in
Tokyo on June 2
Electronic Structure of Dangling Bonds in Amorphous Silicon Studied via a Density-Matrix Functional Method
A structural model of hydrogenated amorphous silicon containing an isolated
dangling bond is used to investigate the effects of electron interactions on
the electronic level splittings, localization of charge and spin, and
fluctuations in charge and spin. These properties are calculated with a
recently developed density-matrix correlation-energy functional applied to a
generalized Anderson Hamiltonian, consisting of tight-binding one-electron
terms parametrizing hydrogenated amorphous silicon plus a local interaction
term. The energy level splittings approach an asymptotic value for large values
of the electron-interaction parameter U, and for physically relevant values of
U are in the range 0.3-0.5 eV. The electron spin is highly localized on the
central orbital of the dangling bond while the charge is spread over a larger
region surrounding the dangling bond site. These results are consistent with
known experimental data and previous density-functional calculations. The spin
fluctuations are quite different from those obtained with unrestricted
Hartree-Fock theory.Comment: 6 pages, 6 figures, 1 tabl
Ballistic versus diffusive magnetoresistance of a magnetic point contact
The quasiclassical theory of a nanosize point contacts (PC) between two
ferromagnets is developed. The maximum available magnetoresistance values in PC
are calculated for ballistic versus diffusive transport through the area of a
contact. In the ballistic regime the magnetoresistance in excess of few
hundreds percents is obtained for the iron-group ferromagnets. The necessary
conditions for realization of so large magnetoresistance in PC, and the
experimental results by Garcia et al are discussedComment: 4 pages, TEX, 1 Figur
Towards unified understanding of conductance of stretched monatomic contacts
When monatomic contacts are stretched, their conductance behaves in
qualitatively different ways depending on their constituent atomic elements.
Under a single assumption of resonance formation, we show that various
conductance behavior can be understood in a unified way in terms of the
response of the resonance to stretching. This analysis clarifies the crucial
roles played by the number of valence electrons, charge neutrality, and orbital
shapes.Comment: 2 figure
The Van der Waals interaction of the hydrogen molecule - an exact local energy density functional
We verify that the van der Waals interaction and hence all dispersion
interactions for the hydrogen molecule given by: W"= -{A/R^6}-{B/R^8}-{C/R^10}-
..., in which R is the internuclear separation, are exactly soluble. The
constants A=6.4990267..., B=124.3990835 ... and C=1135.2140398... (in Hartree
units) first obtained approximately by Pauling and Beach (PB) [1] using a
linear variational method, can be shown to be obtainable to any desired
accuracy via our exact solution. In addition we shall show that a local energy
density functional can be obtained, whose variational solution rederives the
exact solution for this problem. This demonstrates explicitly that a static
local density functional theory exists for this system. We conclude with
remarks about generalising the method to other hydrogenic systems and also to
helium.Comment: 11 pages, 13 figures and 28 reference
Effective action and density functional theory
The effective action for the charge density and the photon field is proposed
as a generalization of the density functional. A simple definition is given for
the density functional, as the functional Legendre transform of the generator
functional of connected Green functions for the density and the photon field,
offering systematic approximation schemes. The leading order of the
perturbation expansion reproduces the Hartree-Fock equation. A renormalization
group motivated method is introduced to turn on the Coulomb interaction
gradually and to find corrections to the Hartree-Fock and the Kohn-Sham
schemes.Comment: New references and a numerical algorithm added, to appear in Phys.
Rev. B. 30 pages, no figure
Spin pumping and magnetization dynamics in metallic multilayers
We study the magnetization dynamics in thin ferromagnetic films and small
ferromagnetic particles in contact with paramagnetic conductors. A moving
magnetization vector causes \textquotedblleft pumping\textquotedblright of
spins into adjacent nonmagnetic layers. This spin transfer affects the
magnetization dynamics similar to the Landau-Lifshitz-Gilbert phenomenology.
The additional Gilbert damping is significant for small ferromagnets, when the
nonmagnetic layers efficiently relax the injected spins, but the effect is
reduced when a spin accumulation build-up in the normal metal opposes the spin
pumping. The damping enhancement is governed by (and, in turn, can be used to
measure) the mixing conductance or spin-torque parameter of the
ferromagnet--normal-metal interface. Our theoretical findings are confirmed by
agreement with recent experiments in a variety of multilayer systems.Comment: 10 pages, 6 figure
Analytic theory of ground-state properties of a three-dimensional electron gas at varying spin polarization
We present an analytic theory of the spin-resolved pair distribution
functions and the ground-state energy of an electron gas
with an arbitrary degree of spin polarization. We first use the Hohenberg-Kohn
variational principle and the von Weizs\"{a}cker-Herring ideal kinetic energy
functional to derive a zero-energy scattering Schr\"{o}dinger equation for
. The solution of this equation is implemented
within a Fermi-hypernetted-chain approximation which embodies the Hartree-Fock
limit and is shown to satisfy an important set of sum rules. We present
numerical results for the ground-state energy at selected values of the spin
polarization and for in both a paramagnetic and a fully
spin-polarized electron gas, in comparison with the available data from Quantum
Monte Carlo studies over a wide range of electron density.Comment: 13 pages, 8 figures, submitted to Phys. Rev.
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