486 research outputs found
A new perturbation treatment applied to the transport through a quantum dot
Resonant tunnelling through an Anderson impurity is investigated by employing
a new perturbation scheme at nonequilibrium. This new approach gives the
correct weak and strong coupling limit in by introducing adjustable
parameters in the self-energy and imposing self-consistency of the occupation
number of the impurity. We have found that the zero-temperature linear response
conductance agrees well with that obtained from the exact sum rule. At finite
temperature the conductance shows a nonzero minimum at the Kondo valley, as
shown in recent experiments. The effects of an applied bias voltage on the
single-particle density of states and on the differential conductances are
discussed for Kondo and non-Kondo systems.Comment: 4 pages, 4 figures, submitted to PRB-Rapid Comm. Email addresses
[email protected], [email protected]
Quantum Mechanics of Multi-Prong Potentials
We describe the bound state and scattering properties of a quantum mechanical
particle in a scalar -prong potential. Such a study is of special interest
since these situations are intermediate between one and two dimensions. The
energy levels for the special case of identical prongs exhibit an
alternating pattern of non-degeneracy and fold degeneracy. It is shown
that the techniques of supersymmetric quantum mechanics can be used to generate
new solutions. Solutions for prongs of arbitrary lengths are developed.
Discussions of tunneling in -well potentials and of scattering for piecewise
constant potentials are given. Since our treatment is for general values of
, the results can be studied in the large limit. A somewhat surprising
result is that a free particle incident on an -prong vertex undergoes
continuously increased backscattering as the number of prongs is increased.Comment: 17 pages. LATEX. On request, TOP_DRAW files or hard copies available
for 7 figure
Effect of the Coulomb repulsion on the {\it ac} transport through a quantum dot
We calculate in a linear response the admittance of a quantum dot out of
equilibrium. The interaction between two electrons with opposite spins
simultaneously residing on the resonant level is modeled by an Anderson
Hamiltonian. The electron correlations lead to the appearence of a new feature
in the frequency dependence of the conductance. For certain parameter values
there are two crossover frequencies between a capacitive and an inductive
behavior of the imaginary part of the admittance. The experimental implications
of the obtained results are briefly discussed.Comment: 13 pages, REVTEX 3.0, 2 .ps figures from [email protected],
NUB-308
On the Inequivalence of Weak-Localization and Coherent Backscattering
We define a current-conserving approximation for the local conductivity
tensor of a disordered system which includes the effects of weak localization.
Using this approximation we show that the weak localization effect in
conductance is not obtained simply from the diagram corresponding to the
coherent back-scattering peak observed in optical experiments. Other diagrams
contribute to the effect at the same order and decrease its value. These
diagrams appear to have no semiclassical analogues, a fact which may have
implications for the semiclassical theory of chaotic systems. The effects of
discrete symmetries on weak localization in disordered conductors is evaluated
and and compared to results from chaotic scatterers.Comment: 24 pages revtex + 12 figures on request; hub.94.
Electron spin operation by electric fields: spin dynamics and spin injection
Spin-orbit interaction couples electron spins to electric fields and allows
electrical monitoring of electron spins and electrical detection of spin
dynamics. Competing mechanisms of spin-orbit interaction are compared, and
optimal conditions for the electric operation of electrons spins in a quantum
well by a gate voltage are established. Electric spin injection into
semiconductors is discussed with a special emphasis on the injection into
ballistic microstructures. Dramatic effect of a long range Coulomb interaction
on transport phenomena in space-quantized low-dimensional conductors is
discussed in conclusion.Comment: A plenary paper at the 11th Intern. Conf. on Narrow Gap
Semiconductors (Buffalo, NY, June 2003). To be published in Physica
Andreev Scattering and the Kondo Effect
We examine the properties of an infinite- Anderson impurity coupled to
both normal and superconducting metals. Both the cases of a quantum dot and a
quantum point contact containing an impurity are considered; for the latter, we
study both one and two-channel impurities. Using a generalization of the
noncrossing approximation which incorporates multiple Andreev reflection, we
compute the impurity spectral function and the linear-response conductance of
these devices. We find generically that the Kondo resonance develops structure
at energies corresponding to the superconducting gap, and that the magnitude of
the resonance at the Fermi energy is altered. This leads to observable changes
in the zero-bias conductance as compared to the case with no superconductivity.Comment: 8 pages, 7 figures; expanded version to appear in PR
Nonequilibrium thermodynamics of interacting tunneling transport: variational grand potential, density-functional formulation, and nature of steady-state forces
The standard formulation of tunneling transport rests on an open-boundary
modeling. There, conserving approximations to nonequilibrium Green function or
quantum-statistical mechanics provide consistent but computational costly
approaches; alternatively, use of density-dependent ballistic-transport
calculations [e.g., Phys. Rev. B 52, 5335 (1995)], here denoted `DBT', provide
computationally efficient (approximate) atomistic characterizations of the
electron behavior but has until now lacked a formal justification. This paper
presents an exact, variational nonequilibrium thermodynamic theory for fully
interacting tunneling and provides a rigorous foundation for frozen-nuclei DBT
calculations as a lowest order approximation to an exact nonequilibrium
thermodynamics density functional evaluation. The theory starts from the
complete electron nonequilibrium quantum statistical mechanics and I identify
the operator for the nonequilibrium Gibbs free energy. I demonstrate a minimal
property of a functional for the nonequilibrium thermodynamic grand potential
which thus uniquely identifies the solution as the exact nonequilibrium density
matrix. I also show that a uniqueness-of-density proof from a closely related
study [Phys. Rev. B 78, 165109 (2008)] makes it possible to provide a
single-particle formulation based on universal electron-density functionals. I
illustrate a formal evaluation of the thermodynamics grand potential value
which is closely related to the variation in scattering phase shifts and hence
to Friedel density oscillations. This paper also discusses the difference
between the here-presented exact thermodynamics forces and the often-used
electrostatic forces. Finally the paper documents an inherent adiabatic nature
of the thermodynamics forces and observes that these are suited for a
nonequilibrium implementation of the Born-Oppenheimer approximation.Comment: 37 pages, 3 Figure
Shot noise of coupled semiconductor quantum dots
The low-frequency shot noise properties of two electrostatically coupled
semiconductor quantum dot states which are connected to emitter/collector
contacts are studied. A master equation approach is used to analyze the bias
voltage dependence of the Fano factor as a measure of temporal correlations in
tunneling current caused by Pauli's exclusion principle and the Coulomb
interaction. In particular, the influence of the Coulomb interaction on the
shot noise behavior is discussed in detail and predictions for future
experiments will be given. Furthermore, we propose a mechanism for negative
differential conductance and investigate the related super-Poissonian shot
noise.Comment: submitted to PR
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