674 research outputs found
Non-adiabatic Josephson Dynamics in Junctions with in-Gap Quasiparticles
Conventional models of Josephson junction dynamics rely on the absence of low
energy quasiparticle states due to a large superconducting gap. With this
assumption the quasiparticle degrees of freedom become "frozen out" and the
phase difference becomes the only free variable, acting as a fictitious
particle in a local in time Josephson potential related to the adiabatic and
non-dissipative supercurrent across the junction. In this article we develop a
general framework to incorporate the effects of low energy quasiparticles
interacting non-adiabatically with the phase degree of freedom. Such
quasiparticle states exist generically in constriction type junctions with high
transparency channels or resonant states, as well as in junctions of
unconventional superconductors. Furthermore, recent experiments have revealed
the existence of spurious low energy in-gap states in tunnel junctions of
conventional superconductors - a system for which the adiabatic assumption
typically is assumed to hold. We show that the resonant interaction with such
low energy states rather than the Josephson potential defines nonlinear
Josephson dynamics at small amplitudes.Comment: 9 pages, 1 figur
Dynamical Coulomb Blockade and the Derivative Discontinuity of Time-Dependent Density Functional Theory
The role of the discontinuity of the exchange-correlation potential of
density functional theory is studied in the context of electron transport and
shown to be intimately related to Coulomb blockade. By following the time
evolution of an interacting nanojunction attached to biased leads, we find
that, instead of evolving to a steady state, the system reaches a dynamical
state characterized by correlation-induced current oscillations. Our results
establish a dynamical picture of Coulomb blockade manifesting itself as a
periodic sequence of charging and discharging of the nanostructure.Comment: to appear in Physical Review Letter
Co-tunneling current through the two-level quantum dot coupled to magnetic leads: A role of exchange interaction
The co-tunneling current through a two-level doubly occupied quantum dot
weakly coupled to ferromagnetic leads is calculated in the Coulomb blockade
regime. It is shown that the dependence of the differrential conductance on
applied voltage has a stair-case structure with different sets of "stairs" for
parallel and anti-parallel configurations of magnetization of the leads.
Contributions to the current from elastic and inelastic processes are
considered distinctly. It is observed that the interference part of the
co-tunneling current involves terms corresponding to inelastic processes.
Dependence of the co-tunneling current on the phases of the tunneling
amplitudes is studied.Comment: LaTex, 14 page
Correlated sequential tunneling through a double barrier for interacting one-dimensional electrons
The problem of resonant tunneling through a quantum dot weakly coupled to
spinless Tomonaga-Luttinger liquids has been studied. We compute the linear
conductance due to sequential tunneling processes upon employing a master
equation approach. Besides the previously used lowest-order golden rule rates
describing uncorrelated sequential tunneling (UST) processes, we systematically
include higher-order correlated sequential tunneling (CST) diagrams within the
standard Weisskopf-Wigner approximation. We provide estimates for the parameter
regions where CST effects can be important. Focusing mainly on the temperature
dependence of the peak conductance, we discuss the relation of these findings
to previous theoretical and experimental results.Comment: replaced with the published versio
Coherent current transport in wide ballistic Josephson junctions
We present an experimental and theoretical investigation of coherent current
transport in wide ballistic superconductor-two dimensional electron
gas-superconductor junctions. It is found experimentally that upon increasing
the junction length, the subharmonic gap structure in the current-voltage
characteristics is shifted to lower voltages, and the excess current at
voltages much larger than the superconducting gap decreases. Applying a theory
of coherent multiple Andreev reflection, we show that these observations can be
explained in terms of transport through Andreev resonances.Comment: 4 pages, 4 figure
Supercurrent noise in quantum point contacts
Spectral density of current fluctuations in a short ballistic superconducting
quantum point contact is calculated for arbitrary bias voltages . Contrary
to a common opinion that the supercurrent flow in Josephson junctions is
coherent process with no fluctuations, we find extremely large current noise
that is {\em caused} by the supercurrent coherence. An unusual feature of the
noise, besides its magnitude, is its voltage dependence: the noise decreases
with increasing , despite the fact that the dc current grows steadily with
. At finite voltages the noise can be qualitatively understood as the shot
noise of the large charge quanta of magnitude equal to the charge
transferred during one period of Josephson oscillations.Comment: 12 pages, revtex, 2 figures by fax/conventional mail upon reques
Properties of Chiral Wilson Loops
We study a class of Wilson Loops in N =4, D=4 Yang-Mills theory belonging to
the chiral ring of a N=2, d=1 subalgebra. We show that the expectation value of
these loops is independent of their shape. Using properties of the chiral ring,
we also show that the expectation value is identically 1. We find the same
result for chiral loops in maximally supersymmetric Yang-Mills theory in three,
five and six dimensions. In seven dimensions, a generalized Konishi anomaly
gives an equation for chiral loops which closely resembles the loop equations
of the three dimensional Chern-Simons theory.Comment: 15 pages, two pictures, some references adde
Coulomb Blockade Peak Spacings: Interplay of Spin and Dot-Lead Coupling
For Coulomb blockade peaks in the linear conductance of a quantum dot, we
study the correction to the spacing between the peaks due to dot-lead coupling.
This coupling can affect measurements in which Coulomb blockade phenomena are
used as a tool to probe the energy level structure of quantum dots. The
electron-electron interactions in the quantum dot are described by the constant
exchange and interaction (CEI) model while the single-particle properties are
described by random matrix theory. We find analytic expressions for both the
average and rms mesoscopic fluctuation of the correction. For a realistic value
of the exchange interaction constant J_s, the ensemble average correction to
the peak spacing is two to three times smaller than that at J_s = 0. As a
function of J_s, the average correction to the peak spacing for an even valley
decreases monotonically, nonetheless staying positive. The rms fluctuation is
of the same order as the average and weakly depends on J_s. For a small
fraction of quantum dots in the ensemble, therefore, the correction to the peak
spacing for the even valley is negative. The correction to the spacing in the
odd valleys is opposite in sign to that in the even valleys and equal in
magnitude. These results are robust with respect to the choice of the random
matrix ensemble or change in parameters such as charging energy, mean level
spacing, or temperature.Comment: RevTex, 11 pages, 9 figures. v2: Conclusions section expanded.
Accepted for publication in PR
Nonequilibrium plasmons and transport properties of a double--junction quantum wire
We study theoretically the current-voltage characteristics, shot noise, and
full counting statistics of a quantum wire double barrier structure. We model
each wire segment by a spinless Luttinger liquid. Within the sequential
tunneling approach, we describe the system's dynamics using a master equation.
We show that at finite bias the non-equilibrium distribution of plasmons in the
central wire segment leads to increased average current, enhanced shot noise,
and full counting statistics corresponding to a super-Poissonian process. These
effects are particularly pronounced in the strong interaction regime, while in
the non-interacting case we recover results obtained earlier using detailed
balance arguments.Comment: 22 pages, RevTex 2-column, 11 figure
Phase-Controlled Force and Magnetization Oscillations in Superconducting Ballistic Nanowires
The emergence of superconductivity-induced phase-controlled forces in the
(0.01-0.1) nN range, and of magnetization oscillations, in nanowire junctions,
is discussed. A giant magnetic response to applied weak magnetic fields, is
predicted in the ballistic Josephson junction formed by a superconducting tip
and a surface, bridged by a normal metal nanowire where Andreev states form.Comment: 5 pages, 3 figure
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