50 research outputs found
On the theory of Josephson effect in a diffusive tunnel junction
Specific features of the equilibrium current-carrying state of a Josephson
tunnel junction between diffusive superconductors are studied theoretically in
the 1D geometry. It is found that the Josephson current induces localized
states of electron excitations in the vicinity of the tunnel barrier, which are
a continuous analog of Andreev levels in a ballistic junction. The depth of the
corresponding ``potential well'' is much greater than the separation between an
Andreev level and the continuous energy spectrum boundary for the same
transmissivity of the barrier. In contrast to a ballistic junction in which the
Josephson current is transported completely by localized excitations, the
contribution to current in a diffusive junction comes from whole spectral
region near the energy gap boundary, where the density of states differs
considerably from its unperturbed value. The correction to the Josephson
current in the second order of the barrier transmissivity, which contains the
second harmonic of the phase jump, is calculated and it is found that the true
expansion parameter of the perturbation theory for a diffusive junction is not
the tunneling probability itself, but a much larger parameter .Comment: 8 pages, 5 Postscript figures, submitted to Low Temp. Phy
Temperature dependence of the superheating field for superconductors in the high-k London limit
We study the metastability of the superheated Meissner state in type II
superconductors with k >> 1 beyond Ginzburg-Landau theory, which is applicable
only in the vicinity of the critical temperature. Within Eilenberger's
semiclassical approximation, we use the local electrodynamic response of the
superconductor to derive a generalized thermodynamic potential valid at any
temperature. The stability analysis of this functional yields the temperature
dependence of the superheating field. Finally, we comment on the implications
of our results for superconducting cavities in particle accelerators.Comment: 7.5 pages, 2 figure
Loss of Andreev Backscattering in Superconducting Quantum Point Contacts
We study effects of magnetic field on the energy spectrum in a
superconducting quantum point contact. The supercurrent induced by the magnetic
field leads to intermode transitions between the electron waves that pass and
do not pass through the constriction. The latter experience normal reflections
which couple the states with opposite momenta inside the quantum channel and
create a minigap in the energy spectrum that depends on the magnetic field
Current-voltage characteristic of narrow superconducting wires: bifurcation phenomena
The current-voltage characteristics of long and narrow superconducting
channels are investigated using the time-dependent Ginzburg-Landau equations
for complex order parameter. We found out that the steps in the current voltage
characteristic can be associated with bifurcations of either steady or
oscillatory solution. We revealed typical instabilities which induced the
singularities in current-voltage characteristics, and analytically estimated
period of oscillations and average voltage in the vicinity of the critical
currents. Our results show that these bifurcations can substantially complicate
dynamics of the order parameter and eventually lead to appearance of such
phenomena as multistability and chaos. The discussed bifurcation phenomena
sheds a light on some recent experimental findings
Vortex nucleation in rotating BEC: the role of the boundary condition for the order parameter
We study the process of vortex nucleation in rotating two-dimensional BEC
confined in a harmonic trap. We show that, within the Gross-Pitaevskii theory
with the boundary condition of vanishing of the order parameter at infinity,
topological defects nucleation occurs via the creation of vortex-antivortex
pairs far from the cloud center, where the modulus of the order parameter is
small. Then, vortices move towards the center of the cloud and antivortices
move in the opposite direction but never disappear. We also discuss the role of
surface modes in this process.Comment: 6 pages, 2 figure
Re-entrant localization of single particle transport in disordered Andreev wires
We study effects of disorder on the low energy single particle transport in a
normal wire surrounded by a superconductor. We show that the heat conductance
includes the Andreev diffusion decreasing with increase in the mean free path
and the diffusive drift produced by a small particle-hole asymmetry,
which increases with increasing . The conductance thus has a minimum as a
function of which leads to a peculiar re-entrant localization as a
function of the mean free path.Comment: 4 pages, 2 figure
Wigner distribution function formalism for superconductors and collisionless dynamics of the superconducting order parameter
A technique to study collisionless dynamics of a homogeneous superconducting
system is developed, which is based on Riccati parametrization of Wigner
distribution function. The quantum evolution of the superconductiung order
parameter, initially deviated from the equilibrium value, is calculated using
this technique. The effect of a time-dependent BCS paring interaction on the
dynamics of the order parameter is also studied.Comment: 14 pages, 5 figure
The dynamics of developing Cooper pairing at finite temperatures
We study the time evolution of a system of fermions with pairing interactions
at a finite temperature. The dynamics is triggered by an abrupt increase of the
BCS coupling constant. We show that if initially the fermions are in a normal
phase, the amplitude of the BCS order parameter averaged over the Boltzman
distribution of initial states exhibits damped oscillations with a relatively
short decay time. The latter is determined by the temperature, the
single-particle level spacing, and the ground state value of the BCS gap for
the new coupling. In contrast, the decay is essentially absent when the system
was in a superfluid phase before the coupling increase.Comment: 4 pages, figure fixe
Superheating fields of superconductors: Asymptotic analysis and numerical results
The superheated Meissner state in type-I superconductors is studied both
analytically and numerically within the framework of Ginzburg-Landau theory.
Using the method of matched asymptotic expansions we have developed a
systematic expansion for the solutions of the Ginzburg-Landau equations in the
limit of small , and have determined the maximum superheating field
for the existence of the metastable, superheated Meissner state as
an expansion in powers of . Our numerical solutions of these
equations agree quite well with the asymptotic solutions for . The
same asymptotic methods are also used to study the stability of the solutions,
as well as a modified version of the Ginzburg-Landau equations which
incorporates nonlocal electrodynamics. Finally, we compare our numerical
results for the superheating field for large- against recent asymptotic
results for large-, and again find a close agreement. Our results
demonstrate the efficacy of the method of matched asymptotic expansions for
dealing with problems in inhomogeneous superconductivity involving boundary
layers.Comment: 14 pages, 8 uuencoded figures, Revtex 3.
Flux penetration and expulsion in thin superconducting disks
Using an expansion of the order parameter over the eigenfunctions of the
linearized first Ginzburg-Landau (GL) equation, we obtain numerically the
saddle points of the free energy separating the stable states with different
number of vortices. In contrast to known surface and geometrical barrier
models, we find that in a wide range of magnetic fields below the penetration
field, the saddle point state for flux penetration into a disk does not
correspond to a vortex located nearby the sample boundary, but to a region of
suppressed superconductivity at the disk edge with no winding of the current,
and which is {\it a nucleus} for the following vortex creation. The height of
this {\it nucleation barrier}, which determines the time of flux penetration,
is calculated for different disk radii and magnetic fields.Comment: Accepted for publication in Physical Review Letter