379 research outputs found
Two-dimensional tunneling in a SQUID
Traditionally quantum tunneling in a static SQUID is studied on the basis of
a classical trajectory in imaginary time under a two-dimensional potential
barrier. The trajectory connects a potential well and an outer region crossing
their borders in perpendicular directions. In contrast to that main-path
mechanism, a wide set of trajectories with components tangent to the border of
the well can constitute an alternative mechanism of multi-path tunneling. The
phenomenon is essentially non-one-dimensional. Continuously distributed paths
under the barrier result in enhancement of tunneling probability. A type of
tunneling mechanism (main-path or multi-path) depends on character of a state
in the potential well prior to tunneling.Comment: 9 pages, 8 figure
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
Spontaneous superconducting islands and Hall voltage in clean superconductors
We study a clean superconductor in the Hall configuration, in the framework
of a purely dissipative time-dependent Ginzburg--Landau theory. We find
situations in which the order parameter differs significantly from zero in a
set of islands that appear to form a periodic structure. When the pattern of
islands becomes irregular, it moves in or against the direction of the current
and a Hall voltage is found. Tiny differences in the initial state may reverse
the sign of the Hall voltage. When the average Hall voltage vanishes, the local
Hall voltage does not necessarily vanish. We examine the influence that several
boundary conditions at the electrodes have on these effects.Comment: 6 pages, Includes additional cases and more detailed result
Wave scattering by discrete breathers
We present a theoretical study of linear wave scattering in one-dimensional
nonlinear lattices by intrinsic spatially localized dynamic excitations or
discrete breathers. These states appear in various nonlinear systems and
present a time-periodic localized scattering potential for plane waves. We
consider the case of elastic one-channel scattering, when the frequencies of
incoming and transmitted waves coincide, but the breather provides with
additional spatially localized ac channels whose presence may lead to various
interference patterns. The dependence of the transmission coefficient on the
wave number q and the breather frequency Omega_b is studied for different types
of breathers: acoustic and optical breathers, and rotobreathers. We identify
several typical scattering setups where the internal time dependence of the
breather is of crucial importance for the observed transmission properties.Comment: 17 pages, 19 figures, submitted to CHAOS (Focus Issue
Quantum and Thermal Depinning of a String from a Linear Defect
The problem of a massive elastic string depinning from a linear defect under
the action of a small driving force is considered. To exponential accuracy the
decay rate is calculated with the help of the instanton method; then,
fluctuations of the quasiclassical solution are taken into account to determine
the preexponential factor. The decay rate exhibits a kind of first order
transition from quantum tunneling to thermal activation with vanishing
crossover region. The model may be applied to describe nucleation in
2-dimensional first order quantum phase transitions.Comment: Revtex. 11 pages + 4 PS figures. Accepted for publication in PR
Evidence of two-dimensional macroscopic quantum tunneling of a current-biased DC-SQUID
The escape probability out of the superconducting state of a hysteretic
DC-SQUID has been measured at different values of the applied magnetic flux. At
low temperature, the escape current and the width of the probability
distribution are temperature independent but they depend on flux. Experimental
results do not fit the usual one-dimensional (1D) Macroscopic Quantum Tunneling
(MQT) law but are perfectly accounted for by the two-dimensional (2D) MQT
behaviour as we propose here. Near zero flux, our data confirms the recent MQT
observation in a DC-SQUID \cite{Li02}.Comment: 4 pages, 4 figures Accepted to PR
Nucleation and Growth of the Superconducting Phase in the Presence of a Current
We study the localized stationary solutions of the one-dimensional
time-dependent Ginzburg-Landau equations in the presence of a current. These
threshold perturbations separate undercritical perturbations which return to
the normal phase from overcritical perturbations which lead to the
superconducting phase. Careful numerical work in the small-current limit shows
that the amplitude of these solutions is exponentially small in the current; we
provide an approximate analysis which captures this behavior. As the current is
increased toward the stall current J*, the width of these solutions diverges
resulting in widely separated normal-superconducting interfaces. We map out
numerically the dependence of J* on u (a parameter characterizing the material)
and use asymptotic analysis to derive the behaviors for large u (J* ~ u^-1/4)
and small u (J -> J_c, the critical deparing current), which agree with the
numerical work in these regimes. For currents other than J* the interface
moves, and in this case we study the interface velocity as a function of u and
J. We find that the velocities are bounded both as J -> 0 and as J -> J_c,
contrary to previous claims.Comment: 13 pages, 10 figures, Revte
Metastability of (d+n)-dimensional elastic manifolds
We investigate the depinning of a massive elastic manifold with internal
dimensions, embedded in a -dimensional space, and subject to an
isotropic pinning potential The tunneling process is
driven by a small external force We find the zero temperature and
high temperature instantons and show that for the case the
problem exhibits a sharp transition from quantum to classical behavior: At low
temperatures the Euclidean action is constant up to exponentially
small corrections, while for The results are universal and do not depend on the detailed shape
of the trapping potential . Possible applications of the problem to
the depinning of vortices in high- superconductors and nucleation in
-dimensional phase transitions are discussed. In addition, we determine the
high-temperature asymptotics of the preexponential factor for the
-dimensional problem.Comment: RevTeX, 10 pages, 3 figures inserte
Quantum instability in a dc-SQUID with strongly asymmetric dynamical parameters
A classical system cannot escape out of a metastable state at zero
temperature. However, a composite system made from both classical and quantum
degrees of freedom may drag itself out of the metastable state by a sequential
process. The sequence starts with the tunneling of the quantum component which
then triggers a distortion of the trapping potential holding the classical
part. Provided this distortion is large enough to turn the metastable state
into an unstable one, the classical component can escape. This process reminds
of the famous baron Muenchhausen who told the story of rescuing himself from
sinking in a swamp by pulling himself up by his own hair--we thus term this
decay the `Muenchhausen effect'. We show that such a composite system can be
conveniently studied and implemented in a dc-SQUID featuring asymmetric
dynamical parameters. We determine the dynamical phase diagram of this system
for various choices of junction parameters and system preparations.Comment: 12 pages, 12 figure
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