522 research outputs found
New Enhanced Tunneling in Nuclear Processes
The small sub-barrier tunneling probability of nuclear processes can be
dramatically enhanced by collision with incident charged particles.
Semiclassical methods of theory of complex trajectories have been applied to
nuclear tunneling, and conditions for the effects have been obtained. We
demonstrate the enhancement of alpha particle decay by incident proton with
energy of about 0.25 MeV. We show that the general features of this process are
common for other sub-barrier nuclear processes and can be applied to nuclear
fission.Comment: RevTex4, 2 figure
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
Vortex liquid crystals in anisotropic type II superconductors
In a type II superconductor in a moderate magnetic field, the superconductor
to normal state transition may be described as a phase transition in which the
vortex lattice melts into a liquid. In a biaxial superconductor, or even a
uniaxial superconductor with magnetic field oriented perpendicular to the
symmetry axis, the vortices acquire elongated cross sections and interactions.
Systems of anisotropic, interacting constituents generally exhibit liquid
crystalline phases. We examine the possibility of a two step melting in
homogeneous type II superconductors with anisotropic superfluid stiffness from
a vortex lattice into first a vortex smectic and then a vortex nematic at high
temperature and magnetic field. We find that fluctuations of the ordered phase
favor an instability to an intermediate smectic-A in the absence of intrinsic
pinning
Negative magnetoresistance and phase slip process in superconducting nanowires
We argue that the negative magnetoresistance of superconducting nanowires,
which was observed in recent experiments, can be explained by the influence of
the external magnetic field on the critical current of the phase slip process.
We show that the suppression of the order parameter in the bulk superconductors
made by an external magnetic field can lead to an enhancement of both the first
and the second critical currents of the phase slip process in
nanowires. Another mechanism of an enhancement of can come from
decreasing the decay length of the charge imbalance at weak
magnetic fields because is inversely proportional to . The
enhancement of the first critical current leads to a larger intrinsic
dissipation of the phase slip process. It suppresses the rate of both the
thermo-activated and/or quantum fluctuated phase slips and results in
decreasing the fluctuated resistance.Comment: 7 pages, 4 figure
Dynamics of the superconducting condensate in the presence of a magnetic field. Channelling of vortices in superconducting strips at high currents
On the basis of the time-dependent Ginzburg-Landau equation we studied the
dynamics of the superconducting condensate in a wide two-dimensional sample in
the presence of a perpendicular magnetic field and applied current. We could
identify two critical currents: the current at which the pure superconducting
state becomes unstable ( \cite{self1}) and the current at which the
system transits from the resistive state to the superconducting state
(). The current decreases monotonically with external
magnetic field, while exhibits a maximum at . For sufficient
large magnetic fields the hysteresis disappears and . In
this high magnetic field region and for currents close to the voltage
appears as a result of the motion of separate vortices. With increasing current
the moving vortices form 'channels' with suppressed order parameter along which
the vortices can move very fast. This leads to a sharp increase of the voltage.
These 'channels' resemble in some respect the phase slip lines which occur at
zero magnetic field.Comment: 5 pages, 4 figures, Proceedings of Third European Conference on
Vortex Matter in Superconductor
Practical dispersion relations for strongly coupled plasma fluids
Very simple explicit analytical expressions are discussed, which are able to
describe the dispersion relations of longitudinal waves in strongly coupled
plasma systems such as one-component plasma and weakly screened Yukawa fluids
with a very good accuracy. Applications to other systems with soft pairwise
interactions are briefly discussed.Comment: 11 pages, 3 figures; Related to arXiv:1711.0615
Dynamics of 2D pancake vortices in layered superconductors
The dynamics of 2D pancake vortices in Josephson-coupled
superconducting/normal - metal multilayers is considered within the
time-dependent Ginzburg-Landau theory. For temperatures close to a
viscous drag force acting on a moving 2D vortex is shown to depend strongly on
the conductivity of normal metal layers. For a tilted vortex line consisting of
2D vortices the equation of viscous motion in the presence of a transport
current parallel to the layers is obtained. The specific structure of the
vortex line core leads to a new dynamic behavior and to substantial deviations
from the Bardeen-Stephen theory. The viscosity coefficient is found to depend
essentially on the angle between the magnetic field and the
axis normal to the layers. For field orientations close to the layers
the nonlinear effects in the vortex motion appear even for slowly moving vortex
lines (when the in-plane transport current is much smaller than the
Ginzburg-Landau critical current). In this nonlinear regime the viscosity
coefficient depends logarithmically on the vortex velocity .Comment: 15 pages, revtex, no figure
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
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
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