101 research outputs found
Pinning and depinning of a classic quasi-one-dimensional Wigner crystal in the presence of a constriction
We studied the dynamics of a quasi-one-dimensional chain-like system of
charged particles at low temperature, interacting through a screened Coulomb
potential in the presence of a local constriction. The response of the system
when an external electric field is applied was investigated. We performed
Langevin molecular dynamics simulations for different values of the driving
force and for different temperatures. We found that the friction together with
the constriction pins the particles up to a critical value of the driving
force. The system can depin \emph{elastically} or \emph{quasi-elastically}
depending on the strength of the constriction. The elastic (quasi-elastic)
depinning is characterized by a critical exponent
(). The dc conductivity is zero in the pinned regime, it has
non-ohmic characteristics after the activation of the motion and then it is
constant. Furthermore, the dependence of the conductivity with temperature and
strength of the constriction was investigated in detail. We found interesting
differences between the single and the multi-chain regimes as the temperature
is increased.Comment: 18 pages, 16 figures, accepted for publication in PR
Non linear flux flow in TiN superconducting thin film
We have studied the superconducting behavior of 100 nm Titanium Nitride (TiN)
thin film in a perpendicular magnetic field. We found a zero field transition
temperature of 4.6 K and a slope in the H-T plane of -0.745 T/K. At 4.2 K, we
have performed careful transport measurements by measuring both the
differential resistivity and voltage as a function of a DC current. Our results
are analyzed in the framework of linear and non linear flux flow behavior. In
particular, we have observed an electronic instability at high vortex
velocities and from its dependence with respect to the applied magnetic field,
we can exctract the inelastic scattering time and diffusion length of the
quasiparticles
Dynamical Phase Transition in a Driven Disordered Vortex Lattice
Using Langevin dynamics, we have investigated the dynamics of vortices in a
disordered two dimensional superconductor subjected to a uniform driving
current. The results provide direct numerical evidence for a dynamical phase
transition between a plastic flow regime and a moving ``hexatic glass." The
simulated current-voltage characteristics are in excellent agreement with
recent transport measurements on amorphous thin film
superconductors.Comment: 13 pages, latex, revtex, 4 figures available upon request from
[email protected]
Plastic energies in layered superconductors
We estimate the energy cost associated with two pancake vortices colliding in
a layered superconductor. It is argued that this energy sets the plastics
energy scale and is the analogue of the crossing energy for vortices in the
continuum case. The starting point of the calculation is the Lawrence-Doniach
version of the Ginzburg-Landau free energy for type-II superconductors. The
magnetic fields considered are along the c-direction and assumed to be
sufficiently high that the lowest Landau level approximation is valid. For
Bi-2212, where it is know that layering is very important, the results are
radically different from what would have been obtained using a
three-dimensional anisotropic continuum model. We then use the plastic energy
for Bi-2212 to successfully explain recent results from Hellerqvist {\em et
al.}\ on its longitudinal resistance.Comment: 5 Pages Revtex, 4 uuencoded postscript figure
Critical depinning force and vortex lattice order in disordered superconductors
We simulate the ordering of vortices and its effects on the critical current
in superconductors with varied vortex-vortex interaction strength and varied
pinning strengths for a two-dimensional system. For strong pinning the vortex
lattice is always disordered and the critical depinning force only weakly
increases with decreasing vortex-vortex interactions. For weak pinning the
vortex lattice is defect free until the vortex-vortex interactions have been
reduced to a low value, when defects begin to appear with a simultaneous rapid
increase in the critical depinning force. In each case the depinning force
shows a maximum for non-interacting vortices. The relative height of the peak
increases and the peak width decreases for decreasing pinning strength in
excellent agreement with experimental trends associated with the peak effect.
We show that scaling relations exist between the distance between defects in
the vortex lattice and the critical depinning force.Comment: 5 pages, 6 figure
Transverse depinning in strongly driven vortex lattices with disorder
Using numerical simulations we investigate the transverse depinning of moving
vortex lattices interacting with random disorder. We observe a finite
transverse depinning barrier for vortex lattices that are driven with high
longitudinal drives, when the vortex lattice is defect free and moving in
correlated 1D channels. The transverse barrier is reduced as the longitudinal
drive is decreased and defects appear in the vortex lattice, and the barrier
disappears in the plastic flow regime. At the transverse depinning transition,
the vortex lattice moves in a staircase pattern with a clear transverse
narrow-band voltage noise signature.Comment: 4 pages, 4 figure
Field-driven topological glass transition in a model flux line lattice
We show that the flux line lattice in a model layered HTSC becomes unstable
above a critical magnetic field with respect to a plastic deformation via
penetration of pairs of point-like disclination defects. The instability is
characterized by the competition between the elastic and the pinning energies
and is essentially assisted by softening of the lattice induced by a
dimensional crossover of the fluctuations as field increases. We confirm
through a computer simulation that this indeed may lead to a phase transition
from crystalline order at low fields to a topologically disordered phase at
higher fields. We propose that this mechanism provides a model of the low
temperature field--driven disordering transition observed in neutron
diffraction experiments on single crystals.Comment: 11 pages, 4 figures available upon request via snail mail from
[email protected]
Gliding dislocations in a driven vortex lattice
The dynamics of dislocations in a two-dimensional vortex lattice is studied
in the presence of a pinning potential and a transport current. In a vortex
lattice drifting with velocity a glide velocity of the dislocation
with respect to the vortex lattice is found to decay like for
large drive. From this result the velocity for the crossover between a regime
of coherent elastic motion and a regime of incoherent plastic motion of
vortices is estimated.Comment: 4 pages Revte
Hall noise and transverse freezing in driven vortex lattices
We study driven vortices lattices in superconducting thin films. Above the
critical force we find two dynamical phase transitions at and
, which could be observed in simultaneous noise measurements of the
longitudinal and the Hall voltage. At there is a transition from plastic
flow to smectic flow where the voltage noise is isotropic (Hall noise =
longitudinal noise) and there is a peak in the differential resistance. At
there is a sharp transition to a frozen transverse solid where the Hall
noise falls down abruptly and vortex motion is localized in the transverse
direction.Comment: 4 pages, 3 figure
Equilibration and Dynamic Phase Transitions of a Driven Vortex Lattice
We report on the observation of two types of current driven transitions in
metastable vortex lattices. The metastable states, which are missed in usual
slow transport measurements, are detected with a fast transport technique in
the vortex lattice of undoped
2H-NbSe. The transitions are seen by following the evolution of these
states when driven by a current. At low currents we observe an equilibration
transition from a metastable to a stable state, followed by a dynamic
crystallization transition at high currents.Comment: 5 pages, 4 figure
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