168 research outputs found
Vortex matter freezing in BiSrCaCuO samples with a very dense distribution of columnar defects
We show that the dynamical freezing of vortex structures nucleated at diluted
densities in BiSrCaCuO samples with a dense
distribution of columnar defects, with
\,kG, results in configurations with liquid-like correlations. We
propose a freezing model considering a relaxation dynamics dominated by
double-kink excitations driven by the local stresses obtained directly from
experimental images. With this model we estimate the relaxation barrier and the
freezing temperature. We argue that the low-field frozen vortex structures
nucleated in a dense distribution of columnar defects thus correspond to an
out-of-equilibrium non-entangled liquid with strongly reduced mobility rather
than to a snapshot of a metastable state with divergent activation barriers as
for instance expected for the Bose-glass phase at equilibrium.Comment: 12 pages, 7 figure
Effective temperature in driven vortex lattices with random pinning
We study numerically correlation and response functions in non-equilibrium
driven vortex lattices with random pinning. From a generalized
fluctuation-dissipation relation we calculate an effective transverse
temperature in the fluid moving phase. We find that the effective temperature
decreases with increasing driving force and becomes equal to the equilibrium
melting temperature when the dynamic transverse freezing occurs. We also
discuss how the effective temperature can be measured experimentally from a
generalized Kubo formula.Comment: 4 pages, 4 figure
Voltage rectification effects in mesoscopic superconducting triangles: experiment and modelling
The interaction of externally applied currents with persistent currents
induced by magnetic field in a mesoscopic triangle is investigated. As a
consequence of the superposition of these currents, clear voltage rectification
effects are observed. We demonstrate that the amplitude of the rectified signal
strongly depends on the configurations of the current leads with the lowest
signal obtained when the contacts are aligned along a median of the triangle.
When the contacts are aligned off-centered compared to the geometrical center,
the voltage response shows oscillations as a function of the applied field,
whose sign can be controlled by shifting the contacts. These results are in
full agreement with theoretical predictions for an analogous system consisting
of a closed loop with a finite number of identical Josephson junctions.Comment: 5 pages, 4 figures, published in Phys. Rev.
Flux melting in BSCCO: Incorporating both electromagnetic and Josephson couplings
Multilevel Monte Carlo simulations of a BSCCO system are carried out
including both Josephson as well as electromagnetic couplings for a range of
anisotropies. A first order melting transition of the flux lattice is seen on
increasing the temperature and/or the magnetic field. The phase diagram for
BSCCO is obtained for different values of the anisotropy parameter .
The best fit to the experimental results of D. Majer {\it et al.} [Phys. Rev.
Lett. {\bf 75}, 1166 (1995)] is obtained for provided one
assumes a temperature dependence of the
penetration depth with . Assuming a dependence
the best fit is obtained for . For finite anisotropy the data is shown to collapse on a straight line
when plotted in dimensionless units which shows that the melting transition can
be satisfied with a single Lindemann parameter whose value is about 0.3. A
different scaling applies to the case. The energy jump is
measured across the transition and for large values of it is found to
increase with increasing anisotropy and to decrease with increasing magnetic
field. For infinite anisotropy we see a 2D behavior of flux droplets with a
transition taking place at a temperature independent of the magnetic field. We
also show that for smaller values of anisotropy it is reasonable to replace the
electromagnetic coupling with an in-plane interaction represented by a Bessel
function of the second kind (), thus justifying our claim in a previous
paper.Comment: 12 figures, revtex
Mode-locking in driven vortex lattices with transverse ac-drive and random pinning
We find mode-locking steps in simulated current-voltage characteristics of
driven vortex lattices with {\it random} pinning when an applied ac-current is
{\it perpendicular} to the dc-current. For low frequencies there is
mode-locking only above a non-zero threshold ac force amplitude, while for
large frequencies there is mode-locking for any small ac force. This is
consistent with the nature of {\it transverse} temporal order in the different
regimes in the absence of an applied ac-drive. For large frequencies the
magnitude of the fundamental mode-locked step depends linearly with the ac
force amplitude.Comment: 4 pages, 4 figures, .tar.gz fil
Heterogeneous dynamics of the three dimensional Coulomb glass out of equilibrium
The non-equilibrium relaxational properties of a three dimensional Coulomb
glass model are investigated by kinetic Monte Carlo simulations. Our results
suggest a transition from stationary to non-stationary dynamics at the
equilibrium glass transition temperature of the system. Below the transition
the dynamic correlation functions loose time translation invariance and
electron diffusion is anomalous. Two groups of carriers can be identified at
each time scale, electrons whose motion is diffusive within a selected time
window and electrons that during the same time interval remain confined in
small regions in space. During the relaxation that follows a temperature quench
an exchange of electrons between these two groups takes place and the
non-equilibrium excess of diffusive electrons initially present decreases
logarithmically with time as the system relaxes. This bimodal dynamical
heterogeneity persists at higher temperatures when time translation invariance
is restored and electron diffusion is normal. The occupancy of the two
dynamical modes is then stationary and its temperature dependence reflects a
crossover between a low-temperature regime with a high concentration of
electrons forming fluctuating dipoles and a high-temperature regime in which
the concentration of diffusive electrons is high.Comment: 10 pages, 9 figure
Transverse phase-locking in fully frustrated Josephson junction arrays: a new type of fractional giant steps
We study, analytically and numerically, phase locking of driven vortex
lattices in fully-frustrated Josephson junction arrays at zero temperature. We
consider the case when an ac current is applied {\it perpendicular} to a dc
current. We observe phase locking, steps in the current-voltage
characteristics, with a dependence on external ac-drive amplitude and frequency
qualitatively different from the Shapiro steps, observed when the ac and dc
currents are applied in parallel. Further, the critical current increases with
increasing transverse ac-drive amplitude, while it decreases for longitudinal
ac-drive. The critical current and the phase-locked current step width,
increase quadratically with (small) amplitudes of the ac-drive. For larger
amplitudes of the transverse ac-signal, we find windows where the critical
current is hysteretic, and windows where phase locking is suppressed due to
dynamical instabilities. We characterize the dynamical states around the
phase-locking interference condition in the curve with voltage noise,
Lyapunov exponents and Poincar\'e sections. We find that zero temperature
phase-locking behavior in large fully frustrated arrays is well described by an
effective four plaquette model.Comment: 12 pages, 11 figure
Crossed-ratchet effects and domain wall geometrical pinning
The motion of a domain wall in a two dimensional medium is studied taking
into account the internal elastic degrees of freedom of the wall and
geometrical pinning produced both by holes and sample boundaries. This study is
used to analyze the geometrical conditions needed for optimizing crossed
ratchet effects in periodic rectangular arrays of asymmetric holes, recently
observed experimentally in patterned ferromagnetic films. Geometrical
calculations and numerical simulations have been used to obtain the anisotropic
critical fields for depinning flat and kinked walls in rectangular arrays of
triangles. The aim is to show with a generic elastic model for interfaces how
to build a rectifier able to display crossed ratchet effects or effective
potential landscapes for controlling the motion of interfaces or invasion
fronts.Comment: 13 pages, 18 figure
Slow relaxations and history dependence of the transport properties of layered superconductors
We study numerically the time evolution of the transport properties of
layered superconductors after different preparations. We show that, in
accordance with recent experiments in BSCCO performed in the second peak region
of the phase diagram (Portier et al, 2001), the relaxation strongly depends on
the initial conditions and is extremely slow. We investigate the dependence on
the pinning center density and the perturbation applied. We compare the
measurements to recent findings in tapped granular matter and we interpret our
results with a rather simple picture.Comment: 4 pages, 4 fig
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