96 research outputs found
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
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
Driven vortices in 3D layered superconductors: Dynamical ordering along the c-axis
We study a 3D model of driven vortices in weakly coupled layered
superconductors with strong pinning. Above the critical force , we find a
plastic flow regime in which pancakes in different layers are uncoupled,
corresponding to a pancake gas. At a higher , there is an ``smectic flow''
regime with short-range interlayer order, corresponding to an entangled line
liquid. Later, the transverse displacements freeze and vortices become
correlated along the c-axis, resulting in a transverse solid. Finally, at a
force the longitudinal displacements freeze and we find a coherent solid
of rigid lines.Comment: 4 pages, 3 postscript figure
Mode-locking in ac-driven vortex lattices with random pinning
We find mode-locking steps in simulated current-voltage characteristics of
ac-driven vortex lattices with {\it random} pinning. For low frequencies there
is mode-locking above a finite ac force amplitude, while for large frequencies
there is mode-locking for any small ac force. This is correlated with the
nature of temporal order in the different regimes in the absence of ac drive.
The mode-locked state is a frozen solid pinned in the moving reference of
frame, and the depinning from the step shows plastic flow and hysteresis.Comment: 4 pages, 4 figure
Static and dynamic coupling transitions of vortex lattices in disordered anisotropic superconductors
We use three-dimensional molecular dynamics simulations of magnetically
interacting pancake vortices to study vortex matter in disordered, highly
anisotropic materials such as BSCCO. We observe a sharp 2D-3D transition from
vortex lines to decoupled pancakes as a function of relative interlayer
coupling strength, with an accompanying large increase in the critical current
remniscent of a second peak effect. We find that decoupled pancakes, when
driven, simultaneously recouple and order into a crystalline-like state at high
drives. We construct a dynamic phase diagram and show that the dynamic
recoupling transition is associated with a double peak in dV/dI.Comment: 4 pages, 4 postscript 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
Transverse Phase Locking for Vortex Motion in Square and Triangular Pinning Arrays
We analyze transverse phase locking for vortex motion in a superconductor
with a longitudinal DC drive and a transverse AC drive. For both square and
triangular arrays we observe a variety of fractional phase locking steps in the
velocity versus DC drive which correspond to stable vortex orbits. The locking
steps are more pronounced for the triangular arrays which is due to the fact
that the vortex motion has a periodic transverse velocity component even for
zero transverse AC drive. All the steps increase monotonically in width with AC
amplitude. We confirm that the width of some fractional steps in the square
arrays scales as the square of the AC driving amplitude. In addition we
demonstrate scaling in the velocity versus applied DC driving curves at
depinning and on the main step, similar to that seen for phase locking in
charge-density wave systems. The phase locking steps are most prominent for
commensurate vortex fillings where the interstitial vortices form symmetrical
ground states. For increasing temperature, the fractional steps are washed out
very quickly, while the main step gains a linear component and disappears at
melting. For triangular pinning arrays we again observe transverse phase
locking, with the main and several of the fractional step widths scaling
linearly with AC amplitude.Comment: 10 pages, 14 postscript figure
Mode locking of vortex matter driven through mesoscopic channels
We investigated the driven dynamics of vortices confined to mesoscopic flow
channels by means of a dc-rf interference technique. The observed mode-locking
steps in the -curves provide detailed information on how the number of rows
and lattice structure in the channel change with magnetic field. Minima in flow
stress occur when an integer number of rows is moving coherently, while maxima
appear when incoherent motion of mixed and row configurations is
predominant. Simulations show that the enhanced pinning at mismatch originates
from quasi-static fault zones with misoriented edge dislocations induced by
disorder in the channel edges.Comment: some minor changes were made, 4 pages, 4 figures, accepted for
publication in Phys. Rev. Let
Dynamic transition in driven vortices across the peak effect in superconductors
We study the zero-temperature dynamic transition from the disordered flow to
an ordered flow state in driven vortices in type-II superconductors. The
transition current is marked by a sharp kink in the
characteristic with a concomitant large increase in the defect concentration.
On increasing magnetic field , the follows the behaviour of the
critical current . Specifically, in the peak effect regime
increases rapidly along with . We also discuss the effect of varying
disorder strength on .Comment: 4 pages, 4 figure
Dynamic ordering and frustration of confined vortex rows studied by mode-locking experiments
The flow properties of confined vortex matter driven through disordered
mesoscopic channels are investigated by mode locking (ML) experiments. The
observed ML effects allow to trace the evolution of both the structure and the
number of confined rows and their match to the channel width as function of
magnetic field. From a detailed analysis of the ML behavior for the case of
3-rows we obtain ({\it i}) the pinning frequency , ({\it ii}) the onset
frequency for ML ( ordering velocity) and ({\it iii}) the
fraction of coherently moving 3-row regions in the channel. The
field dependence of these quantities shows that, at matching, where is
maximum, the pinning strength is small and the ordering velocity is low, while
at mismatch, where is small, both the pinning force and the ordering
velocity are enhanced. Further, we find that , consistent
with the dynamic ordering theory of Koshelev and Vinokur. The microscopic
nature of the flow and the ordering phenomena will also be discussed.Comment: 10 pages, 7 figure, submitted to PRB. Discussion has been improved
and a figure has been adde
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