85 research outputs found
Theory of plastic vortex creep
We develop a theory for plastic flux creep in a topologically disordered
vortex solid phase in type-II superconductors. We propose a detailed
description of the plastic vortex creep of the dislocated, amorphous vortex
glass in terms of motion of dislocations driven by a transport current . The
{\em plastic barriers} show power-law divergence at
small drives with exponents for single dislocation creep and for creep of dislocation bundles. The suppression of the creep rate is a
hallmark of the transition from the topologically ordered vortex lattice to an
amorphous vortex glass, reflecting a jump in from ,
characterizing creep in the topologically ordered vortex lattice near the
transition, to its plastic values. The lower creep rates explain the observed
increase in apparent critical currents in the dislocated vortex glass.Comment: 4 pages, 1 figur
Interacting Arrays of Steps and Lines in Random Media
The phase diagram of two interacting planar arrays of directed lines in
random media is obtained by a renormalization group analysis. The results are
discussed in the contexts of the roughening of reconstructed crystal surfaces,
and the pinning of flux line arrays in layered superconductors. Among the
findings are a glassy flat phase with disordered domain structures, a novel
second-order phase transition with continuously varying critical exponents, and
the generic disappearance of the glassy ``super-rough'' phases found previously
for a single array.Comment: 4 pages, REVTEX 3.0, uses epsf,multicol, 3 .eps-figures, submitted to
PR
Dislocations and the critical endpoint of the melting line of vortex line lattices
We develop a theory for dislocation-mediated structural transitions in the
vortex lattice which allows for a unified description of phase transitions
between the three phases, the elastic vortex glass, the amorphous vortex glass,
and the vortex liquid, in terms of a free energy functional for the dislocation
density. The origin of a critical endpoint of the melting line at high magnetic
fields, which has been recently observed experimentally, is explained.Comment: 4 pages, 1 figur
Non-Universal Quasi-Long Range Order in the Glassy Phase of Impure Superconductors
The structural correlation functions of a weakly disordered Abrikosov lattice
are calculated for the first time in a systematic RG-expansion in d=4-\epsilon
dimensions. It is shown, that in the asymptotic limit the Abrikosov lattice
exhibits still quasi long range translational order described by a
non-universal exponent \bar\eta_{\bf G} which depends on the ratio of the
renormalized elastic constants \kappa =\tilde c_{66}/\tilde c_{11} of the flux
line (FL) lattice. Our calculations show clearly three distinct scaling regimes
corresponding to the Larkin, the manifold and the asymptotic Bragg glass
regime. On a wide range of intermediate length scales the FL displacement
correlation function increases as a power law with twice of the manifold
roughness exponent \zeta_{rm}(\kappa), which is also non-universal. Our
results, in particular the \kappa-dependence of the exponents, are in variance
with those of the variational treatment with replica symmetry breaking which
allows in principle an experimental discrimination between the two approaches.Comment: 4 pages, 3 figure
Point force manipulation and activated dynamics of polymers adsorbed on structured substrates
We study the activated motion of adsorbed polymers which are driven over a
structured substrate by a localized point force.Our theory applies to
experiments with single polymers using, for example, tips of scanning force
microscopes to drag the polymer.We consider both flexible and semiflexible
polymers,and the lateral surface structure is represented by double-well or
periodic potentials. The dynamics is governed by kink-like excitations for
which we calculate shapes, energies, and critical point forces. Thermally
activated motion proceeds by the nucleation of a kink-antikink pair at the
point where the force is applied and subsequent diffusive separation of kink
and antikink. In the stationary state of the driven polymer, the collective
kink dynamics can be described by an one-dimensional symmetric simple exclusion
process.Comment: 7 pages, 2 Figure
Theory of the vortex matter transformations in high Tc superconductor YBCO
Flux line lattice in type II superconductors undergoes a transition into a
"disordered" phase like vortex liquid or vortex glass, due to thermal
fluctuations and random quenched disorder. We quantitatively describe the
competition between the thermal fluctuations and the disorder using the
Ginzburg -- Landau approach. The following T-H phase diagram of YBCO emerges.
There are just two distinct thermodynamical phases, the homogeneous and the
crystalline one, separated by a single first order transitions line. The line
however makes a wiggle near the experimentally claimed critical point at 12T.
The "critical point" is reinterpreted as a (noncritical) Kauzmann point in
which the latent heat vanishes and the line is parallel to the T axis. The
magnetization, the entropy and the specific heat discontinuities at melting
compare well with experiments.Comment: 4 pages 3 figure
Structure of Flux Line Lattices with Weak Disorder at Large Length Scales
Dislocation-free decoration images containing up to 80,000 vortices have been
obtained on high quality BiSrCaCuO superconducting
single crystals. The observed flux line lattices are in the random manifold
regime with a roughening exponent of 0.44 for length scales up to 80-100
lattice constants. At larger length scales, the data exhibit nonequilibrium
features that persist for different cooling rates and field histories.Comment: 4 pages, 3 gif images, to appear in PRB rapid communicatio
Stability of Elastic Glass Phases in Random Field XY Magnets and Vortex Lattices in Type II Superconductors
A description of a dislocation-free elastic glass phase in terms of domain
walls is developed and used as the basis of a renormalization group analysis of
the energetics of dislocation loops added to the system. It is found that even
after optimizing over possible paths of large dislocation loops, their energy
is still very likely to be positive when the dislocation core energy is large.
This implies the existence of an equilibrium elastic glass phase in three
dimensional random field X-Y magnets, and a dislocation free,
bond-orientationally ordered ``Bragg glass'' phase of vortices in dirty Type II
superconductors.Comment: 12 pages, Revtex, no figures, submitted to Phys Rev Letter
Entropic Elasticity of Double-Strand DNA Subject to Simple Spatial Constraints
The aim of the present paper is the study of the entropic elasticity of the
dsDNA molecule, having a cristallographic length L of the order of 10 to 30
persistence lengths A, when it is subject to spatial obstructions. We have not
tried to obtain the single molecule partition function by solving a
Schodringer-like equation. We prefer to stay within a discretized version of
the WLC model with an added one-monomer potential, simulating the spatial
constraints. We derived directly from the discretized Boltzmann formula the
transfer matrix connecting the partition functions relative to adjacent
"effective monomers". We have plugged adequate Dirac delta-functions in the
functional integral to ensure that the monomer coordinate and the tangent
vector are independent variables. The partition function is, then, given by an
iterative process which is both numerically efficient and physically
transparent. As a test of our discretized approach, we have studied two
configurations involving a dsDNA molecule confined between a pair of parallel
plates.Comment: The most formal developments of Section I have been moved into an
appendix and replaced by a direct derivation of the transfer matrix used in
the applications. of Section II. Two paragraphs and two figures have been
added to clarify the physical interpretation of the result
Peak effect, vortex-lattice melting-line and order - disorder transition in conventional and high-T superconductors
We investigate the order - disorder transition line from a Bragg glass to an
amorphous vortex glass in the H-T phase diagram of three-dimensional type-II
superconductors with account of both pinning-caused and thermal fluctuations of
the vortex lattice. Our approach is based on the Lindemann criterion and on
results of the collective pinning theory and generalizes previous work of other
authors. It is shown that the shapes of the order - disorder transition line
and the vortex lattice melting curve are determined only by the Ginzburg
number, which characterizes thermal fluctuations, and by a parameter which
describes the strength of the quenched disorder in the flux-line lattice. In
the framework of this unified approach we obtain the H-T phase diagrams for
both conventional and high-Tc superconductors. Several well-known experimental
results concerning the fishtail effect and the phase diagram of high-Tc
superconductors are naturally explained by assuming that a peak effect in the
critical current density versus H signalizes the order - disorder transition
line in superconductors with point defects.Comment: 15 pages including 11 figure
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