51 research outputs found
Chaos and residual correlations in pinned disordered systems
We study, using functional renormalization (FRG), two copies of an elastic
system pinned by mutually correlated random potentials. Short scale
decorrelation depend on a non trivial boundary layer regime with (possibly
multiple) chaos exponents. Large scale mutual displacement correlation behave
as , the decorrelation exponent proportional to
the difference between Flory (or mean field) and exact roughness exponent
. For short range disorder but small, e.g. for random bond
interfaces , , and for the one component Bragg glass. Random field
(i.e long range) disorder exhibits finite residual correlations (no chaos ) described by new FRG fixed points. Temperature and dynamic chaos
(depinning) are discussed.Comment: 5 page
Critical exponents of the driven elastic string in a disordered medium
We analyze the harmonic elastic string driven through a continuous random
potential above the depinning threshold. The velocity exponent beta = 0.33(2)
is calculated. We observe a crossover in the roughness exponent zeta from the
critical value 1.26 to the asymptotic (large force) value of 0.5. We calculate
directly the velocity correlation function and the corresponding correlation
length exponent nu = 1.29(5), which obeys the scaling relation nu = 1/(2-zeta),
and agrees with the finite-size-scaling exponent of fluctuations in the
critical force. The velocity correlation function is non-universal at short
distances.Comment: 4 pages, 3 figures. corrected references and typo
Domain wall roughness in epitaxial ferroelectric PbZr0.2Ti0.8O3 thin films
The static configuration of ferroelectric domain walls was investigated using
atomic force microscopy on epitaxial PbZr0.2Ti0.8O3 thin films. Measurements of
domain wall roughness reveal a power law growth of the correlation function of
relative displacements B(L) ~ L^(2zeta) with zeta ~ 0.26 at short length scales
L, followed by an apparent saturation at large L. In the same films, the
dynamic exponent mu was found to be ~ 0.6 from independent measurements of
domain wall creep. These results give an effective domain wall dimensionality
of d=2.5, in good agreement with theoretical calculations for a two-dimensional
elastic interface in the presence of random-bond disorder and long range
dipolar interactions.Comment: 5 pages, 4 figure
Short time relaxation of a driven elastic string in a random medium
We study numerically the relaxation of a driven elastic string in a two
dimensional pinning landscape. The relaxation of the string, initially flat, is
governed by a growing length separating the short steady-state
equilibrated lengthscales, from the large lengthscales that keep memory of the
initial condition. We find a macroscopic short time regime where relaxation is
universal, both above and below the depinning threshold, different from the one
expected for standard critical phenomena. Below the threshold, the zero
temperature relaxation towards the first pinned configuration provides a novel,
experimentally convenient way to access all the critical exponents of the
depinning transition independently.Comment: 4.2 pages, 3 figure
2-loop Functional Renormalization for elastic manifolds pinned by disorder in N dimensions
We study elastic manifolds in a N-dimensional random potential using
functional RG. We extend to N>1 our previous construction of a field theory
renormalizable to two loops. For isotropic disorder with O(N) symmetry we
obtain the fixed point and roughness exponent to next order in epsilon=4-d,
where d is the internal dimension of the manifold. Extrapolation to the
directed polymer limit d=1 allows some handle on the strong coupling phase of
the equivalent N-dimensional KPZ growth equation, and eventually suggests an
upper critical dimension of about 2.5.Comment: 4 pages, 3 figure
Contact angle measurements on superhydrophobic Carbon Nanotube Forests : effect of fluid pressure
In this paper the effect of pressure on the contact angle of a water drop on
superhydrophobic Carbon Nanotube (CNT) forests is studied. Superhydrophobic CNT
forests are obtained from a new and simple functionalization strategy, based on
the gold-thiol affinity. Using a specifically devised experimental setup, we
then show that these surfaces are able to withstand high excess pressures
(larger than 10 kPa) without transiting toward a roughness-invaded state,
therefore preserving their low adhesion properties. Together with the
relatively low technical cost of the process, this robustness versus pressure
makes such surfaces very appealing for practical integration into microfluidic
systems.Comment: accepted for publication in Europhysics Letter
Thermal Effects in the dynamics of disordered elastic systems
Many seemingly different macroscopic systems (magnets, ferroelectrics, CDW,
vortices,..) can be described as generic disordered elastic systems.
Understanding their static and dynamics thus poses challenging problems both
from the point of view of fundamental physics and of practical applications.
Despite important progress many questions remain open. In particular the
temperature has drastic effects on the way these systems respond to an external
force. We address here the important question of the thermal effect close to
depinning, and whether these effects can be understood in the analogy with
standard critical phenomena, analogy so useful to understand the zero
temperature case. We show that close to the depinning force temperature leads
to a rounding of the depinning transition and compute the corresponding
exponent. In addition, using a novel algorithm it is possible to study
precisely the behavior close to depinning, and to show that the commonly
accepted analogy of the depinning with a critical phenomenon does not fully
hold, since no divergent lengthscale exists in the steady state properties of
the line below the depinning threshold.Comment: Proceedings of the International Workshop on Electronic Crystals,
Cargese(2008
Width distribution of contact lines on a disordered substrate
We have studied the roughness of a contact line of a liquid meniscus on a
disordered substrate by measuring its width distribution. The comparison
between the measured width distribution and the width distribution calculated
in previous works, extended here to the case of open boundary conditions,
confirms that the Joanny-de Gennes model is not sufficient to describe the
dynamics of contact lines at the depinning threshold. This conclusion is in
agreement with recent measurements which determine the roughness exponent by
extrapolation to large system sizes.Comment: 4 pages, 3 figure
Dynamics and Kinetic Roughening of Interfaces in Two-Dimensional Forced Wetting
We consider the dynamics and kinetic roughening of wetting fronts in the case
of forced wetting driven by a constant mass flux into a 2D disordered medium.
We employ a coarse-grained phase field model with local conservation of
density, which has been developed earlier for spontaneous imbibition driven by
a capillary forces. The forced flow creates interfaces that propagate at a
constant average velocity. We first derive a linearized equation of motion for
the interface fluctuations using projection methods. From this we extract a
time-independent crossover length , which separates two regimes of
dissipative behavior and governs the kinetic roughening of the interfaces by
giving an upper cutoff for the extent of the fluctuations. By numerically
integrating the phase field model, we find that the interfaces are superrough
with a roughness exponent of , a growth exponent of
, and as a function of the
velocity. These results are in good agreement with recent experiments on
Hele-Shaw cells. We also make a direct numerical comparison between the
solutions of the full phase field model and the corresponding linearized
interface equation. Good agreement is found in spatial correlations, while the
temporal correlations in the two models are somewhat different.Comment: 9 pages, 4 figures, submitted to Eur.Phys.J.
Coulombian Disorder in Periodic Systems
We study the effect of unscreened charged impurities on periodic systems. We
show that the long wavelength component of the disorder becomes long ranged and
dominates static correlation functions. On the other hand, because of the
statistical tilt symmetry, dynamical properties such as pinning remain
unaffected. As a concrete example, we focus on the effect of Coulombian
disorder generated by charged impurities, on 3D charge density waves with non
local elasticity. We calculate the x-ray intensity and find that it is
identical to the one produced by thermal fluctuations in a disorder-free
smectic-A. We discuss the consequences of these results for experiments.Comment: 11 pages, 3 figures, revtex
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