77 research outputs found
The role of stationarity in magnetic crackling noise
We discuss the effect of the stationarity on the avalanche statistics of
Barkhuasen noise signals. We perform experimental measurements on a
FeB amorphous ribbon and compare the avalanche distributions
measured around the coercive field, where the signal is stationary, with those
sampled through the entire hysteresis loop. In the first case, we recover the
scaling exponents commonly observed in other amorphous materials (,
). while in the second the exponents are significantly larger
(, ). We provide a quantitative explanation of the
experimental results through a model for the depinning of a ferromagnetic
domain wall. The present analysis shed light on the unusually high values for
the Barkhausen noise exponents measured by Spasojevic et al. [Phys. Rev. E 54
2531 (1996)].Comment: submitted to JSTAT. 11 pages 5 figure
Instanton Analysis of Hysteresis in the Three-Dimensional Random-Field Ising Model
We study the magnetic hysteresis in the random field Ising model in 3D. We
discuss the disorder dependence of the coercive field H_c, and obtain an
analytical description of the smooth part of the hysteresis below and above
H_c, by identifying the disorder configurations (instantons) that are the most
probable to trigger local avalanches. We estimate the critical disorder
strength at which the hysteresis curve becomes continuous. From an instanton
analysis at zero field we obtain a description of local two-level systems in
the ferromagnetic phase.Comment: Phys. Rev. Lett. 96, 117202 (2006
Monte Carlo Dynamics of driven Flux Lines in Disordered Media
We show that the common local Monte Carlo rules used to simulate the motion
of driven flux lines in disordered media cannot capture the interplay between
elasticity and disorder which lies at the heart of these systems. We therefore
discuss a class of generalized Monte Carlo algorithms where an arbitrary number
of line elements may move at the same time. We prove that all these dynamical
rules have the same value of the critical force and possess phase spaces made
up of a single ergodic component. A variant Monte Carlo algorithm allows to
compute the critical force of a sample in a single pass through the system. We
establish dynamical scaling properties and obtain precise values for the
critical force, which is finite even for an unbounded distribution of the
disorder. Extensions to higher dimensions are outlined.Comment: 4 pages, 3 figure
Anisotropic Interface Depinning - Numerical Results
We study numerically a stochastic differential equation describing an
interface driven along the hard direction of an anisotropic random medium. The
interface is subject to a homogeneous driving force, random pinning forces and
the surface tension. In addition, a nonlinear term due to the anisotropy of the
medium is included. The critical exponents characterizing the depinning
transition are determined numerically for a one-dimensional interface. The
results are the same, within errors, as those of the ``Directed Percolation
Depinning'' (DPD) model. We therefore expect that the critical exponents of the
stochastic differential equation are exactly given by the exponents obtained by
a mapping of the DPD model to directed percolation. We find that a moving
interface near the depinning transition is not self-affine and shows a behavior
similar to the DPD model.Comment: 9 pages, 13 figures, REVTe
Absorbing states and elastic interfaces in random media: two equivalent descriptions of self-organized criticality
We elucidate a long-standing puzzle about the non-equilibrium universality
classes describing self-organized criticality in sandpile models. We show that
depinning transitions of linear interfaces in random media and absorbing phase
transitions (with a conserved non-diffusive field) are two equivalent languages
to describe sandpile criticality. This is so despite the fact that local
roughening properties can be radically different in the two pictures, as
explained here. Experimental implications of our work as well as promising
paths for future theoretical investigations are also discussed.Comment: 4 pages. 2 Figure
Origin of the roughness exponent in elastic strings at the depinning threshold
Within a recently developed framework of dynamical Monte Carlo algorithms, we
compute the roughness exponent of driven elastic strings at the
depinning threshold in 1+1 dimensions for different functional forms of the
(short-range) elastic energy. A purely harmonic elastic energy leads to an
unphysical value for . We include supplementary terms in the elastic
energy of at least quartic order in the local extension. We then find a
roughness exponent of , which coincides with the one
obtained for different cellular automaton models of directed percolation
depinning. The quartic term translates into a nonlinear piece which changes the
roughness exponent in the corresponding continuum equation of motion. We
discuss the implications of our analysis for higher-dimensional elastic
manifolds in disordered media.Comment: 4 pages, 2 figure
Drift of a polymer chain in disordered media
We consider the drift of a polymer chain in a disordered medium, which is
caused by a constant force applied to the one end of the polymer, under
neglecting the thermal fluctuations. In the lowest order of the perturbation
theory we have computed the transversal fluctuations of the centre of mass of
the polymer, the transversal and the longitudinal size of the polymer, and the
average velocity of the polymer. The corrections to the quantities under
consideration, which are due to the interplay between the motion and the
quenched forces, are controlled by the driving force and the degree of
polymerization. The transversal fluctuations of the Brownian particle and of
the centre of mass of the polymer are obtained to be diffusive. The transversal
fluctuations studied in the present Letter may also be of relevance for the
related problem of the drift of a directed polymer in disordered media and its
applications.Comment: 11 pages, RevTex, Accepted for publication in Europhysics Letter
Interface Motion in Random Media at Finite Temperature
We have studied numerically the dynamics of a driven elastic interface in a
random medium, focusing on the thermal rounding of the depinning transition and
on the behavior in the pinned phase. Thermal effects are quantitatively
more important than expected from simple dimensional estimates. For sufficient
low temperature the creep velocity at a driving force equal to the
depinning force exhibits a power-law dependence on , in agreement with
earlier theoretical and numerical predictions for CDW's. We have also examined
the dynamics in the pinned phase resulting from slowly increasing the
driving force towards threshold. The distribution of avalanche sizes
decays as , with , in agreement with
recent theoretical predictions.Comment: harvmac.tex, 30 pages, including 9 figures, available upon request.
SU-rm-94073
Scaling in self-organized criticality from interface depinning?
The avalanche properties of models that exhibit 'self-organized criticality'
(SOC) are still mostly awaiting theoretical explanations. A recent mapping
(Europhys. Lett.~53, 569) of many sandpile models to interface depinning is
presented first, to understand how to reach the SOC ensemble and the
differences of this ensemble with the usual depinning scenario. In order to
derive the SOC avalanche exponents from those of the depinning critical point,
a geometric description is discussed, of the quenched landscape in which the
'interface' measuring the integrated activity moves. It turns out that there
are two main alternatives concerning the scaling properties of the SOC
ensemble. These are outlined in one dimension in the light of scaling arguments
and numerical simulations of a sandpile model which is in the quenched
Edwards-Wilkinson universality class.Comment: 7 pages, 3 figures, Statphys satellite meeting in Merida, July 200
Anisotropic Scaling in Threshold Critical Dynamics of Driven Directed Lines
The dynamical critical behavior of a single directed line driven in a random
medium near the depinning threshold is studied both analytically (by
renormalization group) and numerically, in the context of a Flux Line in a
Type-II superconductor with a bulk current . In the absence of
transverse fluctuations, the system reduces to recently studied models of
interface depinning. In most cases, the presence of transverse fluctuations are
found not to influence the critical exponents that describe longitudinal
correlations. For a manifold with internal dimensions,
longitudinal fluctuations in an isotropic medium are described by a roughness
exponent to all orders in , and a
dynamical exponent . Transverse
fluctuations have a distinct and smaller roughness exponent
for an isotropic medium. Furthermore, their
relaxation is much slower, characterized by a dynamical exponent
, where is the
correlation length exponent. The predicted exponents agree well with numerical
results for a flux line in three dimensions. As in the case of interface
depinning models, anisotropy leads to additional universality classes. A
nonzero Hall angle, which has no analogue in the interface models, also affects
the critical behavior.Comment: 26 pages, 8 Postscript figures packed together with RevTeX 3.0
manuscript using uufiles, uses multicol.sty and epsf.sty, e-mail
[email protected] in case of problem
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