226 research outputs found
Comment on ``Two Time Scales and Violation of the Fluctuation-Dissipation Theorem in a Finite Dimensional Model for Structural Glasses''
In cond-mat/0002074 Ricci-Tersenghi et al. find two linear regimes in the
fluctuation-dissipation relation between density-density correlations and
associated responses of the Frustrated Ising Lattice Gas. Here we show that
this result does not seem to correspond to the equilibrium quantities of the
model, by measuring the overlap distribution P(q) of the density and comparing
the FDR expected on the ground of the P(q) with the one measured in the
off-equilibrium experiments.Comment: RevTeX, 1 page, 2 eps figures, Comment on F. Ricci-Tersenghi et al.,
Phys. Rev. Lett. 84, 4473 (2000
Memory effects in response functions of driven vortex matter
Vortex flow in driven type II superconductors shows strong memory and history
dependent effects. Here, we study a schematic microscopic model of driven
vortices to propose a scenario for a broad set of these kind of phenomena
ranging from ``rejuvenation'' and ``stiffening'' of the system response, to
``memory'' and ``irreversibility'' in I-V characteristics
Logarithmic Relaxations in a Random Field Lattice Gas Subject to Gravity
A simple lattice gas model with random fields and gravity is introduced to
describe a system of grains moving in a disordered environment. Off equilibrium
relaxations of bulk density and its two time correlation functions are
numerically found to show logarithmic time dependences and "aging" effects.
Similitudes with dry granular media are stressed. The connections with off
equilibrium dynamics in others kinds of "frustrated" lattice models in presence
of a directional driving force (gravity) are discussed to single out the
appearance of universal features in the relaxation process.Comment: 15 pages, latex, 7 figures include
Dynamical response functions in models of vibrated granular media
In recently introduced schematic lattice gas models for vibrated dry granular
media, we study the dynamical response of the system to small perturbations of
shaking amplitudes and its relations with the characteristic fluctuations.
Strong off equilibrium features appear and a generalized version of the
fluctuation dissipation theorem is introduced. The relations with thermal
glassy systems and the role of Edwards' compactivity are discussed.Comment: 12 pages, 2 postscript figure
The jamming transition of Granular Media
We briefly review the basics ideas and results of a recently proposed
statistical mechanical approach to granular materials. Using lattice models
from standard Statistical Mechanics and results from a mean field replica
approach and Monte Carlo simulations we find a jamming transition in granular
media closely related to the glass transition in super-cooled liquids. These
models reproduce the logarithmic relaxation in granular compaction and
reversible-irreversible lines, in agreement with experimental data. The models
also exhibit aging effects and breakdown of the usual fluctuation dissipation
relation. It is shown that the glass transition may be responsible for the
logarithmic relaxation and may be related to the cooperative effects underlying
many phenomena of granular materials such as the Reynolds transition.Comment: 18 pages with 6 postscript figures. to appear in J.Phys: Cond. Ma
Vortices Clustering: The Origin of the Second Peak in the Magnetisation Loops of High Temperature Superconductors
We study vortex clustering in type II Superconductors. We demonstrate that
the ``second peak'' observed in magnetisation loops may be a dynamical effect
associated with a density driven instability of the vortex system. At the
microscopic level the instability shows up as the clustering of individual
vortices at (rare) preferential regions of the pinning potential. In the limit
of quasi-static ramping the instability is related to a phase transition in the
equilibrium vortex system.Comment: 11 pages + 3 figure
Off equilibrium magnetic properties in a model for vortices in superconductors
We study the properties of a simple lattice model of repulsive particles
diffusing in a pinning landscape. The behaviour of the model is very similar to
the observed physics of vortices in superconductors. We compare and discuss the
equilibrium phase diagram, creep dynamics, the Bean state profiles, hysteresis
of magnetisation loops (including the second peak feature), and, in particular,
``off equilibrium'' relaxations. The model is analytically tractable in replica
mean field theory and numerically via Monte Carlo simulations. It offers a
comprehensive schematic framework of the observed phenomenology
Aging and memory phenomena in magnetic and transport properties of vortex matter: a brief review
There is mounting experimental evidence that strong off-equilibrium
phenomena, such as ``memory'' or ``aging'' effects, play a crucial role in the
physics of vortices in type II superconductors. We give a short review, based
on a recently introduced schematic vortex model, of current progresses in
understanding out of equilibrium vortex behaviours. We develop a unified
description of ``memory'' phenomena in magnetic and transport properties, such
as magnetisation loops and their ``anomalous'' 2nd peak, logarithmic creep,
``anomalous'' finite creep rate in the limit of vanishing temperature,
``memory'' and ``irreversibility'' in I-V characteristics, time dependent
critical currents, ``rejuvenation'' and ``aging'' of the system response.Comment: updated versio
Force correlations and arches formation in granular assemblies
In the context of a simple microscopic schematic scalar model we study the
effects of spatial correlations in force transmission in granular assemblies.
We show that the parameters of the normalized weights distribution function,
, strongly depend on the spatial extensions,
, of such correlations. We show, then, the connections between
measurable macroscopic quantities and microscopic mechanisms enhancing
correlations. In particular we evaluate how the exponential cut-off,
, and the small forces power law exponent, , depend
on the correlation length, . If correlations go to infinity, weights are
power law distributed.Comment: 6 page
Thermodynamics and statistical mechanics of frozen systems in inherent states
We discuss a Statistical Mechanics approach in the manner of Edwards to the
``inherent states'' (defined as the stable configurations in the potential
energy landscape) of glassy systems and granular materials. We show that at
stationarity the inherent states are distributed according a generalized Gibbs
measure obtained assuming the validity of the principle of maximum entropy,
under suitable constraints. In particular we consider three lattice models (a
diluted Spin Glass, a monodisperse hard-sphere system under gravity and a
hard-sphere binary mixture under gravity) undergoing a schematic ``tap
dynamics'', showing via Monte Carlo calculations that the time average of
macroscopic quantities over the tap dynamics and over such a generalized
distribution coincide. We also discuss about the general validity of this
approach to non thermal systems.Comment: 10 pages, 16 figure
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