249 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
Mechanics and dynamics of X-chromosome pairing at X inactivation
At the onset of X-chromosome inactivation, the vital process whereby female mammalian cells equalize X products with
respect to males, the X chromosomes are colocalized along their Xic (X-inactivation center) regions. The mechanism
inducing recognition and pairing of the X’s remains, though, elusive. Starting from recent discoveries on the molecular
factors and on the DNA sequences (the so-called "pairing sites") involved, we dissect the mechanical basis of Xic
colocalization by using a statistical physics model. We show that soluble DNA-specific binding molecules, such as those
experimentally identified, can be indeed sufficient to induce the spontaneous colocalization of the homologous
chromosomes but only when their concentration, or chemical affinity, rises above a threshold value as a consequence of a
thermodynamic phase transition. We derive the likelihood of pairing and its probability distribution. Chromosome dynamics
has two stages: an initial independent Brownian diffusion followed, after a characteristic time scale, by recognition and
pairing. Finally, we investigate the effects of DNA deletion/insertions in the region of pairing sites and compare model
predictions to available experimental data
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
Internal avalanches in models of granular media
We study the phenomenon of internal avalanching within the context of
recently introduced lattice models of granular media. The avalanche is produced
by pulling out a grain at the base of the packing and studying how many grains
have to rearrange before the packing is once more stable. We find that the
avalanches are long-ranged, decaying as a power-law. We study the distriution
of avalanches as a function of the density of the packing and find that the
avalanche distribution is a very sensitive structural probe of the system.Comment: 12 pages including 9 eps figures, LaTeX. To appear in Fractal
Aging and multiscaling in out of equilibrium dynamical processes in granular media
In the framework of recently introduced frustrated lattice gas models, we
study the out of equilibrium dynamical processes during the compaction process
in granular media. We find irreversible-reversible cycles in agreement with
recent experimental observations. Moreover in analogy with the phenomenology of
the glass transition we find aging effects during the compaction process In
particular we find that the two time density correlation function
asymptotically scales as a function of the single variable .
This result is interpreted in terms of multiscaling properties of the system.Comment: 4 page
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
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
Two-phase densification of cohesive granular aggregates
When poured into a container, cohesive granular materials form low-density,
open granular aggregates. If pressed upon with a ram, these aggregates densify
by particle rearrangement. Here we introduce experimental evidence to the
effect that particle rearrangement is a spatially heterogeneous phenomenon,
which occurs in the form of a phase transformation between two configurational
phases of the granular aggregate. We then show that the energy landscape
associated with particle rearrangement is consistent with our interpretation of
the experimental results. Besides affording insight into the physics of the
granular state, our conclusions are relevant to many engineering processes and
natural phenomena.Comment: 7 pages, 3 figure
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
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
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