372 research outputs found
Relaxation processes and entropic traps in the Backgammon model
We examine the density-density correlation function in a model recently
proposed to study the effect of entropy barriers in glassy dynamics. We find
that the relaxation proceeds in two steps with a fast beta process followed by
alpha relaxation. The results are physically interpreted in the context of an
adiabatic approximation which allows to separate the two processes, and to
define an effective temperature in the off-equilibrium dynamics of the model.
We investigate the behavior of the response function associated to the density,
and find violations of the fluctuation dissipation theorem.Comment: 4 Pages including 3 Figures, Revte
Glass transition in models with controlled frustration
A class of models with self-generated disorder and controlled frustration is
studied. Between the trivial case, where frustration is not present at all, and
the limit case, where frustration is present over every length scale, a region
with local frustration is found where glassy dynamics appears. We suggest that
in this region, the mean field model might undergo a p-spin like transition,
and increasing the range of frustration, a crossover from a 1-step replica
symmetry breaking to a continuous one might be observed.Comment: 4 pages, 6 figure
Jamming transition in granular media: A mean field approximation and numerical simulations
In order to study analytically the nature of the jamming transition in
granular material, we have considered a cavity method mean field theory, in the
framework of a statistical mechanics approach, based on Edwards' original idea.
For simplicity we have applied the theory to a lattice model and a transition
with exactly the same nature of the glass transition in mean field models for
usual glass formers is found. The model is also simulated in three dimensions
under tap dynamics and a jamming transition with glassy features is observed.
In particular two step decays appear in the relaxation functions and dynamic
heterogeneities resembling ones usually observed in glassy systems. These
results confirm early speculations about the connection between the jamming
transition in granular media and the glass transition in usual glass formers,
giving moreover a precise interpretation of its nature.Comment: 11 pages, 12 figure
Phase diagram of glassy systems in an external field
We study the mean-field phase diagram of glassy systems in a field pointing
in the direction of a metastable state. We find competition among a
``magnetized'' and a ``disordered'' phase, that are separated by a coexistence
line as in ordinary first order phase transitions. The coexistence line
terminates in a critical point, which in principle can be observed in numerical
simulations of glassy models.Comment: 4 pages, 5 figure
On the rigidity of a hard sphere glass near random close packing
We study theoretically and numerically the microscopic cause of the
mechanical stability of hard sphere glasses near their maximum packing. We show
that, after coarse-graining over time, the hard sphere interaction can be
described by an effective potential which is exactly logarithmic at the random
close packing . This allows to define normal modes, and to apply recent
results valid for elastic networks: mechanical stability is a non-local
property of the packing geometry, and is characterized by some length scale
which diverges at [1, 2]. We compute the scaling of the bulk and
shear moduli near , and speculate on the possible implications of these
results for the glass transition.Comment: 7 pages, 4 figures. Figure 4 had a wrong unit in abscissa, which was
correcte
DYNAMICAL SOLUTION OF A MODEL WITHOUT ENERGY BARRIERS
In this note we study the dynamics of a model recently introduced by one of
us, that displays glassy phenomena in absence of energy barriers. Using an
adiabatic hypothesis we derive an equation for the evolution of the energy as a
function of time that describes extremely well the glassy behaviour observed in
Monte Carlo simulations.Comment: 11 pages, LaTeX, 3 uuencoded figure
Intermittency of glassy relaxation and the emergence of a non-equilibrium spontaneous measure in the aging regime
We consider heat exchange processes between non-equilibrium aging systems (in
their activated regime) and the thermal bath in contact. We discuss a scenario
where two different heat exchange processes concur in the overall heat
dissipation: a stimulated fast process determined by the temperature of the
bath and a spontaneous intermittent process determined by the fact that the
system has been prepared in a non-equilibrium state. The latter is described by
a probability distribution function (PDF) that has an exponential tail of width
given by a parameter , and satisfies a fluctuation theorem (FT)
governed by that parameter. The value of is proportional to the
so-called effective temperature, thereby providing a practical way to
experimentally measure it by analyzing the PDF of intermittent events.Comment: Latex file, 8 pages + 5 postscript figure
Glasslike Arrest in Spinodal Decomposition as a Route to Colloidal Gelation
Colloid-polymer mixtures can undergo spinodal decomposition into colloid-rich
and colloid-poor regions. Gelation results when interconnected colloid-rich
regions solidify. We show that this occurs when these regions undergo a glass
transition, leading to dynamic arrest of the spinodal decomposition. The
characteristic length scale of the gel decreases with increasing quench depth,
and the nonergodicity parameter exhibits a pronounced dependence on scattering
vector. Mode coupling theory gives a good description of the dynamics, provided
we use the full static structure as input.Comment: 14 pages, 4 figures; replaced with published versio
Glass transition in the quenched and annealed version of the frustrated lattice gas model
In this paper we study the 3d frustrated lattice gas model in the annealed
version, where the disorder is allowed to evolve in time with a suitable
kinetic constraint. Although the model does not exhibit any thermodynamic
transition it shows a diverging peak at some characteristic time in the
dynamical non-linear susceptibility, similar to the results on the p-spin model
in mean field and Lennard-Jones mixture recently found by Donati et al.
[cond-mat/9905433]. Comparing these results to those obtained in the model with
quenched interactions, we conclude that the critical behavior of the dynamical
susceptibility is reminiscent of the thermodynamic transition present in the
quenched model, and signaled by the divergence of the static non-linear
susceptibility, suggesting therefore a similar mechanism also in supercooled
glass-forming liquids.Comment: 8 pages, 14 figure
Evidence of a Critical time in Constrained Kinetic Ising models
We study the relaxational dynamics of the one-spin facilitated Ising model
introduced by Fredrickson and Andersen. We show the existence of a critical
time which separates an initial regime in which the relaxation is exponentially
fast and aging is absent from a regime in which relaxation becomes slow and
aging effects are present. The presence of this fast exponential process and
its associated critical time is in agreement with some recent experimental
results on fragile glasses.Comment: 20 Pages + 7 Figures, Revte
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