452 research outputs found
Are defect models consistent with the entropy and specific heat of glass-formers?
We show that point-like defect model of glasses cannot explain thermodynamic
properties of glass-formers, as for example the excess specific heat close to
the glass transition, contrary to the claim of J.P. Garrahan, D. Chandler
[Proc. Natl. Acad. Sci. 100, 9710 (2003)]. More general models and approaches
in terms of extended defects are also discussed.Comment: 4 pages, version to appear in J. Chem. Phys with a Note Adde
Super-diffusion around the rigidity transition: Levy and the Lilliputians
By analyzing the displacement statistics of an assembly of horizontally
vibrated bidisperse frictional grains in the vicinity of the jamming transition
experimentally studied before, we establish that their superdiffusive motion is
a genuine Levy flight, but with `jump' size very small compared to the diameter
of the grains. The vibration induces a broad distribution of jumps that are
random in time, but correlated in space, and that can be interpreted as
micro-crack events at all scales. As the volume fraction departs from the
critical jamming density, this distribution is truncated at a smaller and
smaller jump size, inducing a crossover towards standard diffusive motion at
long times. This interpretation contrasts with the idea of temporally
persistent, spatially correlated currents and raises new issues regarding the
analysis of the dynamics in terms of vibrational modes.Comment: 7 pages, 6 figure
Cluster Dynamical Mean Field analysis of the Mott transition
We investigate the Mott transition using a cluster extension of dynamical
mean field theory (DMFT). In the absence of frustration we find no evidence for
a finite temperature Mott transition. Instead, in a frustrated model, we
observe signatures of a finite temperature Mott critical point in agreement
with experimental studies of kappa-organics and with single site DMFT. As the
Mott transition is approached, a clear momentum dependence of the electron
lifetime develops on the Fermi surface with the formation of cold regions along
the diagonal direction of the Brillouin zone. Furthermore the variation of the
effective mass is no longer equal to the inverse of the quasi particle residue,
as in DMFT, and is reduced approaching the Mott transition.Comment: 4 page
On the Adam-Gibbs-Wolynes scenario for the viscosity increase in glasses
We reformulate the interpretation of the mean-field glass transition scenario
for finite dimensional systems, proposed by Wolynes and collaborators.
This allows us to establish clearly a temperature dependent length xi* above
which the mean-field glass transition picture has to be modified. We argue in
favor of the mosaic state introduced by Wolynes and collaborators, which leads
to the Adam-Gibbs relation between the viscosity and configurational entropy of
glass forming liquids.
Our argument is a mixture of thermodynamics and kinetics, partly inspired by
the Random Energy
Model: small clusters of particles are thermodynamically frozen in low energy
states, whereas large clusters are kinetically frozen by large activation
energies. The relevant relaxation time is that of the smallest `liquid'
clusters. Some physical consequences are discussed.Comment: 8 page
Dynamic criticality at the jamming transition
We characterize vibrational motion occurring at low temperatures in dense
suspensions of soft repulsive spheres over a broad range of volume fractions
encompassing the jamming transition at (T = 0, phi = phi_J). We find that
characteristic time and length scales of thermal vibrations obey critical
scaling in the vicinity of the jamming transition. We show in particular that
the amplitude and the time scale of dynamic fluctuations diverge symmetrically
on both sides of the transition, and directly reveal a diverging correlation
length. The critical region near phi_J is divided in three different regimes
separated by a characteristic temperature scale T*(phi) that vanishes
quadratically with the distance to phi_J. While two of them, (T < T*(phi), phi
> phi_J) and (T < T*(phi), phi < phi_J), are described by harmonic theories
developed in the zero temperature limit, the third one for T > T*(phi) is
inherently anharmonic and displays new critical properties. We find that the
quadratic scaling of T*(phi) is due to nonperturbative anharmonic
contributions, its amplitude being orders of magnitude smaller than the
perturbative prediction based on the expansion to quartic order in the
interactions. Our results show that thermal vibrations in colloidal assemblies
directly reveal the critical nature of the jamming transition. The critical
region, however, is very narrow and has not yet been attained experimentally,
even in recent specifically-dedicated experiments.Comment: 18 pages; submitted to J. Chem. Phys. for "Special Topic Issue on the
Glass Transition
Critical fluctuations and breakdown of Stokes-Einstein relation in the Mode-Coupling Theory of glasses
We argue that the critical dynamical fluctuations predicted by the
mode-coupling theory (MCT) of glasses provide a natural mechanism to explain
the breakdown of the Stokes-Einstein relation. This breakdown, observed
numerically and experimentally in a region where MCT should hold, is one of the
major difficulty of the theory, for which we propose a natural resolution based
on the recent interpretation of the MCT transition as a bona fide critical
point with a diverging length scale. We also show that the upper critical
dimension of MCT is d_c=8.Comment: Proceedings of the workshop on non-equilibrium phenomena in
supercooled fluids, glasses and amorphous materials (17-22 September, 2006,
Pisa
Patch-repetition correlation length in glassy systems
We obtain the patch-repetition entropy Sigma within the Random First Order
Transition theory (RFOT) and for the square plaquette system, a model related
to the dynamical facilitation theory of glassy dynamics. We find that in both
cases the entropy of patches of linear size l, Sigma(l), scales as s_c l^d+A
l^{d-1} down to length-scales of the order of one, where A is a positive
constant, s_c is the configurational entropy density and d the spatial
dimension. In consequence, the only meaningful length that can be defined from
patch-repetition is the cross-over length xi=A/s_c. We relate xi to the typical
length-scales already discussed in the literature and show that it is always of
the order of the largest static length. Our results provide new insights, which
are particularly relevant for RFOT theory, on the possible real space structure
of super-cooled liquids. They suggest that this structure differs from a mosaic
of different patches having roughly the same size.Comment: 6 page
Kibble-Zurek mechanism and infinitely slow annealing through critical points
We revisit the Kibble-Zurek mechanism by analyzing the dynamics of phase
ordering systems during an infinitely slow annealing across a second order
phase transition. We elucidate the time and cooling rate dependence of the
typical growing length and we use it to predict the number of topological
defects left over in the symmetry broken phase as a function of time, both
close and far from the critical region. Our results extend the Kibble-Zurek
mechanism and reveal its limitations.Comment: 5 pages, 4 fig
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