254 research outputs found
Dynamics and structure of an aging binary colloidal glass
We study aging in a colloidal suspension consisting of micron-sized particles
in a liquid. This system is made glassy by increasing the particle
concentration. We observe samples composed of particles of two sizes, with a
size ratio of 1:2.1 and a volume fraction ratio 1:6, using fast laser scanning
confocal microscopy. This technique yields real-time, three-dimensional movies
deep inside the colloidal glass. Specifically, we look at how the size, motion
and structural organization of the particles relate to the overall aging of the
glass. Particles move in spatially heterogeneous cooperative groups. These
mobile regions tend to be richer in small particles, and these small particles
facilitate the motion of nearby particles of both sizes.Comment: 7 pages; submitted to Phys. Rev. E. Revised with 1 new figure,
improved tex
Glassy freezing of orbital dynamics in FeCr2S4 and FeSc2S4
We report on a thorough dielectric investigation of the glass-like freezing
of the orbital reorientation-dynamics, recently found for the crystalline
sulpho-spinels FeCr2S4 and FeSc2S4. As the orbital reorientations are coupled
to a rearrangement of the surrounding ionic lattice via the Jahn-Teller effect,
the freezing of the orbital moments is revealed by a relaxational behaviour of
the complex dielectric permittivity. Additional conductivity (both dc and ac)
and contact contributions showing up in the spectra are taken into account by
an equivalent circuit description. The orbital relaxation dynamics continuously
slows down over six decades in time, before at the lowest temperatures the
glass transition becomes suppressed by quantum tunnelling.Comment: J. Non-Cryst. Solids, in press. 6 pages, 4 figure
Segmental relaxation in semicrystalline polymers: a mean field model for the distribution of relaxation times in confined regimes
The effect of confinement in the segmental relaxation of polymers is
considered. On the basis of a thermodynamic model we discuss the emerging
relevance of the fast degrees of freedom in stimulating the much slower
segmental relaxation, as an effect of the constraints at the walls of the
amorphous regions. In the case that confinement is due to the presence of
crystalline domains, a quasi-poissonian distribution of local constraining
conditions is derived as a result of thermodynamic equilibrium. This implies
that the average free energy barrier for conformational
rearrangement is of the same order of the dispersion of the barrier heights,
, around . As an example, we apply the results to
the analysis of the -relaxation as observed by dielectric broad band
spectroscopy in semicrystalline poly(ethylene terephthalate) cold-crystallized
from either an isotropic or an oriented glass. It is found that in the latter
case the regions of cooperative rearrangement are significantly larger than in
the former.Comment: 10 pages, 4 figures .ep
Monte-Carlo simulation of supercooled liquids using a self-consistent local temperature
We combine Creutz energy conservation with Kawasaki spin exchange to simulate
the microcanonical dynamics of a system of interacting particles. Relaxation
occurs via Glauber spin-flip activation using a self-consistent temperature.
Heterogeneity in the dynamics comes from finite-size constraints on the spin
exchange that yield a distribution of correlated regions. The simulation
produces a high-frequency response that can be identified with the boson peak,
and a lower-frequency peak that contains non-Debye relaxation and non-Arrhenius
activation, similar to the primary response of supercooled liquids.Comment: 16 pages, 4 figure
Reply to ``Comment on `Hole-burning experiments within glassy models with infinite range interactions' ''
This is a reply to the comments by Richter and Chamberlin, and Diezemann and
Bohmer to our paper (Phys. Rev. Lett. 85, 3448 (2000)). As further evidence for
the claims in this Letter, we here reproduce the nonlinear spectral
hole-burning experimental protocol in an equilibrated fully connected
spin-glass model and we exhibit frequency selectivity, together with a shift in
the base of the spectral hole.Comment: 1 page, two figures, to appear in Phys. Rev. Let
Molecular motion in concentrated solutions of spherical polystyrene microgels studied with the pulsed field gradient n.m.r.
Results of a pulsed field gradient n.m.r. study of the motion of swollen spherical microgels in solution are presented. We have measured the echo attenuation (or the incoherent dynamic structure function) of the protons in the microgels in the dynamic range from qR≪1 up to qR ≈ 1.8 (where q=scattering vector and R = particle radius), and in the timescale from a few milliseconds up to 100 ms. Rotational diffusion of the microgel spheres could not be detected with certainty. However, restricted diffusion of the spheres within a cage was observed, in particular for the large microgel with R =125 nm, where the short-time diffusion could be monitored. For apparent volume fractions (φ > 0.6, the diffusion is restricted within a space scale of root mean square displacement, 〈z2〉 1 2 120 nm. With increasing volume fraction of the microgels in solution, (φ > 0.6, the diffusion becomes increasingly restricted. This crossover corresponds to the dynamic glass transition observed by Bartsch et al. for a similar system using quasielastic light scattering. © 1994
On the static length of relaxation and the origin of dynamic heterogeneity in fragile glass-forming liquids
The most puzzling aspect of the glass transition observed in laboratory is an
apparent decoupling of dynamics from structure. In this paper we recount the
implication of various theories of glass transition for the static correlation
length in an attempt to reconcile the dynamic and static lengths associate with
the glass problem. We argue that a more recent characterization of the static
relaxation length based on the bond ordering scenario, as the typical length
over which the energy fluctuations are correlated, is more consistent with, and
indeed in perfect agreement with the typical linear size of the dynamically
heterogeneous domains observed in deeply supercooled liquids. The correlated
relaxation of bonds in terms of energy is therefore identified as the physical
origin of the observed dynamic heterogeneity.Comment: 6 pages, 1 figur
Sticky Spheres, Entropy barriers and Non-equilibrium phase transitions
A sticky spheres model to describe slow dynamics of a non-equilibrium system
is proposed. The dynamical slowing down is due to the presence of entropy
barriers. We present an exact mean field analysis of the model and demonstrate
that there is a non-equilibrium phase transition from an exponential cluster
size distribution to a powerlaw.Comment: 10pages text and 2 figure
Hole-burning experiments within solvable glassy models
We reproduce the results of non-resonant spectral hole-burning experiments
with fully-connected (equivalently infinite-dimensional) glassy models that are
generalizations of the mode-coupling approach to nonequilibrium situations. We
show that an ac-field modifies the integrated linear response and the
correlation function in a way that depends on the amplitude and frequency of
the pumping field. We study the effect of the waiting and recovery-times and
the number of oscillations applied. This calculation will help descriminating
which results can and which cannot be attributed to dynamic heterogeneities in
real systems.Comment: 4 pages, 8 figures, RevTe
Dynamic heterogeneities in the out-of-equilibrium dynamics of simple spherical spin models
The response of spherical two-spin interaction models, the spherical
ferromagnet (s-FM) and the spherical Sherrington-Kirkpatrick (s-SK) model, is
calculated for the protocol of the so-called nonresonant hole burning
experiment (NHB) for temperatures below the respective critical temperatures.
It is shown that it is possible to select dynamic features in the
out-of-equilibrium dynamics of both models, one of the hallmarks of dynamic
heterogeneities. The behavior of the s-SK model and the s-FM in three
dimensions is very similar, showing dynamic heterogeneities in the long time
behavior, i.e. in the aging regime. The appearence of dynamic heterogeneities
in the s-SK model explicitly demonstrates that these are not necessarily
related to {\it spatial} heterogeneities. For the s-FM it is shown that the
nature of the dynamic heterogeneities changes as a function of dimensionality.
With incresing dimension the frequency selectivity of the NHB diminishes and
the dynamics in the mean-field limit of the s-FM model becomes homogeneous.Comment: 16 pages, 8 figure
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