851 research outputs found
Numerical Investigation of Glassy Dynamics in Low Density Systems
Vitrification in colloidal systems typically occurs at high densities driven
by sharply varying, short-ranged interactions. The possibility of glassy
behavior arising from smoothly varying, long-ranged particle interactions has
received relatively little attention. Here we investigate the behavior of
screened charged particles, and explicitly demonstrate that these systems
exhibit glassy properties in the regime of low temperature and low density.
Properties close to this low density (Wigner) glass transition share many
features with their hard-sphere counterparts, but differ in quantitative
aspects that may be accounted for via microscopic theoretical considerations.Comment: 4 pages, 4 figures, revised versio
Systematic characterization of thermodynamic and dynamical phase behavior in systems with short-ranged attraction
In this paper we demonstrate the feasibility and utility of an augmented
version of the Gibbs ensemble Monte Carlo method for computing the phase
behavior of systems with strong, extremely short-ranged attractions. For
generic potential shapes, this approach allows for the investigation of
narrower attractive widths than those previously reported. Direct comparison to
previous self-consistent Ornstein-Zernike approximation calculations are made.
A preliminary investigation of out-of-equilibrium behavior is also performed.
Our results suggest that the recent observations of stable cluster phases in
systems without long-ranged repulsions are intimately related to gas-crystal
and metastable gas-liquid phase separation.Comment: 10 pages, 8 figure
Inhomogeneous Mode-Coupling Theory and Growing Dynamic Length in Supercooled Liquids
We extend Mode-Coupling Theory (MCT) to inhomogeneous situations, relevant
for supercooled liquid in pores, close to a surface, or in an external field.
We compute the response of the dynamical structure factor to a static
inhomogeneous external potential and provide the first direct evidence that the
standard formulation of MCT is associated with a diverging length scale. We
find in particular that the so called ``cages'' are in fact extended objects.
Although close to the transition the dynamic length grows as |T-T_c|^-1/4 in
_both_ the beta and alpha regimes, our results suggest that the fractal
dimension of correlated clusters is larger in the alpha regime. We also derive
inhomogeneous MCT equations valid to second order in gradients.Comment: 4 page
Numerically Exact Long Time Behavior of Nonequilibrium Quantum Impurity Models
A Monte Carlo sampling of diagrammatic corrections to the non-crossing
approximation is shown to provide numerically exact estimates of the long-time
dynamics and steady state properties of nonequilibrium quantum impurity models.
This `bold' expansion converges uniformly in time and significantly ameliorates
the sign problem that has heretofore limited the power of real-time Monte Carlo
approaches to strongly interacting real-time quantum problems. The new approach
enables the study of previously intractable problems ranging from generic long
time nonequilibrium transport characteristics in systems with large onsite
repulsion to the direct description of spectral functions on the real frequency
axis in Dynamical Mean Field Theory
Cooperativity Beyond Caging: Generalized Mode Coupling Theory
The validity of mode coupling theory (MCT) is restricted by an uncontrolled
factorization approximation of density correlations. The factorization can be
delayed and ultimately avoided, however, by explicitly including higher order
correlations. We explore this approach within a microscopically motivated
schematic model. Analytic tractability allows us to discuss in great detail the
impact of factorization at arbitrary order, including the limit of avoided
factorization. Our results indicate a coherent picture for the capabilities as
well as limitations of MCT. Moreover, including higher order correlations
systematically defers the transition and ultimately restores ergodicity.
Power-law divergence of the relaxation time is then replaced by continuous but
exponential growth.Comment: 4 pages, 2 figure
Mode coupling theory in the FDR-preserving field theory of interacting Brownian particles
We develop a renormalized perturbation theory for the dynamics of interacting
Brownian particles, which preserves the fluctuation-dissipation relation order
by order. We then show that the resulting one-loop theory gives a closed
equation for the density correlation function, which is identical with that in
the standard mode coupling theory.Comment: version to be published in Fast Track Communication in Journal of
Physics A:Math. Theo
Spontaneous and induced dynamic correlations in glass-formers II: Model calculations and comparison to numerical simulations
We study in detail the predictions of various theoretical approaches, in
particular mode-coupling theory (MCT) and kinetically constrained models
(KCMs), concerning the time, temperature, and wavevector dependence of
multi-point correlation functions that quantify the strength of both induced
and spontaneous dynamical fluctuations. We also discuss the precise predictions
of MCT concerning the statistical ensemble and microscopic dynamics dependence
of these multi-point correlation functions. These predictions are compared to
simulations of model fragile and strong glass-forming liquids. Overall, MCT
fares quite well in the fragile case, in particular explaining the observed
crucial role of the statistical ensemble and microscopic dynamics, while MCT
predictions do not seem to hold in the strong case. KCMs provide a simplified
framework for understanding how these multi-point correlation functions may
encode dynamic correlations in glassy materials. However, our analysis
highlights important unresolved questions concerning the application of KCMs to
supercooled liquids.Comment: 23 pages, 12 fig
Understanding How Femtosecond Laser Waveguide Fabrication in Glasses Works
In order to understand the physical processes associated with fs-laser waveguide writing in glass, the effects of the laser repetition rate, the material composition and feature size were studied. The resulting material changes were observed by collecting Raman and fluorescence spectra with a confocal microscope. The guiding behavior of the waveguides was evaluated by measuring near field laser coupling profiles in combination with white light microscopy. Waveguides and Bragg gratings were fabricated in fused silica using pulse repetition rates from 1 kHz to 1 MHz and a wide range of scan speeds and pulse energies. Two types of fluorescence were detected in fused silica, depending on the fabrication conditions. Fluorescence from self trapped exciton (E{prime}{sub {delta}}) defects, centered at 550 nm, were dominant for conditions with low total doses, such as using a 1 kHz laser with a scan speed of 20 {micro}m/s and pulse energies less than 1 {micro}J. For higher doses a broad fluorescence band, centered at 650 nm, associated with non-bridging oxygen hole center (NBOHC) defects was observed. Far fewer NBOHC defects were formed with the 1 MHz laser than with the kHz lasers possibly due to annealing of the defects during writing. We also observed an increase in the intensity of the 605 cm{sup -1} Raman peak relative to the total Raman intensity, corresponding to an increase in the concentration of 3-membered rings for all writing conditions. The magnitude of this increase in waveguides fabricated with a 1 MHz laser was nearly twice that of waveguides fabricated with a 1 kHz laser. Additional waveguides were fabricated in soda lime silicate glasses to assess the effects of changing the glass composition. These waveguides formed around, not inside the exposed regions. This is distinctly different from fused silica in which the waveguides are inside the exposed regions. A comprehensive analysis of all the experimental results indicates that good waveguides are formed below the actual damage threshold of the glass. The rapid quenching model, which correlates the refractive index of the modified material to its cooling rate, explains the effect of composition on waveguide behavior
Dynamical heterogeneity in a glass forming ideal gas
We conduct a numerical study of the dynamical behavior of a system of
three-dimensional crosses, particles that consist of three mutually
perpendicular line segments rigidly joined at their midpoints. In an earlier
study [W. van Ketel et al., Phys. Rev. Lett. 94, 135703 (2005)] we showed that
this model has the structural properties of an ideal gas, yet the dynamical
properties of a strong glass former. In the present paper we report an
extensive study of the dynamical heterogeneities that appear in this system in
the regime where glassy behavior sets in. On the one hand, we find that the
propensity of a particle to diffuse is determined by the structure of its local
environment. The local density around mobile particles is significantly less
than the average density, but there is little clustering of mobile particles,
and the clusters observed tend to be small. On the other hand, dynamical
susceptibility results indicate that a large dynamical length scale develops
even at moderate densities. This suggests that propensity and other mobility
measures are an incomplete measure of dynamical length scales in this system.Comment: 11 pages, 7 figure
Nonlinear viscoelasticity of metastable complex fluids
Many metastable complex fluids such as colloidal glasses and gels show
distinct nonlinear viscoelasticity with increasing oscillatory-strain
amplitude; the storage modulus decreases monotonically as the strain amplitude
increases whereas the loss modulus has a distinct peak before it decreases at
larger strains. We present a qualitative argument to explain this ubiquitous
behavior and use mode coupling theory (MCT) to confirm it. We compare
theoretical predictions to the measured nonlinear viscoelasticity in a dense
hard sphere colloidal suspensions; reasonable agreement is obtained. The
argument given here can be used to obtain new information about linear
viscoelasticity of metastable complex fluids from nonlinear strain
measurements.Comment: 7 pages, 3 figures, accepted for publication in Europhys. Let
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