69 research outputs found
Fluctuations in glassy systems
We summarize a theoretical framework based on global time-reparametrization
invariance that explains the origin of dynamic fluctuations in glassy systems.
We introduce the main ideas without getting into much technical details. We
describe a number of consequences arising from this scenario that can be tested
numerically and experimentally distinguishing those that can also be explained
by other mechanisms from the ones that we believe, are special to our proposal.
We support our claims by presenting some numerical checks performed on the 3d
Edwards-Anderson spin-glass. Finally, we discuss up to which extent these ideas
apply to super-cooled liquids that have been studied in much more detail up to
present.Comment: 33 pages, 7 figs, contribution to JSTAT special issue `Principles of
Dynamical Systems' work-shop at Newton Institute, Univ. of Cambridge, U
Modification of the aging dynamics of glassy polymers due to a temperature step
Molecular dynamics simulations are used to investigate the connection between
thermal history and physical aging in polymer glasses, in particular the
effects of a temperature square step. Measurements of two-time correlation
functions show that a negative temperature step causes "rejuvenation" of the
sample: the entire spectrum of relaxation times appears identical to a younger
specimen that did not experience a temperature step. A positive temperature
step, however, leads to significant changes in the relaxation times. At short
times, the dynamics are accelerated (rejuvenation), whereas at long times the
dynamics are slowed (over-aging). All findings are in excellent qualitative
agreement with recent experiments. The two regimes can be explained by the
competing contributions of dynamical heterogeneities and faster aging dynamics
at higher temperatures. As a result of this competition, the transition between
rejuvenation and over-aging depends on the length of the square step, with
shorter steps causing more rejuvenation and longer steps causing more
over-aging. Although the spectrum of relaxation times is greatly modified by a
temperature step, the van Hove functions, which measure the distribution of
particle displacements, exhibit complete superposition at times when the
mean-squared displacements are equal.Comment: 13 pages, 8 figure
Growing dynamical length, scaling and heterogeneities in the 3d Edwards-Anderson model
We study numerically spatio-temporal fluctuations during the
out-of-equilibrium relaxation of the three-dimensional Edwards-Anderson model.
We focus on two issues. (1) The evolution of a growing dynamical length scale
in the glassy phase of the model, and the consequent collapse of the
distribution of local coarse-grained correlations measured at different pairs
of times on a single function using {\it two} scaling parameters, the value of
the global correlation at the measuring times and the ratio of the coarse
graining length to the dynamical length scale (in the thermodynamic limit). (2)
The `triangular' relation between coarse-grained local correlations at three
pairs of times taken from the ordered instants .
Property (1) is consistent with the conjecture that the development of
time-reparametrization invariance asymptotically is responsible for the main
dynamic fluctuations in aging glassy systems as well as with other mechanisms
proposed in the literature. Property (2), we stress, is a much stronger test of
the relevance of the time-reparametrization invariance scenario.Comment: 24 pages, 12 fig
Fluctuations in the coarsening dynamics of the O(N) model: are they similar to those in glassy systems?
We study spatio-temporal fluctuations in the non-equilibrium dynamics of the
d dimensional O(N) in the large N limit. We analyse the invariance of the
dynamic equations for the global correlation and response in the slow ageing
regime under transformations of time. We find that these equations are
invariant under scale transformations. We extend this study to the action in
the dynamic generating functional finding similar results. This model therefore
falls into a different category from glassy problems in which full
time-reparametrisation invariance, a larger symmetry that emcompasses time
scale invariance, is expected to be realised asymptotically. Consequently, the
spatio-temporal fluctuations of the large N O(N) model should follow a
different pattern from that of glassy systems. We compute the fluctuations of
local, as well as spatially separated, two-field composite operators and
responses, and we confront our results with the ones found numerically for the
3d Edwards-Anderson model and kinetically constrained lattice gases. We analyse
the dependence of the fluctuations of the composite operators on the growing
domain length and we compare to what has been found in super-cooled liquids and
glasses. Finally, we show that the development of time-reparametrisation
invariance in glassy systems is intimately related to a well-defined and finite
effective temperature, specified from the modification of the
fluctuation-dissipation theorem out of equilibrium. We then conjecture that the
global asymptotic time-reparametrisation invariance is broken down to time
scale invariance in all coarsening systems.Comment: 57 pages, 5 figure
Local influence of boundary conditions on a confined supercooled colloidal liquid
We study confined colloidal suspensions as a model system which approximates
the behavior of confined small molecule glass-formers. Dense colloidal
suspensions become glassier when confined between parallel glass plates. We use
confocal microscopy to study the motion of confined colloidal particles. In
particular, we examine the influence particles stuck to the glass plates have
on nearby free particles. Confinement appears to be the primary influence
slowing free particle motion, and proximity to stuck particles causes a
secondary reduction in the mobility of free particles. Overall, particle
mobility is fairly constant across the width of the sample chamber, but a
strong asymmetry in boundary conditions results in a slight gradient of
particle mobility.Comment: For conference proceedings, "Dynamics in Confinement", Grenoble,
March 201
The Physics of the Colloidal Glass Transition
As one increases the concentration of a colloidal suspension, the system
exhibits a dramatic increase in viscosity. Structurally, the system resembles a
liquid, yet motions within the suspension are slow enough that it can be
considered essentially frozen. This kinetic arrest is the colloidal glass
transition. For several decades, colloids have served as a valuable model
system for understanding the glass transition in molecular systems. The spatial
and temporal scales involved allow these systems to be studied by a wide
variety of experimental techniques. The focus of this review is the current
state of understanding of the colloidal glass transition. A brief introduction
is given to important experimental techniques used to study the glass
transition in colloids. We describe features of colloidal systems near and in
glassy states, including tremendous increases in viscosity and relaxation
times, dynamical heterogeneity, and ageing, among others. We also compare and
contrast the glass transition in colloids to that in molecular liquids. Other
glassy systems are briefly discussed, as well as recently developed synthesis
techniques that will keep these systems rich with interesting physics for years
to come.Comment: 56 pages, 18 figures, Revie
New Mandates and Imperatives in the Revised ACA Code of Ethics
The first major revision of the ACA Code of Ethics in a decade occurred in late 2005, with the updated edition containing important new mandates and imperatives. This article provides interviews with members of the Ethics Revision Task Force that flesh out seminal changes in the revised ACA Code of Ethics in the areas of confidentiality, romantic and sexual interactions, dual relationships, end-of-life care for terminally ill clients, cultural sensitivity, diagnosis, interventions, practice termination, technology, and deceased clients
State sampling dependence of the Hopfield network inference
The fully connected Hopfield network is inferred based on observed
magnetizations and pairwise correlations. We present the system in the glassy
phase with low temperature and high memory load. We find that the inference
error is very sensitive to the form of state sampling. When a single state is
sampled to compute magnetizations and correlations, the inference error is
almost indistinguishable irrespective of the sampled state. However, the error
can be greatly reduced if the data is collected with state transitions. Our
result holds for different disorder samples and accounts for the previously
observed large fluctuations of inference error at low temperatures.Comment: 4 pages, 1 figure, further discussions added and relevant references
adde
Collective perspective on advances in Dyson-Schwinger Equation QCD
We survey contemporary studies of hadrons and strongly interacting quarks
using QCD's Dyson-Schwinger equations, addressing: aspects of confinement and
dynamical chiral symmetry breaking; the hadron spectrum; hadron elastic and
transition form factors, from small- to large-Q^2; parton distribution
functions; the physics of hadrons containing one or more heavy quarks; and
properties of the quark gluon plasma.Comment: 56 pages. Summary of lectures delivered by the authors at the
"Workshop on AdS/CFT and Novel Approaches to Hadron and Heavy Ion Physics,"
2010-10-11 to 2010-12-03, hosted by the Kavli Institute for Theoretical
Physics, China, at the Chinese Academy of Science
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