9 research outputs found
A Crash Course on Aging
In these lecture notes I describe some of the main theoretical ideas emerged
to explain the aging dynamics. This is meant to be a very short introduction to
aging dynamics and no previous knowledge is assumed. I will go through simple
examples that allow one to grasp the main results and predictions.Comment: Lecture Notes (22 pages) given at "Unifying Concepts in Glass Physics
III", Bangalore (2004); to be published in JSTA
Linear response subordination to intermittent energy release in off-equilibrium aging dynamics
The interpretation of experimental and numerical data describing
off-equilibrium aging dynamics crucially depends on the connection between
spontaneous and induced fluctuations. The hypothesis that linear response
fluctuations are statistically subordinated to irreversible outbursts of
energy, so-called quakes, leads to predictions for averages and fluctuations
spectra of physical observables in reasonable agreement with experimental
results [see e.g. Sibani et al., Phys. Rev. B74:224407, 2006]. Using
simulational data from a simple but representative Ising model with plaquette
interactions, direct statistical evidence supporting the hypothesis is
presented and discussed in this work.
A strict temporal correlation between quakes and intermittent magnetization
fluctuations is demonstrated. The external magnetic field is shown to bias the
pre-existent intermittent tails of the magnetic fluctuation distribution, with
little or no effect on the Gaussian part of the latter. Its impact on energy
fluctuations is shown to be negligible.
Linear response is thus controlled by the quakes and inherits their temporal
statistics. These findings provide a theoretical basis for analyzing
intermittent linear response data from aging system in the same way as thermal
energy fluctuations, which are far more difficult to measure.Comment: 9 pages, 10 figures. Text improve
The relative influences of disorder and of frustration on the glassy dynamics in magnetic systems
The magnetisation relaxations of three different types of geometrically
frustrated magnetic systems have been studied with the same experimental
procedures as previously used in spin glasses. The materials investigated are
YMoO (pyrochlore system), SrCrGaO (piled
pairs of Kagom\'e layers) and (HO)Fe(SO)(OH) (jarosite
compound). Despite a very small amount of disorder, all the samples exhibit
many characteristic features of spin glass dynamics below a freezing
temperature , much smaller than their Curie-Weiss temperature .
The ageing properties of their thermoremanent magnetization can be well
accounted for by the same scaling law as in spin glasses, and the values of the
scaling exponents are very close. The effects of temperature variations during
ageing have been specifically investigated. In the pyrochlore and the
bi-Kagom\'e compounds, a decrease of temperature after some waiting period at a
certain temperature re-initializes ageing and the evolution at the new
temperature is the same as if the system were just quenched from above .
However, as the temperature is raised back to , the sample recovers the
state it had previously reached at that temperature. These features are known
in spin glasses as rejuvenation and memory effects. They are clear signatures
of the spin glass dynamics. In the Kagom\'e compound, there is also some
rejuvenation and memory, but much larger temperature changes are needed to
observe the effects. In that sense, the behaviour of this compound is
quantitatively different from that of spin glasses.Comment: latex VersionCorrigee4.tex, 4 files, 3 figures, 5 pages (Proceedings
of the International Conference on Highly Frustrated Magnetism (HFM2003),
August 26-30, 2003, Institut Laue Langevin (ILL), Grenoble, France
Slow dynamics and aging in spin-glasses
Contribution presented by Eric Vincent in the Conference `Complex Behaviour
of Glassy Systems', Sitges, Barcelona, Spain, June, 1996. It contains a review
of the experimental results on Slow dynamics and aging in spin-glasses. It also
presents their comparison with recent theoretical developments in the
description of the out of equilibrium dynamics of disordered systems; namely,
the trap model and the mean-field theory.Comment: 35 pages, 12 figures, macro lmamult.sty (included
Real spin glasses relax slowly in the shade of hierarchical trees
The Parisi solution of the mean-field spin glass has been widely accepted and
celebrated. Its marginal stability in 3d and its complexity however raised the
question of its relevance to real spin glasses. This paper gives a short
overview of the important experimental results which could be understood within
the mean-field solution. The existence of a true phase transition and the
particular behaviour of the susceptibility below the freezing temperature,
predicted by the theory, are clearly confirmed by the experimental results. The
behaviour of the complex order parameter and of the Fluctuation Dissipation
ratio are in good agreement with results of spontaneous noise measurements. The
very particular ultrametric symmetry, the key feature of the theory, provided
us with a simple description of the rejuvenation and memory effects observed in
experiment. Finally, going a step beyond mean-field, the paper shortly
discusses new analyses in terms of correlated domains characterized by their
length scales, as well as new experiments on superspin glasses which compare
well with recent theoretical simulations.Comment: To appear in the proceedings of "Wandering with Curiosity in Complex
Landscapes", a scientific conference in honour of Giorgio Parisi for his 60th
birthday, Roma, September 8-10 2008 (submitted for the special issue of the
Journal of Statistical Physics, 2009
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
From Linear to Nonlinear Response in Spin Glasses: Importance of Mean-Field-Theory Predictions
Deviations from spin-glass linear response in a single crystal Cu:Mn 1.5 at %
are studied for a wide range of changes in magnetic field, . Three
quantities, the difference , the effective waiting time,
, and the difference are examined in our
analysis. Three regimes of spin-glass behavior are observed as
increases. Lines in the plane, corresponding to ``weak'' and
``strong'' violations of linear response under a change in magnetic field, are
shown to have the same functional form as the de Almeida-Thouless critical
line. Our results demonstrate the existence of a fundamental link between
static and dynamic properties of spin glasses, predicted by the mean-field
theory of aging phenomena.Comment: 9 pages, 10 figure