1,322 research outputs found
Fluctuation dissipation ratio in the one dimensional kinetic Ising model
The exact relation between the response function and the
two time correlation function is derived analytically in the
one dimensional kinetic Ising model subjected to a temperature quench. The
fluctuation dissipation ratio is found to depend on time
through in the time region where scaling holds. The crossover from the nontrivial form
to takes place as the waiting
time is increased from below to above the equilibration time .Comment: 2 figure
Time-energy correlations in solar flare occurrence
The existence of time-energy correlations in flare occurrence is still an
open and much debated problem. This study addresses the question whether
statistically significant correlations are present between energies of
successive flares as well as energies and waiting times. We analyze the GOES
catalog with a statistical approach based on the comparison of the real catalog
with a reshuffled one where energies are decorrelated. This analysis reduces
the effect of background activity and is able to reveal the role of
obscuration. We show the existence of non-trivial correlations between waiting
times and energies, as well as between energies of subsequent flares. More
precisely, we find that flares close in time tend to have the second event with
large energy. Moreover, after large flares the flaring rate significantly
increases, together with the probability of other large flares. Results suggest
that correlations between energies and waiting times are a physical property
and not an effect of obscuration. These findings could give important
information on the mechanisms for energy storage and release in the solar
corona
Nonequilibrium fluctuation-dissipation theorem and heat production
We use a relationship between response and correlation function in
nonequilibrium systems to establish a connection between the heat production
and the deviations from the equilibrium fluctuation-dissipation theorem. This
scheme extends the Harada-Sasa formulation [Phys. Rev. Lett. 95, 130602
(2005)], obtained for Langevin equations in steady states, as it also holds for
transient regimes and for discrete jump processes involving small entropic
changes. Moreover, a general formulation includes two times and the new
concepts of two-time work, kinetic energy, and of a two-time heat exchange that
can be related to a nonequilibrium "effective temperature". Numerical
simulations of a chain of anharmonic oscillators and of a model for a molecular
motor driven by ATP hydrolysis illustrate these points.Comment: 5 pages, 3 figure
Scaling of the linear response function from zero field cooled and thermoremanent magnetization in phase ordering kinetics
In this paper we investigate the relation between the scaling properties of
the linear response function , of the thermoremanent magnetization
(TRM) and of the zero field cooled magnetization (ZFC) in the context of phase
ordering kinetics. We explain why the retrival of the scaling properties of
from those of TRM and ZFC is not trivial. Preasymptotic contributions
generate a long crossover in TRM, while ZFC is affected by a dangerous
irrelevant variable. Lack of understanding of both these points has generated
some confusion in the literature. The full picture relating the exponents of
all the quantities involved is explicitely illustrated in the framework of the
large model. Following this scheme, an assessment of the present status of
numerical simulations for the Ising model can be made. We reach the conclusion
that on the basis of the data available up to now, statements on the scaling
properties of can be made from ZFC but not from TRM. From ZFC data for
the Ising model with we confirm the previously found linear
dependence on dimensionality of the exponent entering . We also find evidence that a recently derived form of the
scaling function , using local scale invariance arguments [M.Henkel,
M.Pleimling, C.Godr\`{e}che and J.M.Luck, Phys.Rev.Lett. {\bf 87}, 265701
(2001)], does not hold for the Ising model.Comment: 26 pages, 14 figure
Phase ordering in 3d disordered systems
We study numerically the phase-ordering kinetics of the site-diluted and
bond-diluted Ising models after a quench from an infinite to a low temperature.
We show that the speed of growth of the ordered domain's size is non-monotonous
with respect to the amount of dilution : Starting from the pure case
the system slows down when dilution is added, as it is usually expected when
disorder is introduced, but only up to a certain value beyond which the
speed of growth raises again. We interpret this counterintuitive fact in a
renormalization-group inspired framework, along the same lines proposed for the
corresponding two-dimensional systems, where a similar pattern was observed.Comment: 8 pages, 4 figures.To appear on Journal of Statistical Mechanics:
Theory and Experiment. arXiv admin note: text overlap with arXiv:1306.514
Quasi-deterministic dynamics, memory effects, and lack of self-averaging in the relaxation of quenched ferromagnets
We discuss the interplay between the degree of dynamical stochasticity,
memory persistence and violation of the self-averaging property in the aging
kinetics of quenched ferromagnets. We show that, in general, the longest
possible memory effects, which correspond to the slowest possible temporal
decay of the correlation function, are accompanied by the largest possible
violation of self-averaging and a quasi-deterministic descent into the ergodic
components. This phenomenon is observed in different systems, such as the Ising
model with long-range interactions, including mean-field, and the short-range
random field Ising model.Comment: Introduction strongly revised, changed figures. Accepted for
publication as a Rapid Communication in Physical Review
Off-equilibrium generalization of the fluctuation dissipation theorem for Ising spins and measurement of the linear response function
We derive for Ising spins an off-equilibrium generalization of the
fluctuation dissipation theorem, which is formally identical to the one
previously obtained for soft spins with Langevin dynamics [L.F.Cugliandolo,
J.Kurchan and G.Parisi, J.Phys.I France \textbf{4}, 1641 (1994)]. The result is
quite general and holds both for dynamics with conserved and non conserved
order parameter. On the basis of this fluctuation dissipation relation, we
construct an efficient numerical algorithm for the computation of the linear
response function without imposing the perturbing field, which is alternative
to those of Chatelain [J.Phys. A \textbf{36}, 10739 (2003)] and Ricci-Tersenghi
[Phys.Rev.E {\bf 68}, 065104(R) (2003)]. As applications of the new algorithm,
we present very accurate data for the linear response function of the Ising
chain, with conserved and non conserved order parameter dynamics, finding that
in both cases the structure is the same with a very simple physical
interpretation. We also compute the integrated response function of the two
dimensional Ising model, confirming that it obeys scaling , with , as previously found with a different
method.Comment: 12 pages, 5 figure
On a relation between roughening and coarsening
We argue that a strict relation exists between two in principle unrelated
quantities: The size of the growing domains in a coarsening system, and the
kinetic roughening of an interface. This relation is confirmed by extensive
simulations of the Ising model with different forms of quenched disorder, such
as random bonds, random fields and stochastic dilution.Comment: 8 pages, 3 figures. To appear on EP
Roughening of an interface in a system with surface or bulk disorder
We study numerically the roughening properties of an interface in a
two-dimensional Ising model with either random bonds or random fields, which
are representative of universality classes where disorder acts only on the
interface or also away from it, in the bulk. The dynamical structure factor
shows a rich crossover pattern from the form of a pure system at large
wavevectors , to a different behavior, typical of the kind of disorder, at
smaller 's. For the random field model a second crossover is observed from
the typical behavior of a system where disorder is only effective on the
surface, as the random bond model, to the truly large scale behavior, where
bulk-disorder is important, that is observed at the smallest wavevectors.Comment: 13 pages, 8 figure
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