7,706 research outputs found
Metastability and Avalanches in a Nonequilibrium Ferromagnetic System
We present preliminary results on the metastable behavior of a nonequilibrium
ferromagnetic system. The metastable state mean lifetime is a non-monotonous
function of temperature; it shows a maximum at certain non-zero temperature
which depends on the strengh of the nonequilibrium perturbation. This is in
contrast with the equilibrium case in which lifetime increases monotonously as
the temperature is decreasesed. We also report on avalanches during the decay
from the metastable state. Assuming both free boundaries and nonequilibrium
impurities, the avalanches exhibit power-law size and lifetime distributions.
Such scale free behavior is very sensible. The chances are that our
observations may be observable in real (i.e. impure) ferromagnetic
nanoparticles.Comment: 6 pages, 4 figures, to be published in 2002 Granada Seminar
Proceeding
Symmetries in Fluctuations Far from Equilibrium
Fluctuations arise universally in Nature as a reflection of the discrete
microscopic world at the macroscopic level. Despite their apparent noisy
origin, fluctuations encode fundamental aspects of the physics of the system at
hand, crucial to understand irreversibility and nonequilibrium behavior. In
order to sustain a given fluctuation, a system traverses a precise optimal path
in phase space. Here we show that by demanding invariance of optimal paths
under symmetry transformations, new and general fluctuation relations valid
arbitrarily far from equilibrium are unveiled. This opens an unexplored route
toward a deeper understanding of nonequilibrium physics by bringing symmetry
principles to the realm of fluctuations. We illustrate this concept studying
symmetries of the current distribution out of equilibrium. In particular we
derive an isometric fluctuation relation which links in a strikingly simple
manner the probabilities of any pair of isometric current fluctuations. This
relation, which results from the time-reversibility of the dynamics, includes
as a particular instance the Gallavotti-Cohen fluctuation theorem in this
context but adds a completely new perspective on the high level of symmetry
imposed by time-reversibility on the statistics of nonequilibrium fluctuations.
The new symmetry implies remarkable hierarchies of equations for the current
cumulants and the nonlinear response coefficients, going far beyond Onsager's
reciprocity relations and Green-Kubo formulae. We confirm the validity of the
new symmetry relation in extensive numerical simulations, and suggest that the
idea of symmetry in fluctuations as invariance of optimal paths has
far-reaching consequences in diverse fields.Comment: 8 pages, 4 figure
Test of the Additivity Principle for Current Fluctuations in a Model of Heat Conduction
The additivity principle allows to compute the current distribution in many
one-dimensional (1D) nonequilibrium systems. Using simulations, we confirm this
conjecture in the 1D Kipnis-Marchioro-Presutti model of heat conduction for a
wide current interval. The current distribution shows both Gaussian and
non-Gaussian regimes, and obeys the Gallavotti-Cohen fluctuation theorem. We
verify the existence of a well-defined temperature profile associated to a
given current fluctuation. This profile is independent of the sign of the
current, and this symmetry extends to higher-order profiles and spatial
correlations. We also show that finite-time joint fluctuations of the current
and the profile are described by the additivity functional. These results
suggest the additivity hypothesis as a general and powerful tool to compute
current distributions in many nonequilibrium systems.Comment: 4 pages, 4 figure
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