529 research outputs found
Functional Optimization in Complex Excitable Networks
We study the effect of varying wiring in excitable random networks in which
connection weights change with activity to mold local resistance or
facilitation due to fatigue. Dynamic attractors, corresponding to patterns of
activity, are then easily destabilized according to three main modes, including
one in which the activity shows chaotic hopping among the patterns. We describe
phase transitions to this regime, and show a monotonous dependence of critical
parameters on the heterogeneity of the wiring distribution. Such correlation
between topology and functionality implies, in particular, that tasks which
require unstable behavior --such as pattern recognition, family discrimination
and categorization-- can be most efficiently performed on highly heterogeneous
networks. It also follows a possible explanation for the abundance in nature of
scale--free network topologies.Comment: 7 pages, 3 figure
Nonequilibrium phase transitions and tricriticality in a three-dimensional lattice system with random-field competing kinetics
We study a nonequilibrium Ising model that stochastically evolves under the
simultaneous operation of several spin-flip mechanisms. In other words, the
local magnetic fields change sign randomly with time due to competing kinetics.
This dynamics models a fast and random diffusion of disorder that takes place
in dilute metallic alloys when magnetic ions diffuse. We performe Monte Carlo
simulations on cubic lattices up to L=60. The system exhibits ferromagnetic and
paramagnetic steady states. Our results predict first-order transitions at low
temperatures and large disorder strengths, which correspond to the existence of
a nonequilibrium tricritical point at finite temperature. By means of standard
finite-size scaling equations, we estimate the critical exponents in the
low-field region, for which our simulations uphold continuous phase
transitions.Comment: 14 pages, 7 figures, accepted for publication in Phys. Rev.
Noise driven dynamic phase transition in a a one dimensional Ising-like model
The dynamical evolution of a recently introduced one dimensional model in
\cite{biswas-sen} (henceforth referred to as model I), has been made stochastic
by introducing a parameter such that corresponds to the
Ising model and to the original model I. The equilibrium
behaviour for any value of is identical: a homogeneous state. We
argue, from the behaviour of the dynamical exponent ,that for any , the system belongs to the dynamical class of model I indicating a
dynamic phase transition at . On the other hand, the persistence
probabilities in a system of spins saturate at a value , where remains constant for all supporting the existence of the dynamic phase transition at .
The scaling function shows a crossover behaviour with for and for
.Comment: 4 pages, 5 figures, accepted version in Physical Review
Unstable Dynamics, Nonequilibrium Phases and Criticality in Networked Excitable Media
Here we numerically study a model of excitable media, namely, a network with
occasionally quiet nodes and connection weights that vary with activity on a
short-time scale. Even in the absence of stimuli, this exhibits unstable
dynamics, nonequilibrium phases -including one in which the global activity
wanders irregularly among attractors- and 1/f noise while the system falls into
the most irregular behavior. A net result is resilience which results in an
efficient search in the model attractors space that can explain the origin of
certain phenomenology in neural, genetic and ill-condensed matter systems. By
extensive computer simulation we also address a relation previously conjectured
between observed power-law distributions and the occurrence of a "critical
state" during functionality of (e.g.) cortical networks, and describe the
precise nature of such criticality in the model.Comment: 18 pages, 9 figure
Revisiting the effect of external fields in Axelrod's model of social dynamics
The study of the effects of spatially uniform fields on the steady-state
properties of Axelrod's model has yielded plenty of controversial results. Here
we re-examine the impact of this type of field for a selection of parameters
such that the field-free steady state of the model is heterogeneous or
multicultural. Analyses of both one and two-dimensional versions of Axelrod's
model indicate that, contrary to previous claims in the literature, the steady
state remains heterogeneous regardless of the value of the field strength.
Turning on the field leads to a discontinuous decrease on the number of
cultural domains, which we argue is due to the instability of zero-field
heterogeneous absorbing configurations. We find, however, that spatially
nonuniform fields that implement a consensus rule among the neighborhood of the
agents enforces homogenization. Although the overall effects of the fields are
essentially the same irrespective of the dimensionality of the model, we argue
that the dimensionality has a significant impact on the stability of the
field-free homogeneous steady state
Absorbing-state phase transitions on percolating lattices
We study nonequilibrium phase transitions of reaction-diffusion systems
defined on randomly diluted lattices, focusing on the transition across the
lattice percolation threshold. To develop a theory for this transition, we
combine classical percolation theory with the properties of the supercritical
nonequilibrium system on a finite-size cluster. In the case of the contact
process, the interplay between geometric criticality due to percolation and
dynamical fluctuations of the nonequilibrium system leads to a new universality
class. The critical point is characterized by ultraslow activated dynamical
scaling and accompanied by strong Griffiths singularities. To confirm the
universality of this exotic scaling scenario we also study the generalized
contact process with several (symmetric) absorbing states, and we support our
theory by extensive Monte-Carlo simulations.Comment: 11 pages, 10 eps figures included, final version as publishe
Assessing Human Eye Exposure to UV Light: A Narrative Review.
Exposure to ultraviolet light is associated with several ocular pathologies. Understanding exposure levels and factors is therefore important from a medical and prevention perspective. A review of the current literature on ocular exposure to ultraviolet light is conducted in this study. It has been shown that ambient irradiance is not a good indicator of effective exposure and current tools for estimating dermal exposure have limitations for the ocular region. To address this, three methods have been developed: the use of anthropomorphic manikins, measurements through wearable sensors and numerical simulations. The specific objective, limitations, and results obtained for the three different methods are discussed
A new approach to the chap LQ regulator exploiting the geometric properties of the Hamiltonian system
The cheap LQ regulator is reinterpreted as an output nulling problem which is a basic problem of the geometric control theory. In fact, solving the LQ regulator problem is equivalent to keep the output of the related Hamiltonian system identically zero. The solution lies on a controlled invariant subspace whose dimension is characterized in terms of the minimal conditioned invariant of the original system, and the optimal feedback gain is computed as the friend matrix of the resolving subspace. This study yields a new computational framework for the cheap LQ regulator, relying only on the very basic and simple tools of the geometric approach, namely the algorithms for controlled and conditioned invariant subspaces and invariant zeros
Reentrant Behavior of the Spinodal Curve in a Nonequilibrium Ferromagnet
The metastable behavior of a kinetic Ising--like ferromagnetic model system
in which a generic type of microscopic disorder induces nonequilibrium steady
states is studied by computer simulation and a mean--field approach. We pay
attention, in particular, to the spinodal curve or intrinsic coercive field
that separates the metastable region from the unstable one. We find that, under
strong nonequilibrium conditions, this exhibits reentrant behavior as a
function of temperature. That is, metastability does not happen in this regime
for both low and high temperatures, but instead emerges for intermediate
temperature, as a consequence of the non-linear interplay between thermal and
nonequilibrium fluctuations. We argue that this behavior, which is in contrast
with equilibrium phenomenology and could occur in actual impure specimens,
might be related to the presence of an effective multiplicative noise in the
system.Comment: 7 pages, 4 figures; Final version to appear in Phys. Rev. E; Section
V has been revise
Nonequilibrium phase transition on a randomly diluted lattice
We show that the interplay between geometric criticality and dynamical
fluctuations leads to a novel universality class of the contact process on a
randomly diluted lattice. The nonequilibrium phase transition across the
percolation threshold of the lattice is characterized by unconventional
activated (exponential) dynamical scaling and strong Griffiths effects. We
calculate the critical behavior in two and three space dimensions, and we also
relate our results to the recently found infinite-randomness fixed point in the
disordered one-dimensional contact process.Comment: 4 pages, 1 eps figure, final version as publishe
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