arXiv subjects

Alexei Vazquez

Publications and source records attributed to Alexei Vazquez.

69 records · Page 4Linked to original sources

arXiv2000

Self-organization in populations of competing agents

A population of heterogenous agents compeeting through a minority rule is investigated. Agents which frequently loose are selected for evolution by changing their strategies. The stationary composition of the population resulting for this self-organization process is computed analytically. Results are compared with numerical simulations of two different minority games and other analytical treatments available in the literature.

cond-mat.dis-nn↗

arXiv2000

Numerical simulations of directed sandpile models

Directed sandpile models with different toppling rules are studied by means of numerical simulations in two dimensions, with the purpose of determining the different universality classes. It is concluded that the random-threshold directed model is in the same universality of the Manna directed model where multiple toppling events plays a determinant role. The BTW model with uniform and noisy driving are found to display the same critical behavior. Moreover it is observed that the Zhang model does not satisfy simple finite size scaling.

cond-mat.stat-mech↗

arXiv1999

Universality classes in the random-storage sandpile model

The avalanche statistics in a stochastic sandpile model where toppling takes place with a probability p is investigated. The limiting case p=1 corresponds to the Bak-Tang-Wiesenfeld (BTW) model with deterministic toppling rule. Based on the moment analysis of the distribution of avalanche sizes we conclude that for 0<p<p_c the model belongs to the DP universality class while for p_c<p<1 it belongs to the BTW universality class, where p_c is identified with the critical probability for directed percolation in the corresponding lattice.

cond-mat↗

arXiv1999

Force fluctuations in granular materials

Force fluctuations in granular materials are investigated. A continuum equation is derived starting from a discrete model proposed in the literature. The influence of boundary conditions is investigated. For periodic boundary conditions the average weight is found to increase linearly with depth while it saturates to a constant value for absorbing boundary conditions, which models the existence of walls. The scale dependencies of the saturation weight, the saturation depth and the average squared fluctuations are obtained. The analytical results are compared with previous works and with numerical simulations in one dimension.

cond-mat↗

arXiv1999

Constant rate linear interface depinning and self organized criticality

The precise determination of the universality classes in self-organized critical phenomena (SOC) is still an unsolved problem. Different SOC models like sandpile, linear interface depinning, and the Barkhausen effect have been investigated independently. In the present work we demonstrate that these models can all be mapped into a linear interface depinning model driven at constant rate. The model is found to belong to the universality class of constant force linear interface depinning above the depinning transition, with an upper critical dimension d_c=4. Results are compared with numerical simulations, experiments, and previous theoretical works reported in the literature. In this way we demonstrate a precise connection between different SOC models which display the same universal beha

cond-mat↗

arXiv1999

Universality in self-organized critical slope models

The dynamics of critical slope self-organized critical models is studied, using a previous mapping into a linear interface depinning model dragged at one end. The model is solved obtaining the complete set of scaling exponents. Some results are supported by previous RG developed for constant force linear interface depinning models but others, like the linear dependency of the susceptibility with system size, are intrinsic of this model which belongs to a different universality class. The comparison of our results with numerical simulations of ricepile and vortexpile models reported in the literature reveals that, as in the constant force case, there are two universality classes corresponding to random and periodic pinning.

cond-mat↗

arXiv1999

Self-organized criticality in linear interface depinning and sandpile models

The dynamics of an elastic interface profile h(x,t) under a driving force increasing at rate c, a restored force -epsilon h, and disorder is investigated. Using perturbation theory and functional renormalization group the phase diagram and the scaling exponents, up to the first order in 4-d, are obtained. The model is found to be critical in the double limit epsilon->0$ and c/epsilon->0$ and belongs to a different universality class as that of constant force models. It is shown that undirected sandpile models with stochastic rules and linear interface models with extremal dynamics belong to this new universality class.

cond-mat↗

arXiv1999

On the equivalence between the Barkhausen effect and directed Abelian sandpile models

The existence of self-organized criticality in the Barkhausen effect and its analogy with sandpile models is investigated. It is demonstrated that a model recently introduced to describe the dynamics of a domain wall [Cizeau et al, Phys. Rev. Lett. 79, 4669 (1997)] belongs to the universality class of undirected Abelian sandpile models. In this way it is shown that the Barhausen effect can be taken as an experimental observation of self-organized critical phenomena.

cond-mat↗

arXiv1999

Domain wall dynamics in soft magnetic materials

The dynamics of a domain wall in magnetostrictive materials is investigated. The domain wall is modeled by a d-dimensional interface moving in a d+1-dimensional environment. Long-range demagnetization effects and quenched disorder are considered, while the external magnetic field is increased at constant rate. Exact expressions for the average interface velocity and susceptibility are obtained, resulting that the system is critical when the demagnetization constant and the average interface velocity vanishes. The critical exponents are computed to O(4-d). Our predictions are compared with numerical simulations and Barkhausen noise measurements reported in the literature.

cond-mat↗

arXiv1999

Aging and Levy distributions in sandpiles

Aging in complex systems is studied via the sandpile model. Relaxation of avalanches in sandpiles is observed to depend on the time elapsed since the begining of the relaxation. Levy behavior is observed in the distribution of characteristic times. In this way, aging and self-organized criticality appear to be closely related.

cond-mat.soft↗

arXiv1998

Self-organized criticality and directed percolation

A sandpile model with stochastic toppling rule is studied. The control parameters and the phase diagram are determined through a MF approach, the subcritical and critical regions are analyzed. The model is found to have some similarities with directed percolation, but the existence of different boundary conditions and conservation law leads to a different universality class, where the critical state is extended to a line segment due to self-organization. These results are supported with numerical simulations in one dimension. The present model constitute a simple model which capture the essential difference between ordinary nonequilibrium critical phenomena, like DP, and self-organized criticality.

cond-mat↗

arXiv1998

Fractal approach to the beta relaxation in supercooled liquids

In the present work we present a fractal model for the beta relaxation in supercooled liquids. The macroscopic dynamics is obtained by superposition of relaxation of independent mesoscopic regions (cages). Scaling relations and exponents are assumed for the distribution of cage sizes and for the size dependent response of the independent cages. In this way we obtain some scaling relations for the average response, in the time and frequency domain. For a particular choice of the scaling exponents we obtain the scaling relations predicted by the MCT. The comparison with recent light scattering data reveals that the scaling exponents of the distribution of cage sizes are universal but the single cage relaxation depends on the detailed material structure

cond-mat.soft↗

arXiv1998

Diffusion regimes in Levy flights with trapping

The diffusion of a walk in the presence of traps is investigated. Different diffusion regimes are obtained considering the magnitude of the fluctuations in waiting times and jump distances. A constant velocity during the jump motion is assumed to avoid the divergence of the mean squared displacement. Using the limit theorems of the theory of Levy stable distributions we have provided a characterization of the different diffusion regimes.

cond-mat.soft↗

arXiv1998

A Bethe lattice representation for sandpiles

Avalanches in sandpiles are represented throughout a process of percolation in a Bethe lattice with a feedback mechanism. The results indicate that the frequency spectrum and probability distribution of avalanches resemble more to experimental results than other models using cellular automata simulations. Apparent discrepancies between experiments are reconciled. Critical behavior is here expressed troughout the critical properties of percolation phenomena.

cond-mat.soft↗

arXiv1998

Avalanche dynamics and nonexponential relaxation

The theory of SOC, and related avalanche dynamics, is proposed as the origin of the ubiquitous nonexponential relaxation observed in complex systems. Introducing some scaling laws and relations we have obtained that the normalized relaxation function follows an stretched exponential decay and that the frequency spectrum follows a "1/f" noise. Moreover, in the MF aproach the relaxation is found to be exponential.

cond-mat.soft↗