We study the early time behavior of the Baxter-Wu model, an Ising model with three-spin interactions on a triangular lattice. Our estimates for the dynamic exponent z are compatible with results recently obtained for two models which belong to the same universality class of the Baxter-Wu model: the two-dimensional four-state Potts model and the Ising model with three-spin interactions in one direction. However, our estimates for the dynamic exponent theta of the Baxter-Wu model are completely different from the values obtained for those models. This discrepancy could be related to the absence of a marginal operator in the Baxter-Wu model.

Mixtures of IrO 2 '/MnO 2 (30:70 mol%) have been electrochemically studied by cyclic voltammetry (CV) in acid solution. The crystalline structure, morphology and the electrochemical properties of the electrodes have been studied as a function of the annealing temperature. X-ray diffraction analysis (XRD), show absence of Mn 2 O 3 phase formation and suggest the possible of formation of a solid solution of IrO 2 and MnO 2 mainly between 400 and 450 8C. The voltammetric behavior depends on the potential cycle number and annealing temperature employed in the preparation of the oxide layer. A good potential window in aqueous H 2 SO 4 and high electroactive area are obtained due to the contribution of Ir redox transitions. Energy-dispersive X-ray (EDX) and scanning electron microscopy (SEM) analysis suggest an enrichment of the Ir content on the surface at the cost of the dissolution of the manganese present in the film when the electrode is submitted to the continuous potential scan. The electrodes have been found to perform well in electrochemical capacitor applications with a specific capacitance close to 550 F g (1. The large capacitance exhibited by this system arises from a combination of the double-layer capacitance and pseudocapacitance associated with surface redox-type reactions.

We study a probabilistic cellular automaton to describe two population biology problems: the threshold of species coexistence in a predator-prey system and the spreading of an epidemic in a population. By carrying out time-dependent simulations we obtain the dynamic critical exponents and the phase boundaries (thresholds) related to the transition between an active state, where prey and predators present a stable coexistence, and a prey absorbing state. The estimates for the critical exponents show that the transition belongs to the directed percolation universality class. In the limit where the cellular automaton maps into a model for the spreading of an epidemic with immunization we observe a crossover from directed percolation class to the dynamic percolation class. Patterns of growing clusters related to species coexistence and spreading of epidemic are shown and discussed.

We study a stochastic lattice model describing the dynamics of coexistence of two interacting biological species. The model comprehends the local processes of birth, death, and diffusion of individuals of each species and is grounded on interaction of the predator-prey type. The species coexistence can be of two types: With self-sustained coupled time oscillations of population densities and without oscillations. We perform numerical simulations of the model on a square lattice and analyze the temporal behavior of each species by computing the time correlation functions as well as the spectral densities. This analysis provides an appropriate characterization of the different types of coexistence. It is also used to examine linked population cycles in nature and in experiment.

In this paper we revisited the Ziff-Gulari-Barshad model to study its phase transitions and critical exponents through time-dependent Monte Carlo simulations. We use a method proposed recently to locate the nonequilibrium second-order phase transitions and that has been successfully used in systems with defined Hamiltonians and with absorbing states. This method, which is based on optimization of the coefficient of determination of the order parameter, was able to characterize the continuous phase transition of the model, as well as its upper spinodal point, a pseudocritical point located near the discontinuous phase transition. The static critical exponents β, ν_{∥}, and ν_{⊥}, as well as the dynamic critical exponents θ and z for the continuous transition point, were also estimated and are in excellent agreement with results found in literature.