Everaldo Arashiro

Voltammetric characterization of an iridium oxide-based system: the pseudocapacitive nature of the Ir0.3Mn0.7O2 electrode

Mixtures of IrO2� /MnO2 (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 Mn2O3 phase formation and suggest the possible of formation of a solid solution of IrO2 and MnO2 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 H2SO4 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. # 2002 Elsevier Science Ltd. All rights reserved.

The threshold of coexistence and critical behaviour of a predator–prey cellular automaton

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 mean-field approximations and numerical simulations we obtain 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 numerical 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.

Short-time critical dynamics of the Baxter-Wu model

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.

Short-time dynamics of polypeptides

The authors study the short-time dynamics of helix-forming polypeptide chains using an all-atom representation of the molecules and an implicit solvation model to approximate the interaction with the surrounding solvent. The results confirm earlier observations that the helix-coil transition in proteins can be described by a set of critical exponents. The high statistics of the simulations allows the authors to determine the exponent values with increased precision and support universality of the helix-coil transition in homopolymers and (helical) proteins.

CRITICAL BEHAVIOR AND THRESHOLD OF COEXISTENCE OF A PREDATOR–PREY STOCHASTIC MODEL IN A 2D LATTICE

We investigate the critical behavior of a stochastic lattice model describing a predator–prey system. By means of Monte Carlo procedure we simulate the model defined on a regular square lattice and determine the threshold of species coexistence, that is, the critical phase boundaries related to the transition between an active state, where both species coexist and an absorbing state where one of the species is extinct. A finite size scaling analysis is employed to determine the order parameter, order parameter fluctuations, correlation length and the critical exponents. Our numerical results for the critical exponents agree with those of the directed percolation universality class. We also check the validity of the hyperscaling relation and present the data collapse curves.

Melanin-like units concentration effects in melanin-like/vanadium pentoxide hybrid compounds

An intercalated material has been prepared consisting of a regular alternation of organic/inorganic layers. The films were obtained by reacting 3,4-dihydroxy-phenylalanine (dl-DOPA) with varying concentrations in a solution with freshly prepared HVO3, which suffered a oxidative polymerization/intercalation ‘in situ’ process, forming a green suspension, in which after de-hydration at room temperature produces a nanocomposite with a dark blue metallic color. Films (C8H9NO2)xV2O5·nH2O were characterized using FT–IR, ESR, powder X-ray diffraction, conductivity, and cyclic voltammetry. The X-ray diffractograms indicate that the lamellar structure of the V2O5 is preserved for low concentrations of melanin-like units (x<0.02), but the inter-planar space increases from 1.2 nm to ca. 1.5 nm. The FT–IR and ESR spectra confirm that the V2O5 structure is preserved, however the presence of melanin-like units induces the reduction of V(V) ions to V(IV) ions. The melanin insertion is observed to increase the stability and reproducibility of the electrochemical insertion/de-insertion of Li+. The incorporation of melanin-like structures increases the conductivity, followed by a decrease in its temperature activation energy.

The stochastic nature of predator-prey cycles

We study by numerical simulations the time correlation function of a stochastic lattice model describing the dynamics of coexistence of two interacting biological species that present time cycles in the number of species individuals. Its asymptotic behavior is shown to decrease in time as a sinusoidal exponential function from which we extract the dominant eigenvalue of the evolution operator related to the stochastic dynamics showing that it is complex with the imaginary part being the frequency of the population cycles. The transition from the oscillatory to the nonoscillatory behavior occurs when the asymptotic behavior of the time correlation function becomes a pure exponential, that is, when the real part of the complex eigenvalue equals a real eigenvalue. We also show that the amplitude of the undamped oscillations increases with the square root of the area of the habitat as ordinary random fluctuations.

Stochastic dynamics of coupled systems and damage spreading

We study the damage spreading in the one-dimensional Ising model by means of the stochastic dynamics resulting from coupling the system and its replica by a family of algorithms that interpolate between the heat bath and the Hinrichsen-Domany algorithms. At high temperatures the dynamics is exactly mapped to the Domany-Kinzel probabilistic cellular automaton. Using a mean-field approximation and Monte Carlo simulations we find the critical line that separates the phase where the damage spreads from the one where it does not.

Damage spreading in the one-dimensional Hinrichsen-Domany model

In this paper we study damage spreading in a one-dimensional model under two dynamics introduced by Hinrichsen and Domany. In particular, we study the effects of synchronous and asynchronous updating on the spreading properties. We show that the damage does not spread when the second dynamic is implemented in a synchronous way. We find that the rules for updating the damage produced by this dynamics, as the temperature goes to infinity and a certain parameter l is zero, are equivalent to those of Grassbergers well-known model A cellular automaton.