Ezequiel V. Albano

Adsorption on stepped surfaces: a Monte-Carlo simulation

Within the lattice gas model for adsorption on cubic (100) surfaces, the effect of surface steps running along lattice directions is modelled by considering adsorption on terraces L lattice spacings wide, with various types of boundary energies on the right and left edges of the terrace. Both the cases of attractive and of repulsive nearest-neighbor interaction between the adparticles are considered. While adsorption isotherms are not much affected by boundary energies in the repulsive case, a drastic influence of various choices of boundary energies is identified for the case of attractive interactions, where the system separates in phases of low and high coverage, respectively. Then adsorption will occur typically near one of the terrace boundaries, and a “domain wall” separating the high and low coverage phases will run parallel to the steps. A wetting transition is identified between a phase where the domain wall is bound to one of the steps and a phase where it is “unbound”, delocalized in the bulk of the terrace. The phase diagram found in the simulation for this wetting transition agrees with a theoretical prediction due to Abraham. In contrast, for repulsive interactions where the system orders in the c(2 × 2) structure, antiphase boundaries occur which typically run perpendicular to the steps. The generalization of these results to other models as well as the application to experiments is briefly discussed.

Monte-Carlo simulation of the oxidation of carbon monoxide on fractal surfaces

Abstract A model for the oxidation of carbon monoxide on a fractal surface, in a square lattice of size L × L (L ⩽ 240), is simulated by means of the Monte-Carlo technique. The employed model, based on the Langmuir-Hinshelwood reaction mechanism, has a single parameter, namely the mole fraction of CO in the gas phase (pCO). Since simulations start with uncovered surfaces, a transient period always precedes the stationary regime of the reaction. A reactive steady state with CO2 production is only found within a narrow window of pCO which depends on the size of the sample. Outside the reaction window the surface becomes self-poisoned by the reactants. Extrapolation to L = ∞ gives 0.314

On the influence of noise on the critical and oscillatory behavior of a predator-prey model: coherent stochastic resonance at the proper frequency of the system

Abstract Noise-induced changes in the critical and oscillatory behavior of a prey–predator system are studied using power spectrum density and spectral amplification factor (SAF) analysis. In the absence of external noise, the population densities exhibit three kinds of asymptotic behavior, namely: absorbing state, fixed point (FP) and an oscillatory regime with a well defined proper (natural) frequency. The addition of noise destabilizes the FP phase inducing a transition to a new OR. Surprisingly, it is found that when a periodic signal is added to the control parameter, the system responds robustly, without relevant changes in its behavior. Nevertheless, the coherent stochastic resonance phenomenon is found only at the proper frequency. Also, a method based on SAF allows us to locate very accurately the transition points between the different regimes.

Thermal desorption mass spectrometry of alkali metal atoms from transition metal surfaces. The influence of coadsorbed oxygen

The coverage dependence of the activation energy of desorption for a planar array of electrical dipoles has been calculated in order to understand thermal desorption traces of alkali metal atoms from transition metal surfaces. The successful comparison between the computed spectra and available experimental data for K/Fe(110), K/Fe(100), K/Fe(polycrystalline), and K/Pt(111) allows us to demonstrate that the mutual dipolar repulsion within the adlayer is responsible for the coverage dependence of the thermal desorption spectra (TDS) characteristic of the above mentioned systems. Shifts of the TDS peaks up to about 450 K due to the lowering of ∼50% in the energy of desorption are well described by the proposed model. Based on these results a simple and accurate method to assess the coverage dependence of the desorption energy is presented. The coadsorption of electropositive and electronegative atoms has been modeled assuming a planar array of electrical dipoles of opposite directions. It is demonstrated th...

Statistical methods applied to the study of opinion formation models : a brief overview and results of a numerical study of a model based on the social impact theory

The aim of this paper is twofold. On the one hand we present a brief overview on the application of statistical physics methods to the modelling of social phenomena focusing our attention on models for opinion formation. On the other hand, we discuss and present original results of a model for opinion formation based on the social impact theory developed by Latane. The presented model accounts for the interaction among the members of a social group under the competitive influence of a strong leader and the mass media, both supporting two different states of opinion. Extensive simulations of the model are presented, showing that they led to the observation of a rich scenery of complex behaviour including, among others, critical behaviour and phase transitions between a state of opinion dominated by the leader and another dominated by the mass media. The occurrence of interesting finite-size effects reveals that, in small communities, the opinion of the leader may prevail over that of the mass media. This observation is relevant for the understanding of social phenomena involving a finite number of individuals, in contrast to actual physical phase transitions that take place in the thermodynamic limit. Finally, we give a brief outlook of open questions and lines for future work.

Study of phase transitions from short-time non-equilibrium behaviour

We review recent progress in the understanding of phase transitions and critical phenomena, obtained by means of numerical simulations of the early dynamic evolution of systems prepared at well-specified initial conditions. This field has seen exhaustive scientific research during the last decade, when the renormalization-group (RG) theoretical results obtained at the end of the 1980s were applied to the interpretation of dynamic Monte Carlo simulation results. While the original RG theory is restricted to critical phenomena under equilibrium conditions, numerical simulations have been applied to the study of far-from-equilibrium systems and irreversible phase transitions, the investigation of the behaviour of spinodal points close to first-order phase transitions and the understanding of the early-time evolution of self-organized criticality (SOC) systems when released far form the SOC regime. The present review intends to provide a comprehensive overview of recent applications in those fields, which can give the flavour of the main ideas, methods and results, and to discuss the directions for further studies. All of these numerical results pose new and interesting theoretical challenges that remain as open questions to be addressed by new research in the coming years.

Symmetric polymer blend confined into a film with antisymmetric surfaces: interplay between wetting behavior and the phase diagram

We study the phase behavior of a symmetric binary polymer blend that is confined in a thin film. The film surfaces interact with the monomers via short-range potentials. We calculate the phase behavior within the self-consistent field theory of Gaussian chains. Over a wide range of parameters we find strong first-order wetting transitions for the semi-infinite system, and the interplay between the wetting/prewetting behavior and the phase diagram in confined geometry is investigated. Antisymmetric boundaries, where one surface attracts the A component with the same strength as the opposite surface attracts the B component, are applied. The phase transition does not occur close to the bulk critical temperature but in the vicinity of the wetting transition. For very thin films or weak surface fields one finds a single critical point at straight phi(c)=1 / 2. For thicker films or stronger surface fields the phase diagram exhibits two critical points and two concomitant coexistence regions. Only below a triple point there is a single two-phase coexistence region. When we increase the film thickness the two coexistence regions become the prewetting lines of the semi-infinite system, while the triple temperature converges toward the wetting transition temperature from above. The behavior close to the tricritical point, which separates phase diagrams with one and two critical points, is studied in the framework of a Ginzburg-Landau ansatz. Two-dimensional profiles of the interface between the laterally coexisting phases are calculated, and the interfacial and line tensions analyzed. The effect of fluctuations and corrections to the self-consistent field theory are discussed.

Study of a lattice-gas model for a prey–predator system

The study of interacting particle systems (IPS) has recently attracted growing interest. Also, the understanding of irreversible processes is a challenging field in statistical physics. In this work a model of an irreversible IPS, where particles are preys and predators, is presented and studied. The model includes smart pursuit (predators to preys) and evasion (preys from predators). Depending on the birth and death probabilities for prey and predators, the model exhibits irreversible phase transitions (IPTs) between a coexistence state with coexistence of both species and an absorbing state where predators extinction is observed. Second-order IPTs belong to the universality class of directed percolation as it follows from an epidemic analysis at criticality.

The Ising square lattice in aL×M geometry: A model for the effect of surface steps on phase transitions in adsorbed monolayers

Critical phenomena in adsorbed monolayers on surfaces are influenced by limited substrate homogeneity, such as surface steps. We consider the resulting finite-size and boundary effects in the framework of a lattice gas system with nearest neighbor attraction in aL×M geometry, with two free boundaries of lengthM≫L, and periodic boundary conditions in the other direction (along the direction of the steps). This geometry thus models a “terrace” of the stepped surface, and adatoms adsorbed on neighboring terraces are assumed to be non-interacting. Also the effect of boundary “fields” is considered (describing the effects of missing neighbors and changed binding energy to the substrate near the boundary). Extensive Monte Carlo calculations on this model performed on a multi-transputer system are presented and analyzed in terms of phenomenological finite size scaling concepts. The fact that two scaling variables occur (ζ/L,L/M, with ζ being the correlation length in the bulk) is demonstrated explicitly. In the absence of boundary fields, the system forM≫L orders nearTc in a domain state, with domain walls running across the terrace, while at some temperature belowTc a transition to a monodomain state occurs. This domain state slightly belowTc is suppressed, however, by rather weak boundary fields. These results are interpreted in terms of exact theoretical predictions.

Critical behaviour of irreversible reaction systems

An introductory review on the critical behaviour of some irreversible reaction systems is presented. The study of these systems has attracted great attention during the last decades due to, on the one hand, the rich and complex underlying physics, and, on the other, their relevance to numerous technological applications in heterogeneous catalysis, corrosion and coating, development of microelectronic devices, etc. The review focuses on recent advances in the understanding of irreversible phase transitions (IPTs), providing a survey of the theoretical development in the field during the last decade, as well as a detailed discussion of relevant numerical simulations. The Langevin formulation for the treatment of second-order IPTs is discussed. Different Monte Carlo (MC) approaches are also presented in detail and the finite-size-scaling analysis of second-order IPTs is described. Special attention is devoted to the description of the recent progress in the study of first-order IPTs observed upon catalytic oxidation of carbon monoxide and the reduction of nitrogen monoxide, using lattice gas reaction models. Only brief comments are given on other reactions such as the oxidation of hydrogen, ammonia synthesis. Also, a discussion of relevant experiments is presented and measurements are compared with the numerical results. Furthermore, promising areas for further research and open questions are also addressed.