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We study the Galam majority rule dynamics with contrarian behavior and an oscillating external propaganda, in a population of agents that can adopt one of two possible opinions. In an iteration step, a random agent interacts with other three random agents and takes the majority opinion among the agents with probability $p(t)$ (majority behavior) or the opposite opinion with probability $1-p(t)$ (contrarian behavior). The probability of following the majority rule $p(t)$ varies with the temperature $T$ and is coupled to a time-dependent oscillating field that mimics a mass media propaganda, in a way that agents are more likely to adopt the majority opinion when it is aligned with the sign of the field. We investigate the dynamics of this model on a complete graph and find various regimes as $T$ is varied. A transition temperature $T_c$ separates a bimodal oscillatory regime for $TT_c$ in which $m$ oscillates around zero. These regimes are characterized by the distribution of residence times that exhibits a unique peak for a resonance temperature $T^*$, where the response of the system is maximum. An insight into these results is given by a mean-field approach, which also shows that $T^*$ and $T_c$ are closely related., Comment: 10 pages, 5 figures

We study the contrarian voter model for opinion formation in a society under the influence of an external oscillating propaganda and stochastic noise. Each agent of the population can hold one of two possible opinions on a given issue --against or in favor, and interacts with its neighbors following either an imitation dynamics (voter behavior) or an anti-alignment dynamics (contrarian behavior): each agent adopts the opinion of a random neighbor with a time-dependent probability $p(t)$, or takes the opposite opinion with probability $1-p(t)$. The imitation probability $p(t)$ is controlled by the social temperature $T$, and varies in time according to a periodic field that mimics the influence of an external propaganda, so that a voter is more prone to adopt an opinion aligned with the field. We simulate the model in complete graph and in lattices, and find that the system exhibits a rich variety of behaviors as $T$ is varied: opinion consensus for $T=0$, a bimodal behavior for $TT_c$, and full disorder for $T \gg 1$. The transition temperature $T_c$ vanishes with the population size $N$ as $T_c \simeq 2/\ln N$ in complete graph. Besides, the distribution of residence times $t_r$ in the bimodal phase decays approximately as $t_r^{-3/2}$. Within the oscillatory regime, we find a stochastic resonance-like phenomenon at a given temperature $T^*$. Also, mean-field analytical results show that the opinion oscillations reach a maximum amplitude at an intermediate temperature, and that exhibit a lag respect to the field that decreases with $T$., Comment: 15 pages, 7 figures

In the present paper, the connection between surface order-disorder phase transitions and the percolating properties of the adsorbed phase has been studied. For this purpose, four lattice-gas models in presence of repulsive interactions have been considered. Namely, monomers on honeycomb, square and triangular lattices, and dimers (particles occupying two adjacent adsorption sites) on square substrates. By using Monte Carlo simulation and finite-size scaling analysis, we obtain the percolation threshold $\theta_c$ of the adlayer, which presents an interesting dependence with $w/k_BT$ (being $w$, $k_B$ and $T$, the lateral interaction energy, the Boltzmann's constant and temperature, respectively). For each geometry and adsorbate size, a phase diagram separating a percolating and a non-percolating region is determined., Comment: 25 pages, 12 figures, figures missing in previous version

In this paper the percolation of monomers on a square lattice is studied as the particles interact with either repulsive or attractive energies. By means of a finite-size scaling analysis, the critical exponents and the scaling collapsing of the fraction of percolating lattice are found. A phase diagram separating a percolating from a non-percolating region is determined. The main features of the phase diagram are discussed in terms of simple considerations related to the interactions present in the problem. The influence of the phase transitions occurring in the system is reflected by the phase diagram. In addition, a scaling treatment maintaining constant the surface coverage and varying the temperature of the system is performed. In all the considered cases, the universality class of the model is found to be the same as for the random percolation model., Comment: 19 pages, 8 figures

Simulation studies are undertaken for the system Ag/Au(100) by means of grand canonical Monte Carlo applied to a large lattice system. The interactions are calculated using the embedded atom model. The formation of adsorbed Ag phases of low dimensionality on Ag(100) is investigated and the influence of surface defects on the shape of the adsorption isotherms is studied. The results of the simulations are discussed in the light of experimental information available from electrochemical measurements., Comment: 31 pages, 11 figures

The deposition of a metal on a foreign substrate is studied by means of grand canonical Monte Carlo simulations and a lattice-gas model with pair potential interactions between nearest neighbors. The influence of temperature and surface defects on adsorption isotherms and differential heat of adsorption is considered. The general trends can be explained in terms of the relative interactions between adsorbate atoms and substrate atoms. The systems Ag/Au(100), Ag/Pt(100), Au/Ag(100) and Pt/Ag(100) are analyzed as examples., Comment: 26 pages, 9 figures

The adsorption of a metal monolayer or submonolayer for the systems Ag/Au(100), Au/Ag(100), Ag/Pt(100), Pt/Ag(100), Au/Pt(100), Pt/Au(100), Au/Pd(100) and Pd/Au(100) was studied by means of lattice Monte Carlo simulations, in the presence of surface defects., Comment: 22 pages, 13 figures

Based on the fact that the underpotential electrochemical deposition of Ag atoms on the $Au(100)$ surface exhibits sharp first-order phase transitions at well-defined values of the (coexistence) chemical potential ($\mu_{coex}$), we performed extensive simulations aimed at investigating the hysteretic dynamic behavior of the system close to coexistence upon the application of a periodic signal of the form $\mu(t) = \mu_{coex} + \mu_{o}sin(2\Pi t/\tau) $, where $\mu_{o}$ and $\tau$ are the amplitude and the period of the sweep, respectively. For relatively short periods and small enough amplitudes the system becomes trapped either at low or high Ag coverage states, as evidenced by recording hysteresis loops. This scenario is identified as dynamically ordered states (DOS), such that the relaxation time $(\tau_{relax})$ of the corresponding metastable state obeys $\tau_{relax} > \tau $. On the other hand, by properly increasing $\mu_{o}$ or/and $\tau$, one finds that the $Ag$ coverage gently follows the external drive (here $\tau_{relax} < \tau $) and the system is said to enter into dynamically disordered states (DDS), where hysteresis loops show almost symmetric limiting cycles. This symmetry breaking between limiting cycles driven by an external signal is discussed in terms of the concept of (out-of-equilibrium) Dynamic Phase Transitions between DOS and DDS, similar to those encountered when a magnetic system is placed in the presence of a variable external magnetic field. However, a careful finite-size scaling study reveals that, at least at $T = 300K$, the $Ag/Au(100)$ system does not exhibit true second-order phase transitions but rather a crossover behavior between states. A diagram showing the location of the ordered and disordered states in the $\mu$ versus $\tau$ plane is obtained and discussed., Comment: 20 pages, 9 figures

Structural and kinetic aspects of 2-D irreversible metal deposition under potentiostatic conditions are analyzed by means of dynamic Monte Carlo simulations employing embedded atom potentials for a model system. Three limiting models, all considering adatom diffusion, were employed to describe adatom deposition.The first model (A) considers adatom deposition on any free substrate site on the surface at the same rate. The second model (B) considers adatom deposition only on substrate sites which exhibit no neighboring sites occupied by adatoms. The third model (C) allows deposition at higher rates on sites presenting neighboring sites occupied by adatoms., Comment: 20 pages, 13 figures

Static calculation and preliminary kinetic Monte Carlo simulation studies are undertaken for the nucleation and growth on a model system which follows a Frank-van der Merwe mechanism. In the present case, we consider the deposition of Ag on Au(100) and Au(111) surfaces. The interactions were calculated using the embedded atom model. The process of formation and growth of 2D Ag structures on Au(100) and Au(111) is investigated and the influence of surface steps on this phenomenon is studied. Very different time scales are predicted for Ag diffusion on Au(100) and Au(111), thus rendering very different regimes for the nucleation and growth of the related 2D phases. These observations are drawn from application of a model free of any adjustable parameter., Comment: 24 pages, LaTeX2e, 10 PS figures, 2 JPEG figures, submitted to Journal of Electrochemical Society Subj-class