Tracer diffusion in ordered lattice-gas systems with defect-controlled transport mechanisms
Tracer diffusion in an ordered two-dimensional lattice-gas system with square lattice symmetry is studied theoretically and by means of computer simulations. It is well known that the ordering with c 2 2 symmetry formed in the vicinity of half-filling of lattice sites is due to strong nearest neighbor repulsive interaction. In this case, the tracer migration occurs mainly by elementary displacements of structural defects. This paper focuses on the dimer transport mechanism which was accounted for with insufficient accuracy in previous publications. The long-time asymptotics of the dimer-tracer correlation function i.e., a three-particle correlation function , which describes concerted tracer-dimer motion, is obtained analytically. We analyze a specific exchange mechanism, relevant to tracer diffusion only. Also, the contribution of generation-recombination jumps to tracer diffusion is taken into account. The theoretical data agree well with the results of intensive Monte Carlo simulations.
Fluctuations in an ordered c(2×2) two-dimensional lattice-gas system with repulsive interactions
Fluctuations of the particle density in an ordered cs232d two-dimensional lattice-gas system are studiedboth analytically and by means of Monte Carlo simulations. The ordering is caused by a strong interparticlerepulsive interaction resulting in the second order phase transition. The lattice of adsorption sites is divided intotwo sublattices salmost ﬁlled and almost empty sublatticesd each of which contains a small number of structural“defects,” i.e., vacancies and excess particles. The relaxation of the correlation function of ﬂuctuations turnsout to be governed by two different functions. This peculiarity is to be contrasted with the traditional ﬂuctua-tion theory which predicts the existence of a single damping constant, determined by the collective diffusioncoefﬁcient. A speciﬁc thesis of the proposed approach is that transport phenomena in ordered systems may bedescribed in terms of both displacements and generation-recombination of structural defects. Accordingly, thecorrelation function of ﬂuctuations depends on diffusion coefﬁcients of two defect species as well as on thegeneration-recombination frequency. Our theory reduces to the usual one when ﬂuctuations occur under localequilibrium conditions, i.e., for a sufﬁciently large size of probe areas and not too great values of interactionparameter. The analytical results agree well with those obtained in the Monte Carlo framework.DOI: 10.1103/PhysRevB.71.224304 PACS numberssd: 63.70.1h, 68.35.Fx, 64.60.Cn, 05.10.LnI. INTRODUCTION
Surface diffusion of long chainlike molecules: The role of memory effects and stiffness on effective diffusion barriers
We study the coverage dependence of surface diffusion for chainlike molecules by the fluctuating-bond model with a Monte Carlo dynamics. The model includes short-ranged excluded volume interactions between different chains as well as an intrachain bond angle potential to describe the chain stiffness. Our primary aim is to consider the role played by chain stiffness and the resulting memory effects in tracer diffusion, and in particular their role in the effective tracer diffusion barrier EAT extracted from the well-known Arrhenius form. We show that the memory effects in tracer diffusion become more pronounced at an increasing coverage as a result of packing requirements. Increasing the chain flexibility furthermore has the same overall effect as increasing the chain length, namely, they both increase EAT. We then analyze the influence of memory effects on EAT and find that, for a single diffusing chain, about 20% of EAT arises from temperature variations in the memory effects, while only the remaining pa...