A Monte Carlo study of surface diffusion coefficients in the presence of adsorbate–adsorbate interactions. III. Repulsive nearest-neighbor and attractive next-nearest-neighbor interactions

Abstract

Tracer, jump, and chemical diffusion coefficients D*, Dj, and D, respectively, were determined on a 50×50 square array of fixed sites by the fluctuation and Kubo–Green methods for the case of nearest-neighbor repulsions and numerically equal next-nearest-neighbor attractions. Both methods give the same D values except at θ=0.5 and T/Tc<0.9, where c(2×2) dominates and simulations are extremely sensitive to very small changes in the small amount of disorder. Activation energies are, in all cases, controlled by next-nearest-neighbor attraction, i.e., are positive relative to the noninteracting case and peak at θ=0.5. However, D has a maximum at θ=0.5 because of a deep minimum in 〈(δN)2〉/〈N〉. D* and Dj are very similar to each other and have a minimum at θ=0.5 and low T, because of the maxima in activation energies at this coverage, caused by the necessity to deform the c(2×2) structure.