Collective surface diffusion of repulsively interacting particles on random alloy surfaces

Tracer and collective diffusion of repulsively interacting particles adsorbed on a random AB alloy surface (Nieto, Uebing, Eur. Phys. J. B 1 (1998) 523) are studied by means of Monte Carlo modelling. Adsorption isotherms and the thermodynamic factors are obtained via simulations in the grand-canonical ensemble. Simulations in the canonical ensemble are used to mimic surface diffusion and to calculate the tracer, jump and chemical diffusion coefficients. The latter is determined utilizing the fluctuation and Kubo–Green methods. Both methods are in good agreement at high temperatures, but produce significantly different results at low temperatures where the repulsive nearest-neighbor interaction favors c(2×2) ordering of the adatoms. These discrepancies are discussed and explained. The effects of both the heterogeneities and the ad–ad interactions on surface diffusion are prominent at low temperatures. The existence of heterogeneities disturbs c(2×2) ordering at θ∼0.5. It is shown that this breakdown of the order substantially affects surface diffusion.