Surface diffusion in the presence of phase transitions. Monte Carlo studies of a simple lattice gas model

Abstract

A comparison of the Kubo-Green and fluctuation methods for determining surface diffusion coefficients of adsorbates was carried out by Monte Carlo simulations on a square lattice for attractive nearest and repulsive next nearest neighbor interactions. Depending on R ≡ ϕ NNN ϕ NN the lattice gas models show first- and second-order phase transitions at low temperatures. In the presence of second-order phase transitions ( R = −2, R = −1) both methods agree, even in p(2 × 1) ordered regions of the relevant phase diagrams. For systems with first-order phase transitions ( R = −0.5, R = 0) we found a substantial remanent “graininess” of the lattice gas at temperatures well above T c. This graininess represents early stages of first-order phase transitions between lattice gas phases of substantially different particle density. It is shown for the first time that such remanent graininess affects the determination of the chemical surface diffusion coefficient, as diffusion measurements may depend to some extent on the length scale of the measuring technique.