A KINETIC LATTICE-GAS MODEL FOR THE TRIANGULAR LATTICE WITH STRONG DYNAMIC CORRELATIONS .2. COLLECTIVE DIFFUSION

The density-density correlation function and the site-occupation autoc orrelation function are calculated for the lattice-gas model with two- vacancy assisted hopping on the triangular lattice, by both Monte Carl o simulation and analytical approximations. The nonexponential time de pendence of the density-density correlation function for short wavelen gths and high concentrations is explained by the strong spatial inhomo geneity of the collective diffusion process. The coefficient of collec tive diffusion decreases very rapidly at the highest concentrations, s imilarly to the self-diffusion coefficient. The analytical calculation s for a two-variable approximation and a mode-coupling approximation g ive good results for lower concentrations. At higher concentrations th e mode-coupling approximation is unstable, leading to a diverging rath er than decaying correlation function for concentrations higher than 0 .34.