Effects of heterogeneity on the surface diffusion of interacting lattice gases: The bivariate trap model

The Monte Carlo method is utilized for the simulation of diffusion of interacting lattice gases on homogeneous and heterogeneous surfaces. Simulations were carried out on a square array of 64 x 64 lattice sites applying periodic boundary conditions. The chemical surface diffusion coefficients are determined via the Kubo-Green method. Surface heterogeneity is introduced by means of a bivariate random trap model. Calculations are performed for a trap concentration theta(trap) = 0.2 and various values of the trap binding energies epsilon(2), epsilon(1) and the nearest neighbour interaction energy phi(NN). For repulsive nearest neighbour interactions phi(N) < 0 the system exhibits c(2 X 2) ordering at half coverage and low temperatures. Due to the low compressibility of the well-ordered c(2 X 2) lattice gas phase the chemical diffusion coefficient is large under these circumstances. At low temperatures the surface heterogeneity causes a partial break down of the c(2 X 2) ordering which is associated with dramatic changes of the diffusion coefficients. The effects of surface heterogeneity are demonstrated by comparing the coverage dependence of D* and D for heterogeneous and homogeneous lattices.