Surface diffusion in an adsorbed system with competing order parameters

A spin-1 lattice gas model with stochastic dynamics is developed and applied to the surface diffusion in an adsorbed system possessing an internal degree of freedom. Both the chemical diffusion coefficient (D) and the relaxation rate (T) associated with the internal degree of freedom are examined. The coverage dependence of these quantities is studied within the conventional approximation, with the various static averages estimated by a Monte Carlo method. A mean field theory treatment is also given. Particular emphasis is given to the effects that the various phase transitions of the system have upon the behavior of D and T. The ordering of the internal degree of freedom is found to have an important influence on diffusion, leading to a highly reduced diffusion coefficient in the ordered regions. The connection to surface diffusion in adsorbed systems with solid orderings is pointed out. The importance of thermodynamic properties in determining the behavior of the diffusion coefficient in such systems is indicated.