Tracer diffusion in ordered lattice-gas systems with defect-controlled transport mechanisms

Tracer diffusion in an ordered two-dimensional lattice-gas system with square lattice symmetry is studied theoretically and by means of computer simulations. It is well known that the ordering with c(2 x 2) symmetry formed in the vicinity of half-filling of lattice sites is due to strong nearest neighbor repulsive interaction. In this case, the tracer migration occurs mainly by elementary displacements of structural defects. This paper focuses on the dimer transport mechanism which was accounted for with insufficient accuracy in previous publications. The long-time asymptotics of the dimer-tracer correlation function (i.e., a three-particle correlation function), which describes concerted tracer-dimer motion, is obtained analytically. We analyze a specific exchange mechanism, relevant to tracer diffusion only. Also, the contribution of generation-recombination jumps to tracer diffusion is taken into account. The theoretical data agree well with the results of intensive Monte Carlo simulations.