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 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.
Mass transport in an ordered three-dimensional lattice-gas system
Diffusion phenomena on a three-dimensional discrete lattice are studied both analytically and by means of computer simulations. The case of repulsive interaction between the particles occupying nearest-neighbor lattice sites is considered. It is shown that in the case of the disordered particle arrangement, transport phenomena can be described within a theory based on the assumption of uncorrelated particle jumps. In contrast, strong correlation in particle motion, which takes place in antiferromagnetically ordered systems results in considerably lowering the diffusion coefficients. Both random walks and generation-recombination processes of ``structural defects'' of the ordered state govern mass transport in this case. It is shown that jumps of individual defects (vacancies and excess particles of the almost filled and empty sublattices, respectively) and dimers contribute to mass flow. In the vicinity of stoichiometric concentration the defect jumps accompanied with their recombination may also contribute significantly. The jump and collective diffusion coefficients are derived analytically. Comparison of the analytical forms with Monte Carlo data shows a good agreement.
Cutaneous myiasis caused by the housefly, Musca domestica
Summary We report a case of cutaneous myiasis by the housefly. Musca domestica.