FIELD-DRIVEN PHASE-TRANSITIONS IN LATTICE GASES ON INTERPENETRATING SQUARE SUBLATTICES

A model of two interacting lattice gases M and X on two interpenetrati ng square sublattices is studied. Pairwise nearest neighbor attraction phi(MX) and next nearest neighbor repulsion phi(XX) are considered, t he latter being restricted to the X sublattice. By calculating ground- state energies of the relevant lattice gas phases LG, M, X, MX, c(2 x 2) X and c(2 x 2) M2X, ground-state phase diagrams are constructed. Ap plying standard Monte Carlo techniques, corresponding phase diagrams f or elevated temperatures are obtained and compared to the ground-state phase diagram. The Monte Carlo simulations are performed in the grand canonical ensemble with the chemical potentials mu(M), mu(X), and tem perature T as independent variables. A simplified mean-field treatment is presented indicating that the behavior of the M sublattice is rela ted to that of a noninteracting Langmuir gas. X represents a lattice g as with nearest neighbor repulsion between adjacent X adatoms. The pha se boundary of the c(2 x 2) ordered lattice gas phases is evaluated an d compared to the interface solution by Muller-Hartmann and Zittartz.