Phase transitions in a two-dimensional lattice gas model of orientable diatomic molecules

Abstract

Using a lattice-gas version of the extended Blume-Emery-Griffiths spin-1 model and Monte Carlo simulation methods, the ordering of heteronuclear diatomic molecules $(AB)$ adsorbed on a corrugated crystal surface with adsorption sites forming a square lattice is studied. The applied lattice model assumes that the adsorbed molecules are all vertically oriented with respect to the surface and takes into account the interaction between the first- as well as between the second-nearest neighbors. First, assuming that all interactions in the system are nonrepulsive, the order-disorder transitions for different possible phases are discussed. In particular, it is demonstrated that the critical exponents of the order-disorder transitions for the two ordered phases ($\mathit{SAF}$ and ${A}_{3}B$) are nonuniversal and change with the model parameters. Then, assuming that the first-nearest neighbor interactions are nonattractive, the evolution of the phase diagram topology with the change of the model parameters is evaluated. The critical exponents associated with the continuous phase transitions have been estimated using the finite size scaling analysis. It is demonstrated that the nature of the order-disorder phase transitions depends on the structure of the ordered states and that they belong to different classes of universality. It is also found that the phase diagrams exhibit the presence of critical, tricritical, and critical end points, depending on the magnitudes of coupling constants.