Phase behavior of unsymmetrical dimers on a square lattice

Monte Carlo simulations in the grand canonical ensemble have been carried out to study phase transitions in monolayers formed by heterogeneous dimers, composed of segments A and B, on a square lattice. The unsymmetrical segment-segment interactions are assumed (u_AA ≠ u_BB). The systems with u_AB = u_BB = −1 and different AA-interactions studied in our previous work [W. R?ysko, M. Borówko, Surf. Sci. 520 (2002) 151] are reconsidered. The structural phase transitions at high temperatures are investigated. It is shown that topology of a phase diagram depends on the energy u_AA. For repulsive AA-interactions a triple point is found and the system belongs to the universality class of a tricritical point. When u_AA < 0 the critical line of the order-disorder transition terminates at a critical end point and the system belongs to the universality class of the 2D Ising model.