Critical behavior of repulsive dimers on square lattices at 2/3 monolayer coverage

Monte Carlo simulations and finite-size scaling theory have been used to study the critical behavior of repulsive dimers on square lattices at 2/3 monolayer coverage. A "zig-zag" (ZZ) ordered phase, characterized by domains of parallel ZZ strips oriented at $\pm 45^o$ from the lattice symmetry axes, was found. This ordered phase is separated from the disordered state by a order-disorder phase transition occurring at a finite critical temperature. Based on the strong axial anisotropy of the ZZ phase, an orientational order parameter has been introduced. All the critical quantities have been obtained. The set of critical exponents suggests that the system belongs to a new universality class.