Thermal phase diagram of a model Hamiltonian for columnar phases of liquid crystals

We present the phase diagram and critical properties of a coupled $XY$-Ising model on a triangular lattice using the mean-field approximation, the Migdal-Kadanoff scheme of renormalization group and Monte-Carlo simulations. The topology of the phase diagram is similar for the three techniques, with the appearance of a phase with $XY$ order and Ising disorder. The results suggest a line of transitions belonging to the 2D-Ising universality class in contrast with previous data indicating a new universality class. This model is relevant to the columnar phases of discotic liquid crystals [such as hexa(hexylthio)triphenylene (HHTT)] in the limit of weak intercolumn coupling.