Nature of the Phase Transition of the Two-Dimensional Antiferromagnetic Plane Rotator Model on the Triangular Lattice

A study is made on the antiferromagnetic plane rotator model on the triangular lattice by Monte Carlo simulations. In sharp contrast to the ferromagnetic case the specific heat shows a logarithmic divergence at a critical temperature and the magnetic susceptibility is almost independent of temperature over a wide temperature range. The origin of the divergent specific heat is ascribed to an order-disorder transition of chirality, which is introduced to characterize the two-fold degeneracy of the ground state. A discussion is made on the possible existence of an intermediate phase, in which the translational spin order vanishes, but the chirality order remains.