Evidence for Kosterlitz-Thouless-type orientational ordering of CF₃Br monolayers physisorbed on graphite

Monolayers of the halomethane CF₃Br adsorbed on graphite have been investigated by x-ray diffraction. The layers crystallize in a commensurate triangular lattice. On cooling they approach a three-sublattice antiferro-electric pattern of the in-plane components of the dipole moments. The ordering is not consistent with a conventional phase transition, but points to Kosterlitz-Thouless behavior. It is argued that the transition is described by a six-state clock model on a triangular lattice with antiferromagnetic nearest-neighbor interactions which is studied with Monte Carlo simulations. A finite-size scaling analysis shows that the ordering transition is indeed in the KT universality class.