MONTE-CARLO INVESTIGATION OF HEAD-TAIL ORDERING OF CO MONOLAYERS ON GRAPHITE

Heat capacity measurements recently showed that CO physisorbed on grap hite undergoes a head-tail ordering transition at roughly 5 K. The pre sent paper is a detailed Monte Carlo study of this phase transition an d the ordered state, The simulations are based on an ab initio pair po tential and rely crucially on a thorough finite-size scaling study of various quantities. In agreement with experiments we find that the tra nsition belongs to the universality class of the Ising model in two di mensions. We go beyond experimental knowledge by revealing the particu lar ferrielectric structure of the ground state, and show that the tra nsition is due to the molecule's shape asymmetry rather than to electr ostatic dipole interactions. The influence of quantum fluctuations on the ground state and on the transition itself are also analyzed.