CLARIFICATION OF THE HEAD-TAIL ORDERING OF CO ON GRAPHITE - A MONTE-CARLO STUDY

Using an ab initio potential we perform classical and path-integral Monte Carlo simulations together with finite-size scaling to investigate the head-tail ordering of CO molecules in a full mono-layer on graphite. In accordance with experiments, the transition is found to belong to the 2 D Ising universality class and occurs in the correct temperature range; the quantum shift of the transition amounts to 10%. Beyond what is known from experiments we predict a particular ferrielectric ground state, the critical amplitude of the order parameter, and that the ordering is caused by molecular shape asymmmetry rather than electrostatic dipole forces.