N2 MONOLAYERS PHYSISORBED ON GRAPHITE - THE HERRINGBONE TRANSITION REVISITED

Monte Carlo simulations were undertaken of the orientational herringbone phase transition of N2 adsorbed on graphite in the complete monolayer (square-root 3 x square-root 3)R30-degrees structure. The non-universal aspects (concentrating here on the transition temperature) were investigated using several models including the anisotropic planar rotor (APR) Hamiltonian, and two empirical as well as two ab initio potentials. The transition temperature depends strongly on the choice of the potential and can vary by a factor of two. Following several previous studies, the order of this transition was investigated in the framework of the APR Hamiltonian. Based on the analysis of energy and order parameter cumulants, but mainly on measuring directly the correlation length near the transition and imposing several scenarios on these data, our conclusion is that the herringbone ordering is a fluctuation driven weak first order transition.