Molecular dynamics simulation of an overfilled Kr monolayer on graphite

A microscopic model for Kr physisorbed on graphite is studied by the molecular dynamics technique. The model contains a Lennard-Jones potential describing the interaction among the Kr atoms and an external potential with sixfold symmetry describing the interaction between the Kr atoms and the graphite surface. The number of Kr atoms and the area of the surface are chosen to correspond to a five per cent overfilling of the perfect Kr monolayer. The Kr atoms are confined to move in a plane implying that second-layer promotion is inhibited in this study. Most simulations are performed with rectangular lattices of dimensions up to 170.4 AA*147.6 AA with periodic boundary conditions. A few simulations have also been performed with a circular lattice with reflecting walls in order to study the effects of the boundary conditions on the domain structures. The structure factors indicate two phases for the model: a low-temperature incommensurate phase and a high-temperature fluid phase. The transition between the two phases proceeds via a first-order transition with coexistence of the two phases in a temperature interval. No commensurate phase is seen in the simulation, in contrast to the experiments.