This calculation examines the properties of partial monolayers of argon deposited on graphite, over an interval $20\le T\le 90\mathrm{K}$. In particular, we focus upon the phase transitions. Two peaks in the specific heat are found. The first peak is very sharp and narrow. It has been identified as a rotation of the adlayer from an angle off the substrate symmetry axis, to that axis. The calculated orientational behavior is compared with predictions of the two dimensional Ising model. The broad peak around $49.5\mathrm{K}$ is identified as a melting transition which is characterized by a loss of local sixfold symmetry.

• Received 15 August 2001

DOI:https://doi.org/10.1103/PhysRevLett.88.106101