Melting and orientational epitaxy in argon and xenon monolayers on graphite

We have studied the melting behavior for Ar at a coverage of {ρ}{~}0.81 and the orientational-epitaxy effect for Ar and Xe. We find that the Ar melts continuously from a rotated incommensurate solid to a rotated fluid phase; the fluid with positional correlations of 400 e/rA{} is rotated by {~}2^∘{}. The melting behavior agrees well with that observed by x-ray diffraction using a ZYX-graphite substrate, and is consistent with the heat-capacity data of Migone et al. For Xe, the orientational-epitaxy data show a rotation to zero for increasing misfit magnitude, contrary to the Ar and Kr behavior. This is discussed in light of the role of entropy in the structure of this system.