P. A. Heiney, P. W. Stephens, R. J. Birgeneau, P. M. Horn, and D. E. Moncton, Phys. Rev. B28, 6416-6434 (1983).
We have performed an extensive x-ray scattering study of the freezing transition of incommensurate xenon physisorbed on graphite as a function of temperature and chemical potential. At a coverage of 1.1 monolayers the transition is continuous to within the substrate-determined resolution, with correlation lengths in the fluid phase exceeding 50 atomic spacings. At coverages less than one monolayer the transition is first order. A comparison is made with current theories of two-dimensional melting.