Evolution of multilayer Ar and Ne films from two-dimensional to bulk behavior

Argon and neon films on graphite have been studied in detail by high-resolution calorimetry, at temperatures from 60% of the bulk triple point ($T_t$) to several degrees above, and at coverages ranging from two to more than 20 molecular layers. Both adsorbates undergo a series of transformations, including layer formation, layer order-disorder transitions, individual layer melting, surface roughening, and surface melting. Layer transitions at thicknesses between two and six layers display Ising-like critical points, which evolve with increasing thickness to a broad roughening anomaly $T_R$=0.8$T_t$. A distinct series of peaks extending from the region of ordering temperatures to temperatures above $T_t$ is associated with the formation of each of the first four solid layers next to the substrate. Higher layers exhibit no individuality in their thermal behavior, and melting evolves into continuous surface melting. The thickness of the melt liquid at the surface of thick films corresponds to the predicted temperature dependence of surface melting in solids governed by dispersion forces. In thinner films the temperature dependence indicates short-range effective interactions, attributed to the intrusion of solid crystalline order into the liquid at the crystal-melt interface. A novel technique is developed for determining the profile of the crystal-melt interface, and applied to the Ar and Ne film systems. The profiles of both systems can be fitted by exponential or hyperbolic tangent functions. The experimental interface widths are found to be about six layers in Ne and four layers in Ar.