PHASE-TRANSITIONS IN ARGON FILMS

We present heat-capacity data detailing the evolution of the first six layers of argon adsorbed on graphite foam. The second and third layer s have liquid-solid-gas triple points similar to the first layer. Thes e layers exhibit a phase diagram consisting of two-dimensional solid, liquid, and gas phases on top of a solid film. Above the temperatures of the individual-layer triple points, the melting transition for each layer appears to be first order, and the first two layers show eviden ce of registry transitions prior to melting. For films of a total thic kness of about four layers and up, the melting of each of the first th ree layers occurs at temperatures above the bulk triple point, as repo rted by Zhu and Dash [Phys. Rev. B 38, 11 673 (1988)]. Our results con firm those of an ellipsometry study [H. S. Youn and G. B. Hess, Phys. Rev. Lett. 64, 918 (1990)] that found layering transitions above what were believed to be the layering critical-point temperatures. We obser ve heat-capacity peaks identified with these transitions and with melt ing transitions that join them with the low-temperature layering trans itions. A phase diagram based on these data may represent the signatur e of a preroughening transition and a disordered flat phase in the bul k-crystal interface.