We report ellipsometric coverage-vapor-pressure isotherm measurements for argon on graphite over the temperature range 65 to 84 K, which resolve up to eight layers. We confirm previous observations that the first-order layering transitions broaden near 67 K. However, in the fourth and higher layers, condensation again becomes sharp and apparently first order in the range between about 73 K and a new series of layer critical points near 77 K. Thus the surface-roughening transition is above 77 K and the behavior near 67 K represents disordering of only the top layer of the film.PACS numbers: 68.55.Jk, 64.70.Dv, 68.45.Gd A face of a crystal is said to exhibit surface melting l if a mobile "quasiliquid" layer exists on the equilibrium surface at a temperature below the bulk melting point T m , and its thickness increases with temperature, diverging as T approaches T m . Surface roughening 2,3 is the unlocking of the crystal surface from a particular lattice plane through the proliferation of steps, resulting in the loss of faceting in thermal equilibrium. 3 This transition is preceded on the low-temperature side by the appearance and proliferation of vacancies in and adatoms on the initially smooth surface plane. As the temperature increases towards the roughening temperature TR, the characteristic size of clusters of adatoms and vacancies diverges, and the clusters gain their own (divergent) adatom and vacancy clusters. Roughening is thus an essentially many-layer phenomenon, but it must be preceded by short-range disordering of the surface layer. Simulations show a strong correlation between disordering of the top layer and onset of a large surface mobility. 4 Therefore, while surface roughening and surface melting are quite distinct many-layer phenomena, their precursors in the top layer may be coupled.The properties of the surfaces of bulk crystals should be reflected in the properties of the surfaces of multilayer wetting films of the same solid on a foreign substrate. 5 " 7 Disordering of the (partially filled) top layer of a film is a transition of the 2D Ising class. A succession of Ising critical temperatures T Ct " is expected for films of different coverages near n-j layers, but always the transition is in the top layer. Using different models, Huse 8 and Nightingale, Saam, and Schick 9 have shown that the sequence T Cyn should converge to the roughening temperature, with TR -T c , n~-(lnn) ~2.Zhu and Dash 7 (ZD) have reported evidence for both surface melting and surface roughening in heat-capacity studies of two-layer to about twenty-layer films of both neon and argon on graphite. The evidence for surface roughening is small heat-capacity peaks near 0.8T m , attributed to disordering of the partially filled top layer. Previous volumetric isotherm measurements for argon on graphite by Gilquin 10 found broadening of the second-
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