THEORY OF FIRST-ORDER LAYERING TRANSITIONS IN THIN HELIUM FILMS

Thin liquid He-4 films on graphite show evidence of layered growth wit h increasing number density via a succession of first-order phase tran sitions. These so-called ''layering transitions'' separate uniformly c overing phases, such as monolayers and bilayers. The present work is a detailed theoretical study of such layering transitions using a Maxwe ll construction. We model the graphite surface by a strong substrate p otential, and using a microscopic variational theory we obtain the uni form coverage solutions for liquid helium. For each layer, the theory yields the chemical potential mu and surface tension alpha as function s of coverage n, and from this we deduce mu(a). For each set of adjace nt layers, we then obtain the crossing point in the curves of mu(alpha ). In this way we obtain the values of mu, alpha, and surface coverage s for the transition. Particular attention is paid to the monolayer-bi layer transition.