Phase transitions in multilayer helium films

Multilayer helium films have, until recently, been predicted and observed to exhibit a standard form of structural behavior: one or two solid layers reside adjacent to the substrate and continuous wetting liquid extends outward thereafter. We report contrary theoretical results in the case of weak-binding surfaces, the alkali metals and H2 being particularly important examples. Depending on the substrate potential, the predicted behavior can be either nonwetting or prewetting. The focus is on He4, but some results for He3 are presented. Compound substrates, helium layer solidification, third sound, superfluid onset, and possible phase diagrams for T>0 are discussed. The calculations utilize semiempirical density-functional methods which have proven to be accurate in other applications. Recent experiments with alkali metals and H2 are discussed in relation to theoretical results.