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 ${\mathrm{H}}_2$ being particularly important examples. Depending on the substrate potential, the predicted behavior can be either nonwetting or prewetting. The focus is on ${}_{}{}^4{}_{}{}^{}\mathrm{He}$, but some results for ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ 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 ${\mathrm{H}}_2$ are discussed in relation to theoretical results.