HE-4 ON WEAKLY ATTRACTIVE SUBSTRATES - STRUCTURE, STABILITY, AND WETTING BEHAVIOR

Using a microscopic variational approach we examine the structure and the excitation spectrum of layered He-4 liquids absorbed to alkali met al and graphite substrates. We find that the alkali metal substrates p roduce a less pronounced layering structure than the shorter-range gra phite/solid helium potential. For the excitations, three features are in common to the substrates: First, for coverages of a monolayer or mo re, a surface mode is present. Second, a bulk mode which gains strengt h as the coverage is increased, is identifiable for films with suffici ently high coverage. Finally, a two-dimensional mode that propagates w ithin the first layer is, observed for the more attractive substrates. We also present results that we obtain by using the non-local density functional theory. We document the reliability and shortcomings of th is approach by making a detailed comparison of experimental, Monte Car lo, and variational theory results for the structure, energetics, and excitations. We also give a brief discussion on the wetting properties of helium on alkali metal and graphite/solid helium substrates.