4He on weakly attractive substrates: Structure, stability, and wetting behavior

Using a microscopic variational approach the authors examine the structure and the excitation spectrum of layered 4He liquids absorbed to alkali metal and graphite substrates. The authors find that the alkali metal substrates produce a less pronounced layering structure than the shorter-range graphite/solid helium potential. For the excitations, three features are in common to the substrates: First, for coverages of a monolayer or more, a surface mode is present. Second, a bulk mode which gains strength as the coverage is increased, is identifiable for films with sufficiently high coverage. Finally, a two-dimensional mode that propagates within the first layer is observed for the more attractive substrates. The authors also present results that they obtained by using the nonlocal density functional theory. The authors document the reliability and shortcomings of this approach by making a detailed comparison of experimental, Monte Carlo, and variational theory results for the structure, energetics, and excitations. The authors also give a brief discussion on the wetting properties of helium on alkali metal and graphite/solid helium substrates