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

Using a microscopic variational approach we examine the structure and the excitation spectrum of layered⁴He liquids absorbed to alkali metal and graphite substrates. We 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. We also present results that we obtain by using the nonlocal density functional theory. We 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. We also give a brief discussion on the wetting properties of helium on alkali metal and graphite/solid helium substrates.