Adsorption of Helium on Rare-Gas-Plated Graphite

The single-particle quantum states for ${\mathrm{He}}^3$ and ${\mathrm{He}}^4$ atoms adsorbed on rare-gas-plated graphite are analyzed in detail and compared to the corresponding states for adsorption on graphite and on argon-plated copper. The nature of the adsorption states depends primarily on the type of preplating gas and little on the type of underlying base. The adatoms on these substrates have high surface mobility, but the periodic surface potential does produce strong band-structure effects. The mass difference between ${\mathrm{He}}^3$ and ${\mathrm{He}}^4$ is found to be more important than the difference in statistics. The specific heat, which is calculated neglecting helium-helium interactions, is used to further compare the nature of adsorption on the various substrates.