Neutron-Scattering Study of the Structure of Adsorbed Helium Monolayers and of the Excitation Spectrum of Few-Atomic-Layer Superfluid Films

Elastic diffraction of neutrons from helium adsorbed on Grafoil shows that the surface monolayer forms as a highly compressed, ordered triangular lattice incommensurate with the structure of the underlying graphite basal planes. Inelastic scattering from thin superfluid films on top of the surface monolayers reveals the existence of bulklike rotons in films containing on average as few as three layers of atoms.