ORGAN-PIPE MODES OF SODIUM EPITAXIAL MULTILAYERS ON CU(001) OBSERVED BY INELASTIC HELIUM-ATOM SCATTERING

Inelastic helium-atom scattering from epitaxial films (2-20 monolayers) of sodium on Cu(001) reveals a large number of nearly dispersionless phonon modes whose frequencies depend on the thickness and scale as the lower harmonics of an open-ended organ pipe. These data provide evidence for confined acoustic resonances, corresponding to longitudinal standing waves normal to the surface with frequencies about 20% larger than in the bulk.