EXCITATIONS OF SUPERFLUID He IN CONFINEMENT

Neutron inelastic scattering studies of the elementary excitations in con ned super uid He are reviewed. Both recent work on helium in porous silica glass (aerogel, Vycor, etc.) and earlier work on helium lms on graphite surfaces are discussed. The global picture emerging from these studies is that the three-dimensional excitations are essentially the same as in bulk helium. The characteristic feature of con ned helium is the existence of additional layer modes that propagate in the rst few liquid layers near the solid-liquid interface. The dispersion and gap energy of these layers modes depend on the substrate. The layer modes are believed to be at the origin of the di erences in macroscopic properties compared to bulk helium. Experiments suggest the existence of a localized condensate in Vycor.