Elementary excitations in liquid 4 He confined in MCM - 41

We present neutron scattering measurements of the excitations of liquid He confined in MCM-41 at wave vectors 0.35 Å<q<2.15 Å . MCM-41 is a uniform porous material consisting of cylindrical pores, here of 32 Å diameter. The location and phase of He in the pores was well characterized by pressure isotherms and by exploratory neutron scattering measurements. The first 60% of He entering the MCM-41 is tightly bound on the media surface. This He supports no excitations in the energy range investigated (0.3 meV <v <2.0 meV). The He added between 60 and 70% filling is less tightly bound. At 70% filling, He in MCM-41 supports an excitation that is interpreted as a ripplon propagating on the liquid surface. This excitation disappears on further filling. The subsequent liquid He added supports well defined phonon-roton excitations which at low temperatures T<1.4 K, have energies and widths that are the same as those in bulk superfluid He within precision. There is some indication that the phonon energy for q<0.5 Å lies above the bulk value. The liquid also supports layers modes for wavevectors in the roton region (1.90 Å<q< 2.15 Å) as observed in aerogel, Vycor, and Geltech silica. Accurate measurements at higher temperatures were not possible because of the small volume of liquid in the MCM-41 and the present sample cell design.