Elementary excitations in liquid 4 He confined in MCM-41

We present neutron scattering measurements of the excitations of liquid ${}^{4}\mathrm{He}$ confined in MCM-41 at wave vectors $0.35{\AA{}}^{\ensuremath{-}1}l~ql~2.15{\AA{}}^{\ensuremath{-}1}.$ MCM-41 is a uniform porous material consisting of cylindrical pores, here of 32 \AA{} diameter. The location and phase of ${}^{4}\mathrm{He}$ in the pores was well characterized by pressure isotherms and by exploratory neutron scattering measurements. The first 60% of ${}^{4}\mathrm{He}$ entering the MCM-41 is tightly bound on the media surface. This ${}^{4}\mathrm{He}$ supports no excitations in the energy range investigated $(0.3\mathrm{meV}l~\ensuremath{\omega}l~2.0\mathrm{meV}).$ The ${}^{4}\mathrm{He}$ added between 60 and 70% filling is less tightly bound. At 70% filling, ${}^{4}\mathrm{He}$ 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 ${}^{4}\mathrm{He}$ added supports well defined phonon-roton excitations which at low temperatures $Tl~1.4$ K, have energies and widths that are the same as those in bulk superfluid ${}^{4}\mathrm{He}$ within precision. There is some indication that the phonon energy for $ql~0.5{\AA{}}^{\ensuremath{-}1}$ lies above the bulk value. The liquid also supports layers modes for wavevectors in the roton region $(1.90{\AA{}}^{\ensuremath{-}1}l~ql~2.15{\AA{}}^{\ensuremath{-}1})$ 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.