Abstract
We discuss the effects of confinement and disorder on the elementary excitations of superfluid 4 He in porous media as observed by inelastic neutron scattering. The three-dimensional bulk-like phonon-roton excitations of liquid 4 He in aerogel and Vycor are the same as in bulk 4 He at low temperatures, with the same temperature dependence. In addition to these excitations, two-dimensional layer modes, which propagate in the first few liquid layers close to the substrate, are observed in both Vycor and denser aerogels near the roton wave vector, with a roton-like dispersion.
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