Excitations of liquid He-4 in porous media

We report on neutron inelastic scattering measurements of the elementary ex citations in liquid He-4 in porous aerogel and Vycor glass. The measured dy namical structure factor is very similar to bulk He-4, with a similar tempe rature dependence. The small changes in the energy and width of the phonon- roton excitations and their temperature dependences reported earlier in aer ogel are critically examined and re-evaluated. In Vycor, the most important difference with respect to bulk He-4 is the existence of additional low-en ergy excitations, which we tentatively identify with so-called layer modes, i.e. excitations that propagate in the first few layers of the liquid clos e to the substrate. The importance of multiple scattering for the interpret ation of the data is discussed. (C) 2000 Elsevier Science B.V. All rights r eserved.