Excitations of super uid He in porous media: aerogel and Vycor

We present precision neutron scattering measurements of the dynamic structure factor, S(Q;!), and the elementary excitations of liquid He immersed in aerogel and Vycor. In both porous media, liquid He supports two-dimensional (2D) layer modes at wavevectors in the roton region that propagate in the liquid layers adjacent to the media walls. The layer modes have a roton-like energy dispersion relation with energies that are consistent with speci c heat and super uid density measurements at low temperature. Liquid He also supports 3D phonon-roton (p-r) excitations that have energies and lifetimes at temperatures 0:5 T 2:3 K which are the same as in bulk liquid He within current precision (5 eV). The layer modes are the only new excitations observed in these disordering media to date. In Vycor, we observe a well de ned p-r excitation at temperatures above Tc in the normal phase. This suggests that there is a localized condensate (phase coherence on short length scales) above Tc in Vycor.