Excitations of superfluid 4He in porous media: Aerogel and Vycor

We present precision neutron scattering measurements of the dynamic structure factor S(Q,ω) and the elementary excitations of liquid 4He immersed in aerogel and Vycor. In both porous media, liquid 4He supports two-dimensional (2D) layer modes at wave vectors in the roton region that propagate in the liquid layers adjacent to the media walls. The layer modes have a rotonlike energy dispersion relation with energies that are consistent with specific heat and superfluid density measurements at low temperature. Liquid 4He in these media 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 4He within current precision (5μeV). The layer modes are the only new excitations observed in these disordering media to date. In Vycor, we observe a well-defined 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