Inelastic scattering from liquid 4He in aerogel glass

The physics of liquid and solid 4He in restricted geometries has motivated a number of interesting experiments. Recent experiments include detailed measurements of the phase diagram for bulk liquid in vycor, showing a suppression of the superfluid transition and elevation of the melting pressure, and measurements of the superfluid fraction in vycor, aerogel, and zerogel glasses near the lambda point, in which critical exponents differ from the pure 4He values have been observed. Many striking features in several of the experiments on helium in restricted geometries are poorly understood. We have performed inelastic neutron scattering measurements of liquid helium in aerogel glass above and below the superfluid transition for two samples of different porosities. The kinetic energy (KE) of the confined liquid is the same as that of the bulk liquid in the normal phase, but is clearly higher than the bulk values in the superfluid phase. The observed scattering in the superfluid phase is more peaked than in the normal phase: consistent with the presence of a Bose condensate. An estimate of the condensate fraction using a modification of a method due to Sears yields values consistent with those estimated for the bulk liquid. 7 refs., 2 figs., 1 tab