THE COLLECTIVE EXCITATIONS OF SUPERFLUID HE-4 IN AEROGEL GLASS

When liquid He-4 is condensed in porous aerogel glass (typical pore si ze approximate to 500 Angstrom), many of the superfluid properties are significantly altered. Neutron inelastic scattering has been used to measure the collective phonon-roton excitations of liquid He-4 in this restricted geometry. Although at low temperature (1.3 K) the observed dispersion relation is identical to that of bulk He-4, its temperatur e dependence is shown to be different. Previously unexplained superflu id fraction data are shown to be derivable from these microscopic meas urements. An intrinsic broadening of the excitations associated with t he restricted geometry is also present. Possible microscopic explanati ons for the unusual temperature variation of the collective excitation energies are discussed.