The collective excitations of superfluid4He in aerogel glass

When liquid⁴He is condensed in porous aerogel glass (typical pore size ≈ 500 Å), many of the superfluid properties are significantly altered. Neutron inelastic scattering has been used to measure the collective phonon-roton excitations of liquid⁴He in this restricted geometry. Although at low temperature (1.3 K) the observed dispersion relation is identical to that of bulk⁴He, its temperature dependence is shown to be different. Previously unexplained superfluid fraction data are shown to be derivable from these microscopic measurements. An intrinsic broadening of the excitations associated with the restricted geometry is also present. Possible microscopic explanations for the unusual temperature variation of the collective excitation energies are discussed.