Dynamics of liquid ⁴He in Gel-Tek

Abstract
We have studied the excitations in superfluid ⁴He in a new porous silica medium : Gel-Tek. This material has a pore size of 25 Å and a very high specific surface area. We have measured the dynamical structure factor of liquid ⁴He confined in Gel-Tek using inelastic neutron scattering, for several filling fractions between 56% and 105% and several temperatures between 50 mK and 2.5 K. We have observed new features compared to bulk liquid ⁴He. We measured in particular a broad component involving approximately 40% of the helium atoms above the first inert layer. This component appears to be temperature independent between 50 mK and 2.5 K. For filling fractions larger than 55% above the layer, we have observed a maxon-roton type excitation. We have also detected additional intensity at energies below the roton, which we attributed to 2-dimensional excitations propagating in the liquid near the walls of the substrate. We give here a qualitative overview of these different features.