O EDP Sciences, Les Ulis Dynamics of liquid 4 ~ e in Gel-Tek

Abstract. We have studied the excitations in superfluid 4 ~ e in a new porous silica medium: Gel-Tek. This material has a pore size of 25 A and a very high specific surface area. We have measured the dynamical structure factor of liquid 4 ~ e confined in Gel-Tek using inelastic neutron scattering, for s e v d fXing h t i o n s between 56% and 105 % and several temperatures between 50 mK and 2.5 K. We have observed new features compared to bulk liquid 4 ~ e. We measured in particular a broad component involving approximately 40 % of the helium atoms above the iirst inert layer. This component appears to be temperature independent between 50 mK and 2.5 K. For filling fractions larger than 55 % above the first 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. 1