Two-dimensional excitations of superfluid 4He confined in 72% porosity xerogel

We present neutron inelastic-scattering measurements of the low-energy excitation spectrum of superfluid He-4 confined in porous xerogel glass. In addition to the bulk-like excitation, we also observe a broad peak centred at energies of less than that of the bulk-like peak. The dependence an filling fraction of bath peaks has been investigated and we find that the second broad peak exhibits behaviour that is consistent with that of an excitation confined to the high-density liquid layers near the pore walls. i.e, with that of a two-dimensional excitation. (C) 2000 Elsevier Science B.V. All rights reserved.