EFFECTS OF DISORDER ON THE COLLECTIVE EXCITATIONS IN HELIUM

We present inelastic neutron scattering measurements of the roton exci tation of superfluid helium confined in two different porosity aerogel s. The temperature dependence of the roton energy exhibits an anomalou s crossover behavior at T-c = 1.9 K. Below T-c the roton energy is fou nd to be weakly temperature dependent while above T-c it varies more r apidly than the bulk. A simple qualitative model for this behavior in terms of a confined length scale is discussed.