Effects Of Disorder On The Collective Excitations In Helium

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