Neutron Scattering Study of Low-Frequency Vibrations in Silica Aerogels

Low-frequency vibrations in silica aerogels (mass densities 120 kg/m 3 and 250 kg/m 3 ) have been studied in the frequency range from 20 GHz to 2 THz by inelastic neutron scattering for temperatures between 5 and 300 K. The temperature dependence is well explained in terms of harmonic vibrations. The dynamic structure factor displays a sound-wave-like behaviour, i.e. the wavelengths of the modes investigated are large compared to atomic distances. The density of states has been determined on an absolute scale and was found to be the same for both specimens above 50 GHz; it does not show a simple scaling behaviour over the whole frequency range investigated.