COLLECTIVE EXCITATIONS IN DISORDERED-SYSTEMS

After an introduction into the field of collective excitations in disordered matter and into neutron inelastic scattering (NIS) techniques used for their investigation, results obtained by NIS experiments on classical fluids and amorphous solids are reviewed. It is concluded that a reasonably good understanding has been acquired during the years concerning the dispersion of excitations in liquid metals and rare gas fluids and partly also in metallic glasses. However, the damping of these modes and the collective excitations in fluids and in amorphous solids with other than metallic or pure molecular bonds still need to be investigated experimentally in mom detail until a comparable understanding of the collective movements and their decay in these systems has been gained and theoretical predictions can be tested on a quantitative basis. With a few exceptions systematic studies under favorable experimental conditions (dedicated instruments) and high quality data are lacking up to now.