Phonon-density fluctuations and fast relaxation in glasses

It is shown that fluctuations of the phonon density give the main contribution to quasielastic Raman scattering in glasses and supercooled liquids. Due to anharmonic effects these fluctuations couple to vibration modes, giving rise to a memory function term in the vibration susceptibility function. The memory function provides the quasielastic response with a characteristic width corresponding to a relaxation time {τ} of the order of a picosecond. It is shown that τ^-1{~}Ωb/3, where Ωb is the frequency of the boson peak at low temperatures. The intensity of the quasielastic scattering is expressed via the Gr\"unaisen parameter; there is a good agreement with experimental data.