Low-frequency vibrational excitations in vitreous silica: the Ioffe-Regel limit

The behaviour of vibrational plane waves in disordered solids has been studied theoretically, particularly in the vicinity of the Ioffe-Regel crossover. We have analysed the behaviour of vibrational excitations in vitreous silica, for which we have found the Ioffe-Regel crossover frequency to be THz (for both longitudinal and transverse waves), in good agreement with experimental observations. Two methods have been used to obtain : either by determining the decay time of the plane waves, or by analysing the distribution in k -space of plane-wave components in the final scattered state of a plane wave.