Low-frequency dynamics and medium-range order in vitreous silica

Brillouin and Raman scattering experiments have been performed on vitreous silica samples in order to check the validity of several models proposed for the interpretation of the low-frequency part of the Raman spectrum of glasses including the so-called ''boson peak.'' Samples of various origins, various fictive temperatures, and impurities contents as well as irradiated silica have been considered. It is demonstrated that the softening of acoustic modes responsible for the variation of the location of the boson peak maximum omega(max) is not fully accounted by the variation of the sound velocity V. This indicates that the characteristic length l (related to V and omega(max) in several models) is not a constant. Moreover, a clear quantitative correlation between the hypersound attenuation of the phonons and the low-frequency Raman intensity is evidenced.