PHONONS IN VITREOUS SILICA - DISPERSION AND LOCALIZATION

A numerical vibrational-mode analysis has been performed for structura l models of vitreous silica constructed using molecular dynamics. Disp ersion curves, associated with remnants of phonon-like behaviour, exhi bit pseudo-zone boundaries revealing evidence for two pseudo-periods i n the structure, the dominant one being equal to the height of SiQ(4) tetrahedra, and the other to the corresponding height of SiSi4 tetrahe dra. A new model for the boson peak is proposed: the excess modes are the lowest optic band states hybridized with acoustic states.