Abstract
A numerical vibrational-mode analysis has been performed for structural models of vitreous silica constructed using molecular dynamics. Dispersion curves, associated with remnants of phonon-like behaviour, exhibit pseudo-zone boundaries revealing evidence for two pseudo-periods in the structure, the dominant one being equal to the height of SiO4 tetrahedra, and the other to the corresponding height of SiSi4 tetrahedra. A new model for the boson peak is proposed: the excess modes are the lowest optic band states hybridized with acoustic states.