Nature of the short wavelength excitations in vitreous silica: An x-ray Brillouin scattering study

The dynamical structure factor [S(Q, E)] of vitreous silica has been measur ed by inelastic x-ray scattering varying the exchanged wave vector (Q) at F ixed exchanged energy (E)-an experimental procedure that, contrary to the u sual one at constant Q, provides spectra with much better identified inelas tic features. This allows us to obtain the first direct evidence of Brillou in peaks in the S(Q, E) of SiO2 at energies above the boson peak (BP) energ y, a finding that excludes the possibility that the BP marks the transition from propagating to localized dynamics in glasses.