Résumé:

The dynamic structure factor of lithium-diborate glass has been measured at several values of the momentum transfer Q using high resolution inelastic X-ray scattering. Much attention has been devoted to the low-Q-range, below the observed Ioffe-Regel crossover q(IR) similar or equal to 2.1 nm(-1). We find that below q(IR), the linewidth of longitudinal acoustic waves increases with a high power of either Q, or of the frequency Omega, up to the crossover frequency Omega(IR) similar or equal to 9 meV that nearly coincides with the center of the boson peak. This new finding strongly supports the view that resonance and hybridization of acoustic waves with a distribution of rather local low frequency modes forming the boson peak is responsible for the end of acoustic branches in strong covalent glasses. Further, we present high resolution Brillouin light-scattering data obtained at much lower frequencies on the same sample. These clearly rule out a simple Omega(2)-dependence of the acoustic damping over the entire frequency range. (c) 2006 Elsevier B.V. All rights reserved.