Evidence of High Frequency Propagating Modes in Vitreous Silica

High energy resolution inelastic x-ray scattering measurements in the $1–6{\mathrm{nm}}^{-1\mathrm{}}$ momentum transfer ( $Q$) region in vitreous silica ( $v{-\mathrm{S}\mathrm{i}\mathrm{O}}_2$) at $T=1050\mathrm{}\mathrm{K}$ show the existence of collective excitations propagating with a sound velocity of $5800\pm 200\mathrm{m}/\mathrm{s}$ up to $Q=3.5\mathrm{}{\mathrm{nm}}^{-1\mathrm{}}$. The linewidths of the excitations, which are found to obey a $Q^2$ law, are consistent with previous determinations made at low $Q$ and low $T$. The picture of the atomic dynamics in $v{-\mathrm{S}\mathrm{i}\mathrm{O}}_2$ emerging from this study indicates that these propagating modes must contribute to the boson peak.