Evidence of high frequency propagating modes in vitreous silica

High energy resolution inelastic x-ray scattering measurements in the 1-6 nm(-1) momentum transfer (Q) region in vitreous silica (upsilon-SiO2) at T = 1050 K show the existence of collective excitations propagating with a sound velocity of 5800 +/- 200 m/s up to Q = 3.5 nm(-1). 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 upsilon-SiO2 emerging from this study indicates that these propagating modes must contribute to the boson peak.