NEUTRON-SCATTERING ANALYSIS OF LOW-FREQUENCY MODES IN SILICA

We report coherent inelastic neutron scattering results in vitreous si lica at temperatures up to 1673 K, 200 K above the glass transition. A s in earlier low-temperature data, clear evidence is seen for non-soun d-wave modes at the boson peak. Contrary to other glass formers, silic a shows nearly no fast relaxations at the glass transition. The combin ation of the neutron spectra with recent X-ray Brillouin data allows t he identification of the scattering mechanisms for high-frequency soun d waves in the glass. Below 1 THz, one has resonant scattering from lo w-frequency vibrations and above 1 THz, Rayleigh scattering from the a tomic disorder.