The dynamic structure factor S(Q,ω), of vitreous silica, has been measured by inelastic x-ray scattering in the exchanged wave-vector (Q) region Q=4 16.5 nm-1 and up to energies ħω=115 meV in the Stokes side. The unprecedented statistical accuracy in such an extended energy range allows us to accurately determine the longitudinal current spectra and the energies of the vibrational excitations. The simultaneous observation of two excitations in the acoustic region and the persistence of propagating sound waves up to Q values comparable with the (pseudo-)Brillouin-zone edge allow us to observe a positive dispersion in the generalized sound velocity that, around Q≈5 nm-1, varies from ≈6500 to ≈9000 m/s; this phenomenon was never experimentally observed in a glass.
Physical Review B
- Pub Date:
- March 2004
- Vibrational states in disordered systems;
- X-ray scattering;
- Condensed Matter - Disordered Systems and Neural Networks
- 5 pages, 3 figures. To appear in Phys. Rev. B