The Raman coupling function in amorphous silica and the nature of the long-wavelength excitations in disordered systems

New Raman and incoherent neutron scattering data at various temperatures and molecular-dynamic simulations in amorphous silica are compared to obtain the Raman coupling coefficient C (ω) and, in particular, its low-frequency limit. This study indicates that in the ω → 0 limit C (ω) extrapolates to a nonvanishing value, giving important indications on the characteristics of the vibrational modes in disordered materials; in particular our results indicate that even in the limit of very long wavelength the local disorder implies nonregular local atomic displacements.