Isotropic to anisotropic elasticity transition and amorphous order in network glasses

I will discuss the results of a series of inelastic x-rays scattering experiments performed on vitreous silica at various densities and on its parent polycrystalline aggregates, ?-quartz and ?-cristobalite. The main result of our work is the observation of a particularly large static length, interatomic distances, connected to the size of the elastic heterogeneities in the glass. The glass behaves elastically as a continuum medium at large wavelengths, . The atomic vibrations feel the microscopic details of the structure when and their spectrum becomes almost indistinguishable from that of the corresponding polycrystal as the frequency is increased further. Here the glass shows a rich vibrational spectrum, which includes the three acoustics and the optic modes of the crystal averaged over the wave-vector orientations.