Emergence of crystal-like atomic dynamics in glasses at the nanometre scale

I will present the results of a series of inelastic x-rays scattering experiments performed on vitreous silica at various densities [1-3] and on its parent polycrystalline aggregates [4], ?-quartz and ?-cristobalite. The comparison of the experimental results with ab-initio numerical calculations provides compelling evidence of a transition, in the glass, from the isotropic elastic response at long wavelengths to a microscopic regime as the wavelength decreases below a characteristic size ? of a few nanometres, corresponding to about 20 inter-atomic distances. The glass behaves elastically as a continuum medium at large wavelengths, ? >> ?. As the wavelength gets close to ?, the atomic vibrations feel the microscopic details of the structure and eventually their spectrum becomes almost indistinguishable from that of the corresponding polycrystal of similar density. This similarity in the spectrum of the glass and the corresponding polycrystal indicates that the glass vibrations closely resemble those of the polycrystal, with excitations related to the acoustic and optic modes of the crystal. A coherent description of the experimental results is obtained assuming that the length ? corresponds to the size of spatial heterogeneities in the elastic modulus of the glass. [1] G. Baldi, V. M. Giordano, G. Monaco, & B. Ruta, Phys. Rev. Lett. 104, (2010) 195501. [2] G. Baldi, V. M. Giordano, & G. Monaco, Phys. Rev. B 83, (2011) 174203. [3] M. Zanatta et al., Phys. Rev. B 81, (2010). [4] G. Baldi, M. Zanatta, E. Gilioli, V. Milman, K. Refson, B. Wehinger, B. Winkler, A. Fontana, & G. Monaco, Phys. Rev. Lett. (submitted).