Observation of Large Momentum Phononlike Modes in Glasses

The dynamic structure factor of glycerol and LiCl:6${\mathrm{H}}_2$O glasses, measured by inelastic x-ray scattering, shows modes dispersing linearly with momentum in the 2-8 ${\mathrm{nm}}^{-1\mathrm{}}$ region. This finding demonstrates that acousticlike propagating modes can exist in glasses up to wavelengths comparable to interparticle separations. It also shows that the boson peak, a feature systematically observed in the same energy region spanned by these excitations, must have a contribution from this propagating collective dynamics, and not only from possible localized modes.