High Frequency Dynamics of Glass Forming Liquids at the Glass Transition

The dynamic structure factor of the glass formers glycerol, o-terphenyl, and n-butyl-benzene has been studied by inelastic x-ray scattering across the liquid-glass transition. Data taken at selected temperatures in the $1–8{\mathrm{nm}}^{-1\mathrm{}}$ momentum transfer ( $Q$) range show a propagating soundlike mode in both liquid and glass phases. The energy of this mode, studied at fixed $Q$ ( $\approx 2{\mathrm{nm}}^{-1\mathrm{}}$) on a fine temperature grid, shows a cusp behavior in a region either comparable or higher than the macroscopic structural arrest temperature $T_g$. This suggests the existence of a correlation between the glass structure and the arrest taking place at the molecular scale.