High-frequency transverse dynamics in glasses

The improvement in performance of the inelastic x-ray scattering technique has led to accurate determination of the dynamic structure factors of glassy and liquid glycerol, in the region of exchanged momentum Q = 2- 23 nm^-1 and in the temperature range 80-570 K. Thanks to the improved statistical accuracy, it has been possible to identify, in the spectra at the lower temperatures, besides the propagating longitudinal mode, a second non-Q-dispersing peak at hbarOmega_T approx 8.5 meV. We interpret this peak as a signature of the transverse dynamics that, in topologically disordered systems, acquires a longitudinal symmetry component. This assignment is substantiated by the observation that this peak still survives, across the glass transition, in the liquid state, and vanishes when the structural relaxation time tau_alpha approaches Omega_T^-1: a behaviour consistent with the condition tau_alpha Omega_T gg 1 required for the existence of a non-relaxational transverse-like dynamics in the liquid state.