Experimental evidence for high frequency transverse-like excitations in glasses

The dynamic structure factor of glassy and liquid glycerol has been measured by inelastic X-ray scattering in the exchanged momentum (Q) region Q=2÷23 nm 1 and in the temperature range 80÷570 K. Beside the propagating longitudinal excitations modes, at low temperature the spectra show a second non Q-dispersing peak at ~T ≈8.5 meV. We assign this peak to the transverse dynamics that, in topologically disordered systems, acquires a longitudinal symmetry component. This assignment is substantiated by the observation that, in the liquid, this peak vanishes when the structural relaxation time �� approaches 1 T , a behavior consistent with the condition �� 1 T >>1 required for the existence of a transverse-like dynamics in the liquid state.