Experimental evidence for high frequency transverse-like excitations in glasses

Abstract

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 $\hbar Ω_T$$\approx$8.5 meV. We assign this peak to the transverse dynamics that, in topologically disordered systems, acquires a longitudinal symmetry component. This assignment is substantiated by the observation that, in the liquid, this peak vanishes when the structural relaxation time $τ_α$ approaches $Ω_T^{-1}$, a behavior consistent with the condition $τ_αΩ_T^{-1}$$>>$1 required for the existence of a transverse-like dynamics in the liquid state.