Experimental evidence for high frequency transverse-like excitations in glasses

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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 ℏ Ωₜ≈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 τα approaches Ωₜ⁻¹, a behavior consistent with the condition ταΩₜ⁻¹>>1 required for the existence of a transverse-like dynamics in the liquid state.