Experimental evidence for high frequency transverse-like excitations in glasses

TL;DR

This study provides experimental evidence of high-frequency transverse-like excitations in glasses and liquids, revealing a non-dispersing peak in the dynamic structure factor linked to transverse dynamics in disordered systems.

Contribution

First direct measurement of transverse-like excitations in glasses and liquids using inelastic X-ray scattering, demonstrating their existence and behavior across different phases.

Findings

Identification of a non Q-dispersing peak at 8.5 meV in glycerol

Transverse-like excitations vanish in liquids when structural relaxation time decreases

Transverse dynamics in disordered systems acquire a longitudinal component

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$\div$23 nm$^{-1}$ and in the temperature range 80$\div$570 K. Beside the propagating longitudinal excitations modes, at low temperature the spectra show a second non $Q$-dispersing peak at $\hbar \Omega_T$$\approx$8.5 meV. We assign this peak to the transverse dynamics that, in topologically disordered systems, acquires a longitudinal symmetry component. This assignment is substantiated by the observation that, in the liquid, this peak vanishes when the structural relaxation time $\tau_\alpha$ approaches $\Omega_T^{-1}$, a behavior consistent with the condition $\tau_\alpha \Omega_T^{-1}$$>>$1 required for the existence of a transverse-like dynamics in the liquid state.