Phonon-like dynamics in glasses: Coupling between damping and fragility

The high-frequency dynamics of two strong glasses, v-GeO2 and v-ZnCl2, were studied by means of inelastic neutron scattering experiments carried out with three triple-axis spectrometers in order to selectively access either a wide kinematic region or a high-energy resolution. The experimental spectra show well-defined dispersive acoustic excitations coexisting with non-dispersive waves. A thorough analysis of the inelastic line shape of longitudinal acoustic excitations provides estimates of the damping factors, which can be compared to the results of several earlier inelastic X-ray estimates for a wide class of inorganic glasses. A striking relation is observed between the microscopic damping of phonon-like modes in these glasses well below Tg and the fragility of the supercooled liquids approaching the glass transition, thus suggesting new means of investigation of the glass transition phenomenology.