Anharmonic vs. relaxational sound damping in glasses: II. Vitreous silica

TL;DR

This paper reanalyzes sound damping in vitreous silica, showing thermally activated relaxation dominates at low temperatures, while anharmonic effects become significant at hypersonic frequencies, indicating structural changes below room temperature.

Contribution

It distinguishes the roles of relaxational and anharmonic damping mechanisms in vitreous silica across different frequency regimes, highlighting the importance of anharmonicity at hypersonic frequencies.

Findings

Thermally activated relaxation explains sound attenuation above 10 K.

Anharmonic coupling becomes significant at 35 GHz and room temperature.

Silica undergoes gradual structural changes starting below room temperature.

Abstract

The temperature dependence of the frequency dispersion in the sound velocity and damping of vitreous silica is reanalyzed. Thermally activated relaxation accounts for the sound attenuation observed above 10 K at sonic and ultrasonic frequencies. Its extrapolation to the hypersonic regime reveals that the anharmonic coupling to the thermal bath becomes important in Brillouin-scattering measurements. At 35 GHz and room temperature, the damping due to this anharmonicity is found to be nearly twice that produced by thermally activated relaxation. The analysis also reveals a sizeable velocity increase with temperature which is not related with sound dispersion. This suggests that silica experiences a gradual structural change that already starts well below room temperature.