Thermally activated relaxation processes in vitreous silica: An investigation by Brillouin scattering at high pressures

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Abstract
We have carried out Brillouin-scattering experiments on vitreous silica at temperatures between 50 and 300 K and pressures up to 3 GPa. Application of pressure leads to considerable changes of the acoustic absorption and the velocity of sound. In order to explain our observations, we start from the so-called ‘‘tunneling model,’’ which is often used to explain the low-temperature anomalies of amorphous solids. With an extension of this model, we are not only able to describe our experimental results but also the acoustic behavior of vitreous silica at higher temperatures in general. The quantitative fits of the experimental data achieved in this way give a hint as to the nature of the low-energy excitations existing in glasses.