We report the first measurements of Brillouin frequency shifts and linewidths in a solid below 1 K. The resonant attenuation of 35-GHz phonons by two-level systems in vitreous silica and the related anomalous behavior of the velocity are observed down to 0.3 K, where the phonon energy $\hslash \omega \simeq 5kT$. The low-temperature data are fully described by current theory based on the existence of two-level systems in glasses. Above 10 K, in contrast, a qualitative agreement between theory and experiment is found.

• Received 17 September 1979

DOI:https://doi.org/10.1103/PhysRevB.21.5850