Nondynamic Origin of the High-Frequency Acoustic Attenuation in Glasses

The sound attenuation in the THz region is studied down to $T=16\mathrm{}\mathrm{K}$ in glassy glycerol by inelastic x-ray scattering. At striking variance with the decrease found below $\approx 100\mathrm{K}$ in the GHz data, the attenuation in the THz range does not show any $T$ dependence. This result (i) indicates the presence of two different attenuation mechanisms, active, respectively, in the high- and low-frequency limits, (ii) demonstrates the nondynamic origin of the attenuation of THz sound waves, and confirms a similar conclusion obtained in ${\mathrm{SiO}}_2$ glass by molecular dynamics, and (iii) supports the low-frequency attenuation mechanism proposed by Fabian and Allen [Phys. Rev. Lett. 82, 1478 (1999)].