On the role of atomic motion soft modes in acoustic phonon broadening below the boson peak in glasses

This Letter presents a mechanism of acoustic phonon broadening for frequencies lower than the boson peak frequency in glasses exhibiting a high-frequency sound above the boson peak. The mechanism is based on a resonant interaction of an acoustic phonon with harmonic vibrational excitations of soft modes in such glasses. The related width of the phonon is found to be independent of temperature and characterized by a power-law frequency dependence νϰ, with the exponent ϰ varying from ϰ≈2 below the boson peak to ϰ≈4 at lower frequencies. The dependencies do not appear to contradict some recent experimental data, for the glasses under discussion.