Vibration anharmonicity and picosecond relaxation in glasses

摘要

It is shown that vibration anharmonicity can be responsible for the quasielastic line in the low-frequency Raman spectra of glasses. The mechanism is based on the damping of vibrations by the phonon density fluctuations. They are coupled to vibrations due to third-order anharmonicity. This coupling produces the quasielastic response with a characteristic width determined by the relaxation time of the phonon density fluctuations, which is of the order of a picosecond. It is shown that the ratio of the integral quasielastic to vibration spectrum is determined by the Gruneisen parameter squared. Comparison with experimental data shows that this mechanism gives the dominant contribution to the quasielastic Raman scattering in glasses and supercooled liquids near the glass transition temperature. It is shown that in various glasses the inverse relaxation time linearly correlates with the frequency of the boson peak, in good agreement with an empirical observation. [References: 33]