Low-frequency dynamics in an optical strong glass: Vibrational and relaxational contributions

A detailed Raman and Brillouin scattering investigation on a strong glass over a wide range of temperature across the glass transition temperature T-g is presented. The aim of the study is to determine whether the low-frequency vibrational behaviour is correlated with the elastic properties. A careful comparison of several models is carried out in order to determine the reliability of theoretical predictions devoted to the understanding of the dynamics of glass-forming systems. A clear correlation between the position of the boson peak and the hypersonic velocity in both polarizations is demonstrated; in particular, the connection is in better accordance if one considers the transverse velocity. On the other hand, the quasielastic Raman scattering has been analysed in terms of excitations directly coupled to the acoustic phonons. The correlation between the quasielastic Raman intensity and the hypersonic attenuation predicted for the low-temperature regime has been found to hold over a broad range below and above T-g. We found a strong correlation between the Raman features (at 10-50 cm(-1)) and the Brillouin data (at 1 cm(-1)), suggesting that the same mechanism is responsible for the vibrational dynamics within the 1-50 cm(-1) range. Moreover, we emphasize that the softening of the vibrational modes responsible far the boson peak occurs significantly below T-g, while an increase of the quasielastic intensity sets in, indicating some similarity between the origin of the boson peak and the origin of the growth of the quasielastic line.