Role of vibrational modes in structural relaxations in a supercooled liquid

Abstract

Structural relaxation in a supercooled liquid is studied in terms of a model obtained by incorporating metastability in the fluctuating nonlinear description of the mode coupling theory. As a result of the weak coupling to the very long lived defect modes, the correlation of density fluctuations decays to zero for densities higher than the critical density of the ideal glass transition obtained from earlier models. Such models for the amorphous systems also involves the transverse sound modes. The coupling of the slowly decaying density fluctuations to these vibrational modes gives rise to an extra intensity over the intermediate frequency range in the supercooled liquid. The present analysis demonstrates that the nature of the dynamics of defect densities in the disordered system plays a crucial role for the appearance of this intermediate peak in the scattering function. The results for the dynamic structure factor from the model equations are compared with the scattering data of Sokolov et al. With the relaxation time for the defects becoming longer which is the case more appropriate for the strong Glasses as compared to the fragile glasses where structural degradation occurs more easily, the Boson peak become more pronounced.