TRANSPORT AND VIBRATIONAL LIFETIMES IN AMORPHOUS STRUCTURES

The lifetimes of high energy lattice vibrational states in amorphous o r glassy materials are calculated on the basis of vibrational localiza tion for energies omega > omega(c), where omega(c) signifies the mobil ity edge. Anharmonicity-induced localized vibrational slate hopping, w ith the emission of an extended vibrational state (a phonon), is found to be the dominant decay mechanism. Because of the contribution of th e same vertex to thermal transport via localized vibrational state hop ping, the vibrational lifetimes can be expressed in terms of this hopp ing contribution to the thermal conductivity, with only omega(c) as an undetermined variable. At low temperatures, the high energy vibration al lifetime is found to be proportional to the exponential of (omega/o mega(c))(d phi/D) where d(phi) is the superlocalization exponent of th e localized vibrational state, and D is the mass density scaling expon ent (equal to the Euclidean dimension d for dense systems, and the fra ctal dimension for fractal systems). Excellent numerical agreement is found with the experiments of Scholten et al. which find a lifetime ta u = 70 ns for the 480 cm(-1) TO vibration in a-Si:H.