Low-frequency Raman scattering by defects in glasses

Abstract

We describe the excess Raman intensity recently observed by Winterling in vitreous silica for frequency shifts $\ensuremath{\omega}<20$ ${\mathrm{cm}}^{\ensuremath{-}1}$ in terms of structural relaxation of defects, which can have two distinct states of polarizability differing by an amount $\ensuremath{\Delta}\ensuremath{\alpha}=0.6$ ${\mathrm{\AA{}}}^{3}$. All other properties of the defects (distribution of potential barriers) are taken from ultrasonic data. The correct temperature dependence is obtained. The experimentally observed value for the depolarization ratio in vitreous silica (0.30 \ifmmode\pm\else\textpm\fi{} 0.03) can be explained if the simplest possible form of the polarizability difference between the two states is assumed. We briefly discuss Raman scattering by quantum-mechanical tunneling states, which should be observable at very low temperatures.