PHONON SCATTERING AT FREQUENCIES NEAR THE EXCITON FREQUENCY OF TWO-LEVEL DEFECTS

Abstract
In the diagrammatic temperature-dependent perturbation theory of coupled spin-phonon systems an essential quantity determining the poles of the phonon Green function is the polarisation matrix, M, of the generalised spin susceptibility. Near the spin exciton frequency the resonance contribution from the polarisation matrix to all orders is required. Hitherto only the chain-like diagrams that occur in the theory have been retained. For M₄ we show that an important contribution also arises from other diagrams previously neglected, whose inclusion is essential if M₄ is to remain finite in the high temperature limit.