Strong phonon coupling and the relaxation rate spectrum in glasses

Abstract

The influence of strong phonon coupling on the relaxation rate of two-level systems (TLS) in glasses is investigated. A unitary transformation is employed to express the Hamiltonian in terms of dressed TLS operators corresponding to weakly interacting units. The relaxation rate of these TLS is calculated; for asymmetric TLS, it turns out to be strongly reduced in comparison with the conventional result. Nevertheless, the shape of the distribution of relaxation rates is hardly changed, an effect that renders the previous success of weak-coupling theories understandable. In the light of these new results it is argued that most of the experimentally visible TLS rate spectrum may be dynamically generated by strong phonon coupling.