Interactions of two-level systems in glasses

Using methods from polaron theory, the authors derive the elastic interaction between two-level systems (TLS) in glasses in a nonperturbative manner. They then calculate the relaxation rate of the interacting dressed TLS perturbatively. Although this calculation necessitates some approximations, it leads to the statement that for strong TLS-phonon interaction the TLS relaxation rate 1/T1 is at least for asymmetric TLS, considerably different from the prediction of the usual (weak-coupling) expression. The impact of this result on theory and experiment is discussed.