Two-Phonon Resonances and Hybridization of the Resonance with Single-Phonon States

Considering the anharmonic phonon-phonon interactions, two-phonon resonances are studied by Green's-function methods. The two-phonon spectrum exhibits an asymmetric peak near the top of the two-phonon continuum. In the special case of a resonance consisting of two acoustic phonons, the hybridization of the resonance with a single optical phonon is possible, in agreement with experiment.