Thermal Conductivity of Solids. III. Modification of Three-Phonon Processes by Isotopic Scattering

The experimental observation that the effective strength of the phonon-phonon scattering in Ge-Si mixtures increases with the disorder parameter is explained qualitatively and semi-quantitatively by means of second-order interactions in which the three-phonon processes are modified by isotopic scattering. This effect is large because of the occurrence of small-energy denominators and the increased number of final states resulting from the over-all nonconservation of crystal momentum for such three-phonon processes.