IMPURITY-ANHARMONICITY INTERACTION MODES

Adopting the method of double time temperature dependent Green's funct ions a new mode named 'Impurity-Anharmonicity Interaction Mode' is inv estigated for the anharmonic crystals containing isotopic impurities. The widths and shifts related to this mode show strong dependence on t emperature and impurity concentration. The effect of temperature is fo und to be two-folds: one the explicit temperature dependence and the o ther via renormalized phonon frequencies. The present investigations i nvolve non-perturbative approach and nowhere use the quasi-harmonic ap proximation.