A QUANTUM DISSIPATION THEORY OF ANHARMONIC CRYSTALS .2. POLARITONS

In an earlier paper a quantum-mechanical dissipation theory was used to deduce the effect of anharmonicity on crystal vibration modes. These modes interacting with electromagnetic radiation form a new system which is now investigated in a similar way. The coupled radiation-vibration, or polariton, modes experience damping and frequency shifts which are expressed, for alkali halides, in terms of the corresponding phonon quantities. The expressions for phonons are multiplied by a momentum-dependent factor corresponding in magnitude to the polariton's degree of phononlike character and ranging from zero to unity. Results for the example of potassium bromide are presented. Other excitations interacting with phonons are discussed with regard to such a treatment. © 1969 The American Physical Society.