Abstract
Explicit expression for the mixing interaction due to the lattice anharmonicity between the upper and the lower branch of polaritons is derived, using the generalized Langevin equation. Treating the mixing in a rough approximation, the calculated dispersion of the damped polariton qualitatively coincides with the phenomenological one used by Rimai et al. [Phys. Rev. 168 (1968) 623] in explaining their observed data in BaTiO 3 . Their observed data, however, does not call for the concept of polariton and the frequency shift considered by them is concluded to come from nothing but the coupling between the annihilation and the creation operater for the same Fourier component of the displacement of the ions.
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