Abstract
It is shown that conventional Wigner-Brillouin perturbation theory cannot be applied to the calculation of band structures or phonon spectra of superlattices, it results in a wrong number of bands with incorrect spectral weights. Rayleigh-Schrödinger perturbation theory cannot be applied, too, due to the degeneracies necessarely involved. A new type of perturbation theory based on the exact structure of the corresponding Green functions is developed, which is free of these shortcomings.
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