On the validity of the Franck-Condon principle in the optical spectroscopy: optical conductivity of the Fröhlich polaron

Abstract

The optical absorption of the Fröhlich polaron model is obtained by an approximation-free Diagrammatic Monte Carlo method and compared with two new approximate approaches that treat lattice relaxation effects in different ways. We show that: i) a strong coupling expansion, based on the the Franck-Condon principle, well describes the optical conductivity for large coupling strengths ($\alpha >10$); ii) a Memory Function Formalism with phonon broadened levels reproduces the optical response for weak coupling strengths ($\alpha <6$) taking the dynamic lattice relaxation into account. In the coupling regime $6<\alpha <10$ the optical conductivity is a rapidly changing superposition of both Franck-Condon and dynamic contributions.

accepted for publication in PRL

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