title: 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 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.
authors.affiliation: Coherentia CNR-INFM and Dipartimento di Scienze Fisiche, Università di Napoli Federico II, I-80126 Napoli, Italy.
authors.forename: G
authors.initials: G
authors.lastname: De Filippis
authors.forename: V
authors.initials: V
authors.lastname: Cataudella
authors.forename: A S
authors.initials: AS
authors.lastname: Mishchenko
authors.forename: C A
authors.initials: CA
authors.lastname: Perroni
authors.forename: J T
authors.initials: JT
authors.lastname: Devreese