Exciton-Phonon Interaction in Semiconductors with Intermediate Polaron Coupling

The linear absorption of a two-band semiconductor with Coulomb interaction and intermediate electron-LO-phonon coupling is considered. Special attention is payed to the polaron renormalization of the carrier spectrum. It is shown that this may play an important role in the appearance of phonon sidebands in vertexless calculations. To this end the generalized Kadanoff-Baym ansatz (GKBA) is compared with an alternative ansatz which does not rely on the factorization of the spectral Green's function.