Electronic polarons and bipolarons in iron-based superconductors: The role of anions

We introduce a quantum-mechanical model to describe the low-energy scale electronic structure of the Fe-based pnictides, placing a special importance on the large polarizability of the large anions, such as As3-. These are modeled in terms of an atomic two-level system coupled strongly to charge fluctuations on the nearby Fe ions. This strong coupling results in electronic polaron formation, a strong effective reduction in the Fe on-site Coulomb repulsion, and the appearance of a strong attractive interaction for d electrons on nearest-neighbor Fe sites, which could be an essential component of the pairing mechanism. This quantum model allows us to investigate this phenomenology away from the linear regime and to also study the dynamic properties of these polarons, as well as the binding of polaron pairs and their dynamics.