Weak and strong electronic correlations in Fe superconductors

TL;DR

This paper discusses the coexistence of strongly and weakly correlated electrons in Fe superconductors, emphasizing Hund's coupling's role in orbital decoupling and its impact on electronic properties.

Contribution

It provides a unified scenario supported by experiments and advanced calculations, highlighting Hund's coupling as a key factor in orbital-dependent electronic correlations.

Findings

Hund's coupling causes orbital decoupling in Fe superconductors.

Different orbitals exhibit varying degrees of 'Mottness' depending on their filling.

Electronic correlations vary across orbitals, influencing Fermi surface properties.

Abstract

In this chapter the strength of electronic correlations in the normal phase of Fe-superconductors is discussed. It will be shown that the agreement between a wealth of experiments and DFT+DMFT or similar approaches supports a scenario in which strongly-correlated and weakly-correlated electrons coexist in the conduction bands of these materials. I will then reverse-engineer the realistic calculations and justify this scenario in terms of simpler behaviors easily interpreted through model results. All pieces come together to show that Hund's coupling, besides being responsible for the electronic correlations even in absence of a strong Coulomb repulsion is also the origin of a subtle emergent behavior: orbital decoupling. Indeed Hund's exchange decouples the charge excitations in the different Iron orbitals involved in the conduction bands thus causing an independent tuning of the degree of electronic correlation in each one of them. The latter becomes sensitive almost only to the offset of the orbital population from half-filling, where a Mott insulating state is invariably realized at these interaction strengths. Depending on the difference in orbital population a different 'Mottness' affects each orbital, and thus reflects in the conduction bands and in the Fermi surfaces depending on the orbital content.