Electronic correlations in hole- and electron-doped Fe-based superconductors

High-temperature superconductivity in the cuprates occurs at the crossover from a highly-correlated Mott insulating state to a weaker correlated Fermi liquid as a function of hole doping. The iron pnictides were initially thought to be fairly weakly correlated. However, we have recently shown using transport and thermodynamic measurements that KFe$_{2}$As$_{2}$ is strongly correlated. Both the Sommerfeld coefficient and the Pauli susceptibility are strongly enhanced with respect to their bare DFT values. These correlations are even further enhanced in RbFe$_{2}$As$_{2}$ and CsFe$_{2}$As$_{2}$. The temperature dependence of both the susceptibility and the thermal expansion provides strong experimental evidence for the existence of a coherence-incoherence crossover; similar to what is found in heavy-fermion compounds. Whereas the correlations in the cuprates result from a large value of the Hubbard U, recent works have stressed the particular relevance of Hund's coupling in the pnictides. Our data may be interpreted in terms of a close proximity of KFe$_{2}$As$_{2}$ to an orbital-selective Mott transition. We now have good thermodynamic data covering both the hole and electron sides of the BaFe$_{2}$As$_{2}$ system and we will discuss how these correlations are modified by doping.