High-temperature superconductivity in the copper oxides is almost universally associated with strong electron-electron correlations, and the idea that superconductivity appear doping a Mott insulator. The iron-based superconductors do not seem to follow the same paradigm, as the parent compounds are not insulating despite their magnetic ordering. Another key difference is that, while the cuprates can be described with a single valence band, in the iron materials it appears necessary to consider all five iron d-orbitals. Re-examining the experimental data for the effective mass, we show that indeed the phase diagram of iron superconductors is controlled by a Mott insulating state which would occur in a material with a very large hole-doping, while the parent compound is not close to a Mott state despite the commensurate filling. The reason for this behavior is a large Hund's coupling, which decouples the orbitals turning the multi orbital bandstructure into a collection of single-band Hubbard models. This finding creates the strongest connection between cuprates and iron superconductors. In both cases superconductivity appears around 20% doping of a Mott insulator. As a matter of fact the different single-band models have a different population (or doping) and therefore they have a different effective mass, which simply corresponds to the distance from the Mott insulator. This also implies a strongly "orbital-selective" effective mass, which reconciles contrasting evidences of correlations in these materials.
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