  The effects of electron-electron correlations on the low-energy electronic
structure and their relationship with unconventional superconductivity are
central aspects in the research on the iron-based pnictide superconductors.
Here we use soft X-ray angle-resolved photoemission spectroscopy (SX-ARPES) to
study how electronic correlations evolve in different chemically substituted
iron pnictides. We find that correlations are intrinsically related to the
effective filling of the correlated orbitals, rather than to the filling
obtained by valence counting. Combined density functional theory (DFT) and
dynamical mean-field theory (DMFT) calculations capture these effects,
reproducing the experimentally observed trend in the correlation strength. The
occupation-driven trend in the electronic correlation reported in our work
supports the recently proposed connection between cuprate and pnictides phase
diagrams.
