the OSMP picture in that that the metallic phase in this KFS compound is likely even closer to the boundary of OSMP at low temperatures from the mass-diverging behavior of the [$d_{xy}$]{} bands. For the same interaction strength, calculation from our model also identifies the low temperature ground state of the superconducting, intermediate, and insulating phases to be located close to an OSMP, just at the boundary of an OSMP, and in a MI phase, respectively (see SI).

While we cannot completely rule out alternative explanations for the observations presented above, the consistency of the totality of the observations-including strongly orbital dependent band renormalization for [$d_{xy}$]{} versus [$d_{xz}$/$d_{yz}$]{} at both $\Gamma$ and X points in the low temperature metallic state, the non-trivial temperature-dependent spectral weight change for only the [$d_{xy}$]{} band, systematic doping dependence of the related effects in the intermediate and insulating compounds-and the theoretical calculations makes this understanding a most likely scenario, suggesting that the superconductivity in this AFS family exists in close proximity to Mott behavior.

We thank V. Brouet, W. Ku, B. Moritz and I. Mazin for helpful discussions. ARPES experiments were performed at the Stanford Synchrotron Radiation Lightsource and the Advanced Light Source, which are 