  Establishing the appropriate theoretical framework for unconventional
superconductivity in the iron-based materials requires correct understanding of
both the electron correlation strength and the role of Fermi surfaces. This
fundamental issue becomes especially relevant with the discovery of the iron
chalcogenide (FeCh) superconductors, the only iron-based family in proximity to
an insulating phase. Here, we use angle-resolved photoemission spectroscopy
(ARPES) to measure three representative FeCh superconductors, FeTe0.56Se0.44,
K0.76Fe1.72Se2, and monolayer FeSe film grown on SrTiO3. We show that, these
FeChs are all in a strongly correlated regime at low temperatures, with an
orbital-selective strong renormalization in the dxy bands despite having
drastically different Fermi-surface topologies. Furthermore, raising
temperature brings all three compounds from a metallic superconducting state to
a phase where the dxy orbital loses all spectral weight while other orbitals
remain itinerant. These observations establish that FeChs display universal
orbital-selective strong correlation behaviors that are insensitive to the
Fermi surface topology, and are close to an orbital-selective Mott phase
(OSMP), hence placing strong constraints for theoretical understanding of
iron-based superconductors.
