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While the parent compounds of the cuprate high temperature superconductors(high-Tc's) are Mott insulators, the iron-pnictide high-Tc's are in thevicinity of a metallic spin density wave (SDW) state, which highlights thedifference between these two families. However, insulating parent compoundswere identified for the newly discovered KxFe2-ySe2. This raises an intriguingquestion as to whether the iron-based high-Tc's could be viewed as doped Mottinsulators like the cuprates. Here we report angle-resolved photoemissionspectroscopy (ARPES) evidence of two insulating and one semiconducting phasesof KxFe2-ySe2, and the mesoscopic phase separation between thesuperconducting/semiconducting phase and the insulating phases. The insulatingphases are characterized by the depletion of electronic states over a 0.5 eVwindow below the chemical potential, giving a compelling evidence for thepresence of Mott-like physics. The charging effects and the absence of bandfolding in the superconducting/semiconducting phase further prove that thestatic magnetic and vacancy orders are not related to the superconductivity.Instead, the electronic structure of the superconducting phase is much closerto the semiconducting phase, indicating the superconductivity is likelydeveloped by doping the semiconducting phase rather than the insulating phases.

Electronic identification of the actual parental phase of KxFe2-ySe2 superconductor and its intrinsic mesoscopic phase separation