  While the parent compounds of the cuprate high temperature superconductors
(high-Tc's) are Mott insulators, the iron-pnictide high-Tc's are in the
vicinity of a metallic spin density wave (SDW) state, which highlights the
difference between these two families. However, insulating parent compounds
were identified for the newly discovered KxFe2-ySe2. This raises an intriguing
question as to whether the iron-based high-Tc's could be viewed as doped Mott
insulators like the cuprates. Here we report angle-resolved photoemission
spectroscopy (ARPES) evidence of two insulating and one semiconducting phases
of KxFe2-ySe2, and the mesoscopic phase separation between the
superconducting/semiconducting phase and the insulating phases. The insulating
phases are characterized by the depletion of electronic states over a 0.5 eV
window below the chemical potential, giving a compelling evidence for the
presence of Mott-like physics. The charging effects and the absence of band
folding in the superconducting/semiconducting phase further prove that the
static magnetic and vacancy orders are not related to the superconductivity.
Instead, the electronic structure of the superconducting phase is much closer
to the semiconducting phase, indicating the superconductivity is likely
developed by doping the semiconducting phase rather than the insulating phases.
