KFe2Se2 is the Parent Compound of K-Doped Iron Selenide Superconductors

We elucidate the existing controversies in the newly discovered K-doped iron selenide ( KxFe2-ySe2-z) superconductors. The stoichiometric KFe2Se2 with root 2 x root 2 p charge ordering was identified as the parent compound of KxFe2-ySe2-z superconductor using scanning tunneling microscopy and spectroscopy. The superconductivity is induced in KFe2Se2 by either Se vacancies or interacting with the antiferromagnetic K2Fe4Se5 compound. In total, four phases were found to exist in KxFe2-ySe2-z: parent compound KFe2Se2, superconducting KFe2Se2 with root 2 x root 5 charge ordering, superconducting KFe2Se2-z with Se vacancies, and insulating K2Fe4Se5 with root 5 x root 5 Fe vacancy order. The phase separation takes place at the mesoscopic scale under standard molecular beam epitaxy conditions.