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

We elucidate the existing controversies in the newly discovered K-doped iron selenide (KxFe2ySe2z) superconductors. The stoichiometric KFe2Se2 with ffiffiffi 2 p ffiffiffi 2 p charge ordering was identified as the parent compound of KxFe2ySe2z 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 KxFe2ySe2z: parent compound KFe2Se2, superconducting KFe2Se2 with ffiffiffi 2 p ffiffiffi 5 p charge ordering, superconducting KFe2Se2z with Se vacancies, and insulating K2Fe4Se5 with ffiffiffi 5 p ffiffiffi 5 p Fe vacancy order. The phase separation takes place at the mesoscopic scale under standard molecular beam epitaxy conditions. D