Disordered Fe vacancies and superconductivity in potassium-intercalated iron selenide (K2−xFe4+ySe5)

In the high-T c potassium-intercalated FeSe, there has been significant debate regarding what the exact parent compound is. Here we show that the Fe-vacancy ordered K2Fe4Se5 is the magnetic, Mott insulating parent compound of the superconducting state. Non-superconducting K2Fe4Se5 becomes a superconductor after high-temperature annealing, and the overall picture indicates that superconductivity in K2−x Fe4+y Se5 originates from the Fe-vacancy order-to-disorder transition. Thus, the long-pending question as to whether magnetic and superconducting state are competing or cooperating for cuprate superconductors may also apply to the Fe-chalcogenide superconductors. It is believed that the iron selenides and related compounds will provide essential information to understand the origin of superconductivity in the iron-based superconductors, and possibly to the superconducting cuprates.