Abstract
| - In the high- Tc potassium-intercalated FeSe, there has been significant debate regarding what the exact parent compound is. Here we show that the Fe-vacancy ordered K 2Fe 4Se 5 is the magnetic, Mott insulating parent compound of the superconducting state. Non-superconducting K 2Fe 4Se 5 becomes a superconductor after high-temperature annealing, and the overall picture indicates that superconductivity in K 2− xFe 4+ ySe 5 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.
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