Superconductivity Tuned by the Iron Vacancy Order in K _<i>x</i> Fe _{2− <i>y</i>} Se ₂

Abstract

Combining in-depth neutron diffraction and systematic bulk studies, we discover that the 5 � 5 Fe vacancy order, with its associated block antiferromagnetic order, is the ground state with varying occupancy ratios of the iron 16i and vacancy 4d sites across the phase-diagram of K xFe2-ySe2. The orthorhombic order, with one of the four Fe sites vacant, appears only at intermediate temperatures as a competing phase. The material experiences an insulator to metal crossover when the 5 � 5 order is highly developed. Superconductivity occurs in such a metallic phase. © 2013 Chinese Physical Society and IOP Publishing Ltd.