Local structural disorder and superconductivity in K _xFe _2-ySe ₂

We report significantly enhanced magnetic moment in K _0.69(2)Fe _1.45(1)Se _2.00(1) single crystals with sharp T _c and bulk superconductivity obtained by postannealing and quenching process. There are two Fe sites in the K _0.69(2)Fe _1.45(1)Se _2.00(1) unit cell: Fe1, which has higher symmetry with longer average Fe-Se bond length, and Fe2, which has lower symmetry with shorter average Fe-Se bond length. Temperature-dependent x-ray absorption fine-structure (XAFS) analysis results on quenched and as-grown K _0.69(2)Fe _1.45(1)Se _2.00(1) crystals show that quenched K _0.69(2)Fe _1.45(1)Se _2.00(1) have increased average Fe-Se bond length and decreased static disorder. Our results indicate that nonzero population of Fe1 sites is the key structural parameter that governs the bulk superconductivity. We also show clear evidence that Fe1 sites carry higher magnetic moment than Fe2 sites.