Local structural disorder and superconductivity in K$_{x}$Fe$_{2-y}$Se$_{2}$

We report significantly enhanced magnetic moment on K$_{x}$Fe$_{2-y}$Se$_{2}$ single crystals after post-annealing and quenching process. In K$_{x}$Fe$_{2-y}$Se$_{2}$ unit cell, there are two Fe sites, 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$_{x}$Fe$_{2-y}$Se$_{2}$ materials show that quenched K$_{x}$Fe$_{2-y}$Se$_{2}$ materials have increased average Fe-Se bond length and decreased static disorder. This result indicates that occupancy of Fe1 sites increased after post-annealing and quenching process. This result provides clear evidence that Fe1 sites carry higher magnetic moment than Fe2 sites.