Phase separation and magnetic order in K-doped iron selenide superconductor

The newly discovered alkali-doped iron selenide superconductors(1,2) not only reach a superconducting transition temperature as high as 32 K, but also exhibit unique characteristics that are absent from other iron-based superconductors, such as antiferromagnetically ordered insulating phases(3,4), extremely high Neel transition temperatures(5,6) and the presence of Fe vacancies and ordering(7-10). These features have generated considerable excitement as well as confusion, regarding the delicate interplay between Fe vacancies, magnetism and superconductivity(11-13). Here we report on molecular beam epitaxy growth of high-quality KxFe2-ySe2 thin films and in situ low-temperature scanning tunnelling microscope measurement of their atomic and electronic structures. We demonstrate that a KxFe2-ySe2 sample contains two distinct phases: an insulating phase with well-defined root 5 x root 5 order of Fe vacancies, and a superconducting KFe2Se2 phase containing no Fe vacancies. An individual Fe vacancy can locally destroy superconductivity in a similar way to a magnetic impurity in conventional superconductors. Measurement of the magnetic-field dependence of the Fe-vacancy-induced bound states reveals a magnetically related bipartite order in the tetragonal iron lattice. These findings elucidate the existing controversies on this new superconductor and provide atomistic information on the interplay between magnetism and superconductivity in iron-based superconductors.