Phase Separation in Electron Doped Iron-Selenide K0.8Fe1.6Se2 Superconductor by Scanning X-ray Nano-Diffraction

Abstract

A new family of high temperature superconductors, the heavily electron doped iron-selenides, like K0.8Fe1.6Se2, has been attracting high interest since they show both 30 K superconductivity, with missing hole pockets questioning the s± pairing model, and unusually high magnetic moments. The hot debate is between coexistence versus phase separation and on the possible divergence of surface from bulk structure. Here, we provide direct evidence for a nanoscale phase separation in a single crystal of K0.8Fe1.6Se2, where a first magnetic phase, with superlattice modulation $(\sqrt{5}\times\sqrt{5})$ , coexists with a second nonmagnetic phase, with a second superlattice modulation ( $\sqrt{2} \times\sqrt{2}$ ), below 520 K using transmission X-ray diffraction. The mapping of the spatial distribution of the two phases is measured by scanning X-ray nanodiffraction using a 300×300 nm2 X-ray spot. The complex spatial phase separation clarifies the coexistence of superconductivity and magnetism in the same sample.