Correspondence between the electronic structure and phase separation in a K-doped FeSe system.

Phase separated potassium intercalated FeSe thin films have been synthesized by pulsed laser deposition. The coexistence of FeSe phase and 245 phase was investigated both by x-ray photoemission spectroscopy (XPS) and x-ray diffraction. The volume ratio of these two phases is sensitive to temperatures and amount of extra potassium dosing. The XPS and ultraviolet photoelectron spectroscopy results indicated that these two phases shows the different hybridization strength between adjacent Fe layer and Se layer. We infer that the layered electronic structure is the necessary condition of superconductivity in potassium-doped FeSe system, and the phase separation is driven by competition between quasi-2D and 3D bonding mode within FeSe layer. Similar competition may also be able to interpret the phase seperation in K x Fe2-y Se2 bulk single crystal.