Electronic structure and phase separation of superconducting and nonsuperconductingKxFe2−ySe2revealed by x-ray photoemission spectroscopy

Abstract

We have investigated the electronic structure of superconducting (SC) and nonsuperconducting (non-SC) K${}_{x}$Fe${}_{2\ensuremath{-}y}$Se${}_{2}$ using x-ray photoemission spectroscopy (XPS). The spectral shape of the Fe 2$p$ XPS is found to depend on the amount of Fe vacancies. The Fe 2${p}_{3/2}$ peak of the SC and non-SC Fe-rich samples is accompanied by a shoulder structure on the lower binding energy side, which can be attributed to the metallic phase embedded in the Fe${}^{2+}$ insulating phase. The absence of the shoulder structure in the non-SC Fe-poor sample allows us to analyze the Fe 2$p$ spectra using a FeSe${}_{4}$ cluster model. The Fe 3$d$-Se 4$p$ charge-transfer energy of the Fe${}^{2+}$ insulating phase is found to be $\ensuremath{\sim}$2.3 eV which is smaller than the Fe 3$d$-Fe 3$d$ Coulomb interaction of $\ensuremath{\sim}$3.5 eV. This indicates that the Fe${}^{2+}$ insulating state is the charge-transfer type in the Zaanen-Sawatzky-Allen scheme. We also find a substantial change in the valence-band XPS as a function of Fe content and temperature. The metallic state at the Fermi level is seen in the SC and non-SC Fe-rich samples and tends to be enhanced with cooling in the SC sample.