Coexistence of different electronic phases in the K_(0.8)Fe_(1.6)Se_2 superconductor: A bulk-sensitive hard x-ray spectroscopy study

摘要

We have studied electronic and magnetic properties of the K_(0.8)Fe_(1.6)Se_2 superconductor by x-ray absorption and emission spectroscopy. Detailed temperature-dependent measurements along with a direct comparison with the binary FeSe system have revealed coexisting electronic phases: a majority phase with a high-spin ~(3+)Fe state and a minority phase with an intermediate-spin ~(2+)Fe state. The effect of high-temperature annealing suggests that the compressed phase with a lower-spin ~(2+)Fe state is directly related to the high-T_c superconductivity in the title system. The results clearly underline the glassy nature of superconductivity in the electronically inhomogeneous K_(0.8)Fe_(1.6)Se_2, similar to the superconductivity in granular phases.