Temperature dependence of iron local magnetic moment in phase-separated superconducting chalcogenide

We have studied local magnetic moment and electronic phase separation in superconducting KxFe2-ySe2 by x-ray emission and absorption spectroscopy. Detailed temperature-dependent measurements at the Fe K-edge have revealed coexisting electronic phases and their correlation with the transport properties. By cooling down, the local magnetic moment of Fe shows a sharp drop across the superconducting transition temperature (T-c) and the coexisting phases exchange spectral weights with the low-spin state, gaining intensity at the expense of the higher-spin state. After annealing the sample across the iron-vacancy order temperature, the system does not recover the initial state and the spectral weight anomaly at T-c as well as superconductivity disappear. The results clearly underline that the coexistence of the low-spin and high-spin phases and the transitions between them provide unusual magnetic fluctuations and have a fundamental role in the superconducting mechanism of the electronically inhomogeneous KxFe2-ySe2 system.