Role of the Ce valence in the coexistence of superconductivity and ferromagnetism of CeO1-xFxBiS2 revealed by Ce L-3-edge x-ray absorption spectroscopy

We have performed Ce L-3-edge x-ray absorption spectroscopy (XAS) measurements on CeO1-xFxBiS2, in which the superconductivity of the BiS2 layer and the ferromagnetism of the CeO1-xFx layer are induced by the F doping, in order to investigate the impact of the F doping on the local electronic and lattice structures. The Ce L-3-edge XAS spectrum of CeOBiS2 exhibits coexistence of 4f(1) (Ce3+) and 4f(0) (Ce4+) state transitions revealing Ce mixed valency in this system. The spectral weight of the 4f(0) state decreases with the F doping and completely disappears for x > 0.4 where the system shows the superconductivity and the ferromagnetism. The results suggest that suppression of the Ce-S-Bi coupling channel by the F doping appears to drive the system from the valence fluctuation regime to the Kondo-like regime, leading to the coexistence of the superconducting BiS2 layer and the ferromagnetic CeO1-xFx layer