Role of the Ce valence in the coexistence of superconductivity and ferromagnetism of CeO${}_{1-x}$F${}_x$BiS${}_2$ revealed by Ce $L_3$-edge x-ray absorption spectroscopy

Abstract

We have performed Ce $L_3$-edge x-ray absorption spectroscopy (XAS) measurements on CeO${}_{1-x}$F${}_x$BiS${}_2$, in which the superconductivity of the BiS${}_2$ layer and the ferromagnetism of the CeO${}_{1-x}$F${}_x$ 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 CeOBiS${}_2$ exhibits coexistence of $4f^1$ (Ce${}^{3+}$) and $4f^0$ (Ce${}^{4+}$) 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 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 BiS${}_2$ layer and the ferromagnetic CeO${}_{1-x}$F${}_x$ layer.

5 pages, 5 figures

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