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We have performed Ce $L_3$-edge x-ray absorption spectroscopy (XAS) and Ce$4d$-$4f$ resonant photoemission spectroscopy (PES) on single crystals ofCeO$_{1-x}$F$_x$BiS$_2$ for $x=0.0$ and 0.5 in order to investigate the Ce $4f$electronic states. In the Ce $L_3$-edge XAS, mixed valence of Ce was found inthe $x=0.0$ sample and the F-doping suppresses it, which is consistent with theresults on polycrystalline samples. As for the resonant PES, we found that theCe $4f$ electrons in both $x=0.0$ and $0.5$ systems respectively form a flatband at 1.0 eV and 1.4 eV below the Fermi level and there is no contribution tothe Fermi surfaces. Interestingly, Ce valence in CeOBiS$_2$ deviates fromCe$^{3+}$ even though Ce $4f$ electrons are localized, indicating the Cevalence is not in a typical valence fluctuation regime. We assume thatlocalized Ce $4f$ in CeOBiS$_2$ is mixed with the unoccupied Bi $6p_z$, whichis consistent with the previous local structural study. Based on the analysisof the Ce $L_3$-edge XAS spectra using Anderson's impurity model calculation,we found that the transfer integral becomes smaller increasing the number of Ce$4f$ electrons upon the F substitution for O.

physical review. b./physical review. b

Localized and mixed valence state of Ce $4f$ in superconducting and  ferromagnetic CeO$_{1-x}$F$_{x}$BiS$_{2}$ revealed by x-ray absorption and  photoemission spectroscopy