We have performed Ce $L_3$-edge x-ray absorption spectroscopy (XAS) and Ce $4d-4f$ resonant photoemission spectroscopy (PES) on single crystals of ${\mathrm{CeO}}_{1-x}{\mathrm{F}}_x{\mathrm{BiS}}_2$ for $x=0.0$ and 0.5 in order to investigate the Ce $4f$ electronic states. In Ce $L_3$-edge XAS, a mixed valence of Ce was found in the $x=0.0$ sample, and F doping suppressed it, which is consistent with the results on polycrystalline samples. As for resonant PES, we found that the Ce $4f$ electrons in both $x=0.0$ and 0.5 systems respectively formed a flat band at 1.0 and 1.4 eV below the Fermi level and there was no contribution to the Fermi surfaces. Interestingly, Ce valence in ${\mathrm{CeOBiS}}_2$ deviates from ${\mathrm{Ce}}^{3+}$ even though Ce $4f$ electrons are localized, indicating the Ce valence is not in a typical valence fluctuation regime. We assume that localized Ce $4f$ in ${\mathrm{CeOBiS}}_2$ is mixed with unoccupied Bi $6p_z$, which is consistent with a previous local structural study. Based on the analysis of 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.

• Received 29 March 2016
• Revised 28 July 2016

DOI:https://doi.org/10.1103/PhysRevB.94.081106