Impact of valence fluctuations on the electronic properties of RO1−xFxBiS2(R=Ce and Pr)

We have investigated the electronic properties of BiS2-based superconductors by using x-ray photoemission spectroscopy (XPS). In going from x =0.3 to 0.5 in PrO 1−xFxBiS2, the Pr3d and Pr 4d peaks are shifted by ∼ 0.10±0.05 eV from the Fermi level, partially consistent with the electron doping. In PrO1−xFxBiS2, the Pr3+−Pr4+ mixed valence remains unchanged with the electron doping from x=0.3 to 0.5. In CeO1−xFxBiS2, the doped electrons for x=0.5 almost suppress the Ce3+−Ce4+ valence fluctuation. Although the core-level peaks are also shifted by ∼0.10±0.05 eV towards the higher-binding-energy side with the electron doping from x=0 to 0.5 in CeO1−xFxBiS2, the Bi 4f7/2 binding-energy shift is higher in the Pr system compared with the Ce system. The present results suggest that the doped electrons increase orbital occupations in the rare-earth 4f orbitals at the valence band and show valence fluctuations differently in the two systems.