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

We have investigated the electronic properties of ${\mathrm{BiS}}_{2}$-based superconductors by using x-ray photoemission spectroscopy (XPS). In going from $x=0.3$ to 0.5 in ${\mathrm{PrO}}_{1\ensuremath{-}x}{\mathrm{F}}_{x}{\mathrm{BiS}}_{2}$, the Pr $3d$ and Pr $4d$ peaks are shifted by $\ensuremath{\sim}0.10\ifmmode\pm\else\textpm\fi{}0.05$ eV from the Fermi level, partially consistent with the electron doping. In ${\mathrm{PrO}}_{1\ensuremath{-}x}{\mathrm{F}}_{x}{\mathrm{BiS}}_{2}$, the ${\mathrm{Pr}}^{3+}\ensuremath{-}{\mathrm{Pr}}^{4+}$ mixed valence remains unchanged with the electron doping from $x=0.3$ to 0.5. In ${\mathrm{CeO}}_{1\ensuremath{-}x}{\mathrm{F}}_{x}{\mathrm{BiS}}_{2}$, the doped electrons for $x=0.5$ almost suppress the ${\mathrm{Ce}}^{3+}\ensuremath{-}{\mathrm{Ce}}^{4+}$ valence fluctuation. Although the core-level peaks are also shifted by $\ensuremath{\sim}0.10\ifmmode\pm\else\textpm\fi{}0.05$ eV towards the higher-binding-energy side with the electron doping from $x=0$ to 0.5 in ${\mathrm{CeO}}_{1\ensuremath{-}x}{\mathrm{F}}_{x}{\mathrm{BiS}}_{2}$, the Bi $4{f}_{7/2}$ 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.