Ce $4f$ electronic states of ${\mathrm{CeO}}_{1-x}{\mathrm{F}}_x{\mathrm{BiS}}_2$ studied by soft x-ray photoemission spectroscopy

We use soft x-ray photoemission spectroscopy (SXPES) to investigate Ce $4f$ electronic states of a new ${\mathrm{BiS}}_2$ layered superconductor ${\mathrm{CeO}}_{1-x}{\mathrm{F}}_x{\mathrm{BiS}}_2$, for polycrystalline and single-crystal samples. The Ce $3d$ spectrum of the single crystal of nominal composition $x=0.7$ has no $f^0$ component and the spectral shape closely resembles the ones observed for Ce trivalent insulating compounds, strongly implying that the CeO layer is still in an insulating state even after the F doping. The Ce $3d-4f$ resonant SXPES for both polycrystalline and single-crystal samples shows that the prominent peak is located around 1 eV below the Fermi level $\left(E_{\mathrm{F}}\right)$ with negligible spectral intensity at $E_{\mathrm{F}}$. The F-concentration dependence of the valence band spectra for single crystals shows the increases of the degeneracy in energy levels and of the interaction between Ce $4f$ and S $3p$ states. These results give insight into the nature of the ${\mathrm{CeO}}_{1-x}{\mathrm{F}}_x$ layer and the microscopic coexistence of magnetism and superconductivity in ${\mathrm{CeO}}_{1-x}{\mathrm{F}}_x{\mathrm{BiS}}_2$.

Figure 1

Ce $3d$ SXPES spectra of ${\text{CeO}}_{1-x}{\text{F}}_x{\text{BiS}}_2$ single-crystal samples, $x=0.3$ (light green) and 0.7 (dark green), measured with photon energy of 1200 eV. The inset shows the magnified spectra of the $f^1$ and $f^2$ components for $3d_{5/2}$ where the spectra are normalized to the integrated intensities and the spectrum for $x=0.7$ is shifted by $-0.3$ eV to match the peak positions with the ones for $x=0.3$.

Figure 2

Ce $3d-4f$ resonance SXPES spectra of polycrystalline ${\text{CeO}}_{1-x}{\text{F}}_x{\text{BiS}}_2$ [(a) $x=0.0$ and (b) 0.7] and the single crystal [(c) $x=0.7$]. Off- and on-resonance spectra (thin and thick curves, respectively) are measured at two photon energies 870 eV (A) and 881.7 eV (B) in x-ray absorption spectra near the Ce $M_5$ edge shown in the inset, respectively. Each spectrum is magnified by the scale factor denoted on the right side of the figure.

Figure 3

Comparison of the on-resonance spectra in Fig. 2 with the ones (black solid curves) of polycrystalline ${\text{CeFeAsO}}_{0.89}{\text{F}}_{0.11}$, single-crystal ${\text{CeRhIn}}_5$, and single-crystal ${\text{CeFe}}_4{\text{P}}_{12}$, which are taken from Refs. [27], [45], and [46], respectively. They are normalized to the integrated intensities, and the ${\text{CeFeAsO}}_{0.89}{\text{F}}_{0.11}$ spectrum is enlarged twice for the sake of clarity.

Figure 4

(a) F-concentration dependence of the valence band spectra of single crystals measured with photon energy of 1200 eV and angle-integrated for acceptance angle of 11 degrees along $\mathrm{\Gamma }\text{-X}$ centered at the $\mathrm{\Gamma }$ point (about $200\%$ of the BZ). The nominal concentrations of F $\left(x\right)$ are 0.3 and 0.7, respectively. Gray and green Gaussians show fitting results. The dashed lines are assumed Shirley-type backgrounds. (b) Second-derivative intensity map of the energy distribution curves of the ARPES spectra measured along the $\mathrm{\Gamma }\text{-X}$ direction for $x=0.7$ with photon energy of 70 eV. The negatively intense signals show the peak positions of the ARPES spectra. The thick arrow indicates the $4f\text{-derived}$ band and the thin arrow shows the electron pocket band derived mainly from Bi $6p$ [41, 43]. (c) Overlaid comparison of spectra in (a). The spectra are normalized to the integrated intensities.

Figure 5

Schematic picture of F-concentration dependencies of the valence bands.