Localized and mixed valence state of Ce4fin superconducting and ferromagneticCeO1−xFxBiS2

Abstract

We have performed Ce L3-edge x-ray absorption spectroscopy (XAS) and Ce 4d-4f resonant photoemission spectroscopy (PES) on single crystals of CeO1−xFxBiS2 for x = 0.0 and 0.5 in order to investigate the Ce 4f electronic states. In the Ce L3-edge XAS, mixed valence of Ce was found in the x = 0.0 sample and the F-doping suppresses it, which is consistent with the results on polycrystalline samples. As for the resonant PES, we found that the Ce 4f electrons in both x = 0.0 and 0.5 systems respectively form a flat band at 1.0 eV and 1.4 eV below the Fermi level and there is no contribution to the Fermi surfaces. Interestingly, Ce valence in CeOBiS2 deviates from 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 CeOBiS2 is mixed with the unoccupied Bi 6pz, which is consistent with the previous local structural study. Based on the analysis of the Ce L3-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.

Ce 4f electronic states of CeO1−xFxBiS2 studied by sof

“…This result suggests that Ce 4f electrons hardly contribute to the states near the E F and therefore give a negligible contribution to the superconductivity. This result is, furthermore, consistent with the recent resonant photoemission result for single-crystal CeO 0.5 F 0.5 BiS 2 using 4d-4f resonant photoemission [29].…”

“…More importantly, Ce 3d-4f resonant SXPES can provide Ce 4f partial density of states and therefore directly reveal the relation of Ce 4f states with conducting properties, although there are few reports on the Ce 3d-4f resonant SXPES of layered compounds consisting of alternatively stacked CeO and metallic/semiconducting layers so far [26][27][28]. Recent reports on Ce 4d-4f resonant SXPES for CeO 1−x F x BiS 2 shows 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 E F and there was no contribution to the Fermi surfaces [29]. Ce 4d-4f resonant SXPES is, however, surface sensitive and sometimes shows different results from more bulk-sensitive Ce 3d-4f resonant SXPES [30].…”

Bulk Superconductivity Induced by Se Substitution in BiCh2-Based Layered Compounds Eu0.5Ce0.5FBiS2−xSex

“…However, the valence states of Eu and Ce in Eu0.5Ce0.5FBiS2xSex would be essentially sensitive to its chemical composition and/or crystal structure, as recently reported in Eu0.5La0.5FBiS2xSex [33]. In addition, X-ray absorption spectroscopy revealed that mixed valence of Ce ions was suppressed by F doping for CeO1xFxBiS2 [34,35]. It was suggested that Ce-S-Bi coupling channel is essential to drive the system from the valence fluctuation regime to the Kondo-like regime.…”

Electronic structure of self-doped layeredEu3F4Bi2S4material revealed by x-ray absorption spectroscopy and photoelectron spectromicroscopy

“…Below the Eu 4f flat band, we observed several bands, and they are most likely from F 2p and S 3p contributions. The existence of Eu 4f flat band just below the electronlike pockets is similar to the CeO 1−x F x BiS 2 system, where the Ce 4f flat band exists just below the electronlike pockets [27]. In the CeO 1−x F x BiS 2 system, the Ce 4f electrons are found to be in a well-localized state [28].…”