Role of the Ce valence in the coexistence of superconductivity and ferromagnetism of CeO$_{1-x}$F$_{x}$BiS$_{2}$ revealed by Ce $L_3$-edge x-ray absorption spectroscopy

TL;DR

This study investigates how fluorine doping affects the valence state of cerium in CeO$_{1-x}$F$_x$BiS$_2$, revealing a transition from mixed valency to a stable Ce$^{3+}$ state associated with the coexistence of superconductivity and ferromagnetism.

Contribution

The paper demonstrates that F-doping suppresses Ce valence fluctuation, leading to a Kondo-like regime that enables coexistence of superconductivity and ferromagnetism in CeO$_{1-x}$F$_x$BiS$_2$.

Findings

Ce exhibits mixed valency in CeOBiS$_2$.

F-doping reduces Ce valence fluctuation.

Superconductivity and ferromagnetism coexist for x>0.4.

Abstract

We have performed Ce $L_3$-edge x-ray absorption spectroscopy (XAS) measurements on CeO$_{1-x}$F$_x$BiS$_2$, in which the superconductivity of the BiS$_2$ layer and the ferromagnetism of the CeO$_{1-x}$F$_x$ layer are induced by the F-doping, in order to investigate the impact of the F-doping on the local electronic and lattice structures. The Ce $L_3$-edge XAS spectrum of CeOBiS$_2$ exhibits coexistence of $4f^1$ (Ce$^{3+}$) and $4f^0$ (Ce$^{4+}$) state transitions revealing Ce mixed valency in this system. The spectral weight of the $4f^0$ state decreases with the F-doping and completely disappears for $x>0.4$ where the system shows the superconductivity and the ferromagnetism. The results suggest that suppression of Ce-S-Bi coupling channel by the F-doping appears to drive the system from the valence fluctuation regime to the Kondo-like regime, leading to the coexistence of the superconducting BiS$_2$ layer and the ferromagnetic CeO$_{1-x}$F$_x$ layer.