Pressure-induced competition between superconductivity and Kondo effect in CeFeAsO_1-xF_x (x=0.16 and 0.3)

We studied high-pressure behavior of CeFeAsO_1−xF_x superconductors with x=0.16 and x=0.3 by in situ measurements of electrical resistance, X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS) with diamond anvil cell (DAC). A pressure-induced quantum phase transition from the superconducting state to the non-superconducting Kondo screened phase associated with a volume collapse was discovered in the layered CeFeAsO_1−xF_x compounds. The XAS data of Ce-L₃ in CeFeAsO_0.7F_0.3 clearly show a spectral weight transfer from the main line to the satellite line after the transition, demonstrating that Ce's valence changes under high pressure. Comprehensive experimental results and analysis in this paper provide some insight into the connection among superconductivity, valence change and structural phase transition, which reveals a picture of pressure-induced competition between Kondo singlet and BCS singlet in the Ce-pnictide superconductors.