Pressure-induced Competition between Superconductivity and Kondo Effect in CeFeAsO$_{1-x}$F$_{x}$ (x=0.16 and 0.3)

We studied high-pressure behavior of CeFeAsO$_{1-x}$F$_{x}$ superconductors with x=0.16 and x=0.3 by the \textit{in-situ} measurements of electrical resistance, x-ray diffraction (XRD) and x-ray absorption spectroscopy (XAS) in a diamond anvil cell. 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$_{1-x}$F$_{x}$ compounds. This finding is the first observation of pressure-induced quantum phase transition from an electron pairing state in the conduction band to a Kondo screened state between conducting electrons and local moments, and reveals a physical picture of competition between Kondo singlet and BCS singlet in the Ce-pnictide superconductors.