Insular Superconductivity in a Co-Doped Iron Pnictide CaFe1-xCoxAsF

The presence of a macroscopic phase separation between the superconducting and magnetic phases in CaFe1-xCoxAsF is demonstrated by muon spin rotation measurements conducted across their phase boundaries (x=0.05-0.15). The magnetic phase tends to retain the high transition temperature (T-m > T-c), while Co doping induces strong randomness. The volumetric fraction of the superconducting phase is nearly proportional to the Co content x with a constant superfluid density. These observations suggest the formation of superconducting islands (or domains) associated with Co ions in the Fe2As2 layers, indicating a very short coherence length.