  The observation of non-bulk superconductivity with an unexpectedly high onset
transition temperature Tc up to ~ 49 K in non-superconducting single
crystalline CaFe2As2 upon rare-earth doping has raised interesting questions
concerning its origin. Several possibilities including interfacial mechanism
have been proposed to account for the unusual observation. In an attempt to
differentiate such propositions, we have carried out a systematic compositional
and magnetic study on single crystals of nominal (Ca1-xPrx)Fe2As2 with 0 <= x
<= 0.13 throughout the solubility range of Pr, which covers both the
non-superconducting and superconducting regions. We found the unusual
simultaneous occurrence of superparamagnetism and superconductivity with an
x-independent Tc and a close correlation of the superconducting volume fraction
with the magnetic cluster density and As-defect density. The finding
demonstrates a close relationship among superconductivity, superparamagnetism,
and defects, consistent with the previously proposed interface-mechanism, and
offers a possible future path to higher Tc.
