Defect-induced Superconductivity up to 49 K in (Ca1-xRx)Fe2As2

To explore the origin of the unusual non-bulk superconductivity with a Tc up to 49 K reported in the rare-earth-doped CaFe2As2 , the chemical composition, magnetization, specific heat, resistivity and low temperature annealing effect are systematically investigated on nominal (Ca1-xRx)Fe2As2 single crystals with different x's and R $=$ La, Ce, Pr and Nd. All display a doping independent Tc once superconductivity is induced, a doping dependent low field superconducting volume fraction f, and a large magnetic anisotropy $\eta $ in the superconducting state, suggesting a rather inhomogeneous superconducting state in an otherwise chemically ``homogeneous'' superconductor. The wavelength dispersive spectroscopy, specific heat and magnetization measurements show the presence of defects which form superparamagnetic clusters for R $=$ Ce, Pr and Nd, but not for La and display both inter and intra-cluster interactions, implying that defects are locally self-organized. Low temperature annealing reduces only the residual strain in the samples without varying x and suppresses f profoundly; however, the Tc was unaffected. The above observations are consistent with the interface-enhanced superconductivity recently proposed and also demonstrates the crucial role of defects in the occurrence of the unusually high Tc \textasciitilde 49 K in (Ca1-xRx)Fe2As2.