Evidence for defect-induced superconductivity up to 49 K in (C a1-xRx) F e2 A s2

To explore the origin of the unusual nonbulk superconductivity with a Tc up to 49 K reported in the rare-earth-doped CaFe2As2, the chemical composition, magnetization, specific heat, resistivity, and annealing effect are systematically investigated on nominal (Ca1-xRx)Fe2As2 single crystals with different x 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 η in the superconducting state, suggesting a rather inhomogeneous superconducting state in an otherwise microscale homogenous superconductor. The wavelength dispersive spectroscopy and specific heat show the presence of defects that are closely related to f, regardless of the R involved. The magnetism further reveals that the defects are mainly superparamagnetic clusters for R=Ce, Pr, and Nd with strong intercluster interactions, implying that defects are locally self-organized. Annealing at 500 °C, without varying the doping level x, suppresses f profoundly but not the Tc. The above observations provide evidence for the crucial role of defects in the occurrence of the unusually high Tc∼49K in (Ca1-xRx)Fe2As2 and are consistent with the interface-enhanced superconductivity recently proposed.