Evidence for Defect-Induced Superconductivity up to 49 K in (Ca₁₋ₓ)Fe₂As₂

To explore the origin of the unusual nonbulk superconductivity with a T_c 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₋ₓ Rₓ)Fe₂As₂ single crystals with different x and R = La, Ce, Pr, and Nd. All display a doping-independent T_c once superconductivity is induced, a doping-dependent low field superconducting volume fraction ʄ, and a large magnetic anisotropy η 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 ʄ, 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 ⁰C, without varying the doping level x, suppresses ʄ profoundly but not the T_c. The above observations provide evidence for the crucial role of defects in the occurrence of the unusually high T_c ~ to 49K in Ca₁₋ₓ Rₓ Fe₂As₂ and are consistent with the interface-enhanced superconductivity recently proposed.