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 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 microscale-homogenous superconductor. The wavelength dispersive spectroscopy and specific heat show the presence of defects which 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 {\deg}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 ~ 49 K in (Ca1-xRx)Fe2As2 and are consistent with the interface-enhanced superconductivity recently proposed.
@article{arxiv.1602.04266,
title = {Evidence for Defect-induced Superconductivity up to 49 K in (Ca1-xRx)Fe2As2},
author = {L. Z. Deng and B. Lv and K. Zhao and F. Y. Wei and Y. Y. Xue and Z. Wu and C. W. Chu},
journal= {arXiv preprint arXiv:1602.04266},
year = {2016}
}