Since the discovery of high temperature superconductivity in iron pnictides in 2008, many iron-based superconductors (IBSs) with various crystal structures have been exploited.Among them, superconductivity with the onset transition temperature Tc up to 49 K has been detected in rare earth element doped CaFe2As2 system (CaRE122). The electron doping induced high Tc values clearly exceed the highest Tcs induced by both hole doping (~ 38 K) and chemical or physical pressure (~ 33 K) in the 122-type iron-based superconductors. The occurrence of high-Tc superconductivity in this unique system is quite strange in terms of the much lower Tc observed in the sister compounds bearing structural and chemical similarities. Though the high–Tc superconductivity in Ca1−xRExFe2As2 is of the non-bulk feature, its superconducting origin served as a most important question has attracted tremendous attentions ever since its discovery. In this review, we summarize the previous experimental results observed in this series of superconductors and list the possible key points that may be related to the understanding of the nature for the strange high-Tc superconductivity.
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