Superconductivity in undoped CaFe2As2 single crystals
  Single crystals of undoped CaFe2As2 were grown by a FeAs self-flux method, and the crystals were quenched in ice-water rapidly after high temperature growth. The quenched crystal undergoes a collapsed tetragonal structural phase transition around 80 K revealed by the temperature dependent X-ray diffraction measurements. Superconductivity below 25 K was observed in the collapsed phase by resistivity and magnetization measurements. The isothermal magnetization curve measured at 2 K indicates that this is a typical type-II superconductor. For comparison, we systematically characterized the properties of the furnace cooled, quenched, and post-annealed single crystals, and found strong internal crystallographic strain existing in the quenched samples, which is the key for the occurrence of superconductivity in the undoped CaFe2As2 single crystals. 
Dong-Yun Chen, Jia Yu, Bin-Bin Ruan, Qi Guo, Lei Zhang, Qing-Ge Mu,   Xiao-Chuan Wang, Bo-Jin Pan, Gen-Fu Chen, Zhi-An Ren
cond-mat.supr-con cond-mat.mtrl-sci