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.
@article{arxiv.1604.04964,
title = {Superconductivity in undoped CaFe2As2 single crystals},
author = {Dong-Yun Chen and Jia Yu and Bin-Bin Ruan and Qi Guo and Lei Zhang and Qing-Ge Mu and Xiao-Chuan Wang and Bo-Jin Pan and Gen-Fu Chen and Zhi-An Ren},
journal= {arXiv preprint arXiv:1604.04964},
year = {2017}
}