Following the discovery of superconductivity in an iron-based arsenide LaO1-xFxFeAs with a superconducting transition temperature (Tc) of 26 K[1], Tc was pushed up surprisingly to above 40 K by either applying pressure[2] or replacing La with Sm[3], Ce[4], Nd[5] and Pr[6]. The maximum Tc has climbed to 55 K, observed in SmO1-xFxFeAs[7, 8] and SmFeAsO1-x[9]. The value of Tc was found to increase with decreasing lattice parameters in LnFeAsO1-xFx (Ln stands for the lanthanide elements) at an apparently optimal doping level. However, the F- doping in GdFeAsO is particularly difficult[10,11] due to the lattice mismatch between the Gd2O2 layers and Fe2As2 layers. Here we report observation of superconductivity with Tc as high as 56 K by the Th4+ substitution for Gd3+ in GdFeAsO. The incorporation of relatively large Th4+ ions relaxes the lattice mismatch, hence induces the high temperature superconductivity.
@article{arxiv.0804.4290,
title = {Thorium-doping induced superconductivity up to 56 K in Gd1-xThxFeAsO},
author = {Cao Wang and Linjun Li and Shun Chi and Zengwei Zhu and Zhi Ren and Yuke Li and Yuetao Wang and Xiao Lin and Yongkang Luo and Shuai Jiang and Xiangfan Xu and Guanghan Cao and Zhu'an Xu},
journal= {arXiv preprint arXiv:0804.4290},
year = {2008}
}