Superconductivity at 33 K in “111” single crystals at ambient pressure T.-L. Xia, J. B. He, D. M. Wang, and G. F. Chen ∗ Department of Physics, Renmin University of China, Beijing 100872, P. R. China (Dated: October 16, 2018) We have successfully grown single crystalline NaFeAs with cobalt or phosphor doping. Bulk superconductivity occurs in NaFe 0 . 95 Co 0 . 05 As at 19 K while much higher transition temperature is observed in NaFeAs 0 . 8 P 0 . 2 , in which the Tc of 33 K is even higher than the highest value realized by applying pressure in NaFeAs and all other isoelectronic element substituted samples. We discuss these behaviours by comparison with 122 and 1111 systems. We hope that our findings will help improve our understanding of iron-based superconductivity. PACS numbers: 74.70.Xa, 74.25.F-, 74.25.Op ∗ Electronic address: genfuchen@ruc.edu.cn 1 arXiv:1001.3311v2 [cond-mat.supr-con] 3 Feb 2010 Abstract