Quenching of superconductivity by Co doping in K0.8Fe2Se2
  We synthesized a series of K0.8Fe2-xCoxSe2 samples with nominal compositions 0\leq x\leq 0.035 and investigated their physical properties. The results show that the superconductivity in K0.8Fe2-xCoxSe2 is quenched down to 5 K by 0.5 at. % Co doping, the fastest quenching rate ever-reported. The role played here by Co is in contrast with the one in FeAs based superconductors where Co usually induces superconductivity from parent compounds. Such a rapid quenching favors a localized 3d model against the itinerant one for iron pnictide superconductors. 
Tingting Zhou, Xiaolong Chen, Jiangang Guo, Gang Wang, Xiaofang Lai,   Shunchong Wang, Shifeng Jin, and Kaixing Zhu
cond-mat.supr-con cond-mat.str-el