Linear decrease of critical temperature with increasing Zn substitution   in the iron-based superconductor BaFe1.89-2xZn2xCo0.11As2
  The nonmagnetic impurity effect is studied on the Fe-based BaFe1.89Co0.11As2 superconductor (Tc = 25 K) with Zn substitution for Fe up to 8 at. %, which is achieved by means of high-pressure and high-temperature heating. Tc decreases almost linearly with increasing the Zn content and disappears at ~8 atomic %, being different in the shared phenomenology of the early Zn doping studies, where Tc decreases little. The Tc decreasing rate, however, remains much lower (3.63 K/%) than what is expected for the s(+-)-wave model, implying the model is unlikely. Another symmetry model such as the non-sign reversal s-wave model may better account for the result. 
Jun Li, Yanfeng Guo, Shoubao Zhang, Shan Yu, Yoshihiro Tsujimoto,   Hiroshi Kontani, Kazunari Yamaura, Eiji Takayama-Muromachi
cond-mat.supr-con cond-mat.str-el