Effect of Co Impurities on Superconductivity of FeSe0.4Te0.6 single   crystals
  The effect of Co doping on supercoductivity of FeSe$_{0.4}$Te$_{0.6}$ single crystals is investigated. The superconducting transition temperature decreases linearly for Co doping with the rate -0.75 K/(Co %). On the other hand, the residual resistivity increase is less than 50 $/mu/Omega$cm for 4 % Co doping. These data are consistent with the sign changing interband scattering mechanism of superconductivity ($s_/pm$ symmetry). 
Fuyuki Nabeshima, Yusuke Kobayashi, Yoshinori Imai, Ichiro Tsukada,   and Atsutaka Maeda
cond-mat.supr-con