Influence of Iron Substitutions on the Transport Properties of $FeTe_{0.65}Se_{0.35}$ Single Crystals

We study the ab-plane resistivity and Hall effect in the single crystals of $Fe_{1-y}M_yTe_{0.65}Se_{0.35}$, where M = Co or Ni (0 ≤ y ≤ 0.21). In case of each dopant two types of crystals, with different crystalline quality, are prepared by Bridgman's method using different cooling rates, fast or slow. The impurities suppress the superconducting transition temperature, $T_c$, with different rates. $T_c$ reaches zero at markedly different impurity content: only 3 at.% of Ni, and about 14 at.% of Co. In addition, the suppression is somewhat dependent on the crystal cooling rate. The resistivity at the onset of superconductivity rises only weakly with the Co doping, while it increases 10 times faster for Ni. The Hall coefficient $R_{H}$ is positive for Co doping indicating that hole carriers dominate the transport. For Ni $R_{H}$ changes sign into negative at low temperatures for crystals with the Ni content exceeding 6 at.%. The implications of these results are discussed.