Transition-metal substitutions in iron chalcogenides

The $ab$-plane resistivity and Hall effect are studied in Fe_1-yMyTe0.65Se0.35 single crystals doped with two transition-metal elements, M = Co or Ni, over a wide doping range, 0≤y≤0.2. The superconducting transition temperature, Tc, reaches zero for Co at y0.14 and for Ni at y0.032, while the resistivity at the Tc onset increases weakly with Co doping, and strongly with Ni doping. The Hall coefficient RH, positive for $y=0$, remains so at high temperatures for all y, while it changes sign to negative at low T for $y>0.135$ (Co) and $y>0.06$ (Ni). The analysis based on a two-band model suggests that at high T residual hole pockets survive the doping, but holes get localized upon the lowering of T, so that the effect of the electron doping on the transport becomes evident. The suppression of the Tc by Co impurity is related to electron doping, while in the case of the Ni impurity strong electron localization most likely contributes to fast decrease of the Tc.