Effects on superconductivity of transition-metal doping in FeSe(0.5)Te(0.5)

We investigate superconductivity in the compound FeSe(0.5)Te(0.5) and in its transition-metal-substituted derivatives Fe(1-x)TM(x)Se(0.5)Te(0.5), where x = 5% and the substituent ions studied were Mn, Co, Ni, Cu and Zn. Electronic and magnetic measurements indicate that doping with Mn or by Co acts respectively to cause a slight enhancement or suppression of the transition temperature. However, doping with this concentration of Ni or Cu destroys the superconductivity completely, and leads to semiconducting behaviour. Zn ions cannot be incorporated properly into the parent compound. The reasons for these contrasting effects are associated with the differing magnetic properties of the substituent ions, which determine their local impurity moments and the net carrier concentrations in the doped 11 system. The effects of magnetic ion substitution on superconductivity suggest that the pairing symmetry may not be either pure s wave or pure d wave.