Contrasting effects of magnetic ions on the superconductivity in Tl0.4K0.4Fe2-y-xMxSe2 (M = Mn and Ni).

The transport and magnetic properties together with the superconductivity of Tl0.4K0.4Fe2-y-xMxSe2 (M = Mn and Ni) single crystals are studied. On the transport side, both Mn doping and Ni doping lead to a decrease of charge carrier density. However, contrasting effects on superconductivity are found, i.e. substitution of Fe by Ni results in a drastic depression of the onset superconducting transition temperature (Tconset ), while the Tconset value remains nearly unchanged in Mn-doped samples. A remarkable ferromagnetic interaction is found in Ni-doped samples but is absent in Mn-doped samples. The present results suggest that the magnetic pair-breaking effect alone cannot explain the different behavior of depression of superconductivity in Mn-doped and Ni-doped samples. The occurrence of ferromagnetic interaction is responsible for the rapid decrease of Tc onset in Ni-doped samples. [ABSTRACT FROM AUTHOR]