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In this paper, we perform Fe-site substitution experiments on K0.8Fe2-ySe2 superconductor by choosing Cr, Mn, Co, and Zn as dopants. It is found that the doping of Cr, Co, and Zn leads to a drastic depression of superconductivity, while the introduction of Mn does not decrease the superconducting transition temperature (Tc) when x ⩽ 0.067. Electron-spin-resonance measurements reveal the existence of strong magnetic fluctuation in both the parent compound and the doped samples. In the Cr-, Co-, and Zn-doped samples, the magnetic pair-breaking effect severely affects the superconductivity, resulting in drastic depression of superconductivity. On the other hand, in the Mn-doped case, it does not break the cooper pairs and thus hardly affects the Tc value. [ABSTRACT FROM AUTHOR]

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]