Superconducting symmetry studied from impurity effects in single-crystal Fe_1-yMySe_0.3Te_0.7 (M = Co, Ni, Zn)

We have investigated the suppression of the superconducting transition temperature, T_c, with an increase of the residual resistivity, ρ₀, through the substitution of M (M = Co, Ni, Zn) for Fe in Fe_1-yMySe_0.3Te_0.7 single crystals, in order to clarify the symmetry of the superconducting gap in FeSe_1-xTe _x. Small, large and very small suppression of Tc have been observed through the Co, Ni, and Zn substitution, respectively. The magnitude of the suppression rate is hardly explained in terms of the pair-breaking effect due to potential scattering calculated based on the Abrikosov- Gor'kov theory, even if errors in the estimation of the carrier concentration, effective mass and ρ₀ were taken into account. Accordingly, these results suggest that the superconducting symmetry is the s₊₊-wave in FeSe _1-xTe_x.