Impurity Effects on the Superconducting Transition Temperature of Na_zCoO₂·yH₂O

Abstract

Superconducting transition temperature T_c of Na_zCo_1−xM_xO₂·yH₂O has been studied for M=Ir and Ga, which can substitute for Co. It has been found that T_c is suppressed by the M-atom doping. The decreasing rates |dT_c⁄dx| are of the order of 1 K/% and too small to be explained by the pair breaking mechanism for the anisotropic order parameters by non-magnetic impurities. Brief arguments are given on possible origins of the T_c suppression.