Role of impurity oxygen in superconductivity of “non-doped” T′-(La, RE)2CuO4

We have systematically investigated the effect of oxygen non-stoichiometry in a nominally undoped superconductor T′-(La, Y)2CuO4+y. In the experiments, the reduction condition was changed after the sample growth by MBE. The superconductivity is very sensitive to the reduction condition. With systematically increasingly reduced atmospheres, resistivity shows a continuous drop and no discontinuity is observed even until the appearance of superconductivity. The absence of the highly insulating state expected for Mott insulators around y ∼ 0 suggests that T′-(La, Y)2CuO4 has intrinsic carriers. The role of residual apical oxygen, which is detrimental to superconductivity, is also discussed based on the resistivity-temperature characteristics in insufficiently reduced samples.