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)(2)CuO4+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 similar to 0 suggests that T'-(La,Y)(2)CuO4 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. (c) 2005 Elsevier B.V. All rights reserved.