  We have recently achieved superconductivity in undoped T'-RE2CuO4 (RE: Pr,
Nd, Sm, Eu, and Gd), using epitaxial thin films by metal organic decomposition.
The key recipes to achieve superconductivity are low-PO2 firing and subsequent
vacuum reduction to minimize the amount of impurity oxygen atoms, which are
very harmful to high-Tc superconductivity. In this article, we report our
investigation on the reduction dependence of superconductivity of T'-RE2CuO4.
For thin films, the amount of remnant Oap atoms is difficult to evaluate but we
propose that one good measure for this may be the c-axis lattice constant,
which tells us whether the reduction is insufficient or excessive.
