Superconductivity in undoped $T$'-RE$_{2}$CuO$_{4}$ with $T_{C }>$ 30 K

We report the superconductivity in $T'$-\textit{RE}$_{2}$CuO$_{4}$ (\textit{RE} = Pr, Nd, Sm, Eu, and Gd), which have been for a long time believed as a Mott insulator. The highest $T_{c}$ of undoped $T'$-\textit{RE}$_{2}$CuO$_{4}$ is over 30 K, substantially higher than that of `electron-doped' analogs. Remarkably, Gd$_{2}$CuO$_{4}$, even the derivatives of which have not shown superconductivity so far, gets superconducting with $T_{c}^{onset}$ as high as 20 K. Our discovery was achieved by using metal-organic decomposition (MOD), an inexpensive and easy-to-implement thin-film process. The keys to prepare the superconducting films are rather simple, namely low-$P_{O2}$ firing and subsequent low-temperature reduction. One point to be emphasized is that low-$P_{O2}$ phase field has been almost unexplored in the search for new superconductors because of the belief that high $P_{O2}$ \textit{should} be required in the synthesis of Cu$^{2+}$ compounds. Our discovery contradicts with the past results supporting undoped mother compounds, $T'$-\textit{RE}$_{2}$CuO$_{4}$, to be insulating. The clue to understanding the sharp contrast between the past and our results is impurity oxygen (O$_{ap})$ at the apical site, which has to be cleaned up in order to reach the `generic' electronic phase diagram.