  In this article, we report the superconductivity in T'-RE2CuO4 (RE = Pr, Nd,
Sm, Eu, and Gd), which have been for a long time believed as a Mott insulator.
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 firing with low partial-pressure of oxygen and
reduction at low temperatures. The highest Tc of undoped T'-RE2CuO4 is over 30
K, substantially higher than "electron-doped" analogs. Remarkably, Gd2CuO4,
even the derivatives of which have not shown superconductivity so far, gets
superconducting with Tconset as high as 20 K. The implication of our discovery
is briefly discussed.
