  Undoped cuprates have long been considered to be antiferromagnetic
insulators. In this article, however, we report that superconductivity is
achieved in undoped T'-RE2CuO4 (RE = Pr, Nd, Sm, Eu, and Gd). Our discovery was
performed 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.
