Renaissance of high-Tc superconductivity - Undoped cuprates are a Mott insulator?

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It has been believed for a long time that high-Tc superconductivity in cuprates develops only upon doping holes or electrons in the parent antiferromagnetic Mott insulators. Recently, however, we discovered that T'-RE2CuO4 becomes superconducting without doping. The purpose of this research project is to establish superconductivity in the true "undoped" state of T'-RE2CuO4 and thereby to renovate the previous "doped Mott-insulator" scenario for high-Tc superconductivity. In the course of the research, we achieved, for the first time, superconductivity in bulk samples of undoped T'-La2-xSmxCuO4, which was synthesized via a molten alkaline hydroxide route. The bulk nature of superconductivity was also confirmed by low-energy muSR. Furthermore the systematic comparison of the Hall effect and upper critical fields between undoped T'-RE2CuO4 and electron-doped T'-(RE,Ce)2CuO4 revealed no discontinuity with doping, leading to the conclusion that both are in the same ground state.