Novel Electronic State and Superconductivity in the Electron-Doped High-Tc T'-Superconductors

摘要

In this review article, we show our recent results relating to the undoped(Ce-free) superconductivity in the electron-doped high-Tc cuprates with theso-called T' structure. For an introduction, we briefly mention thecharacteristics of the electron-doped T'-cuprates, including the reductionannealing, conventional phase diagram and undoped superconductivity. Then, ourtransport and magnetic results and results relating to the superconductingpairing symmetry of the undoped and underdoped T'-cuprates are shown.Collaborating spectroscopic and nuclear magnetic resonance results are alsoshown briefly. It has been found that, through the reduction annealing, astrongly localized state of carriers accompanied by an antiferromagneticpseudogap in the as-grown samples changes to a metallic and superconductingstate with a short-range magnetic order in the reduced superconducting samples.The formation of the short-range magnetic order due to a very small amount ofexcess oxygen in the reduced superconducting samples suggests that theT'-cuprates exhibiting the undoped superconductivity in the parent compoundsare regarded as strongly correlated electron systems, as well as the hole-dopedhigh-Tc cuprates. We show our proposed electronic structure model to understandthe undoped superconductivity. Finally, unsolved future issues of theT'-cuprates are discussed.