  In this review article, we show our recent results relating to the undoped
(Ce-free) superconductivity in the electron-doped high-Tc cuprates with the
so-called T' structure. For an introduction, we briefly mention the
characteristics of the electron-doped T'-cuprates, including the reduction
annealing, conventional phase diagram and undoped superconductivity. Then, our
transport and magnetic results and results relating to the superconducting
pairing symmetry of the undoped and underdoped T'-cuprates are shown.
Collaborating spectroscopic and nuclear magnetic resonance results are also
shown briefly. It has been found that, through the reduction annealing, a
strongly localized state of carriers accompanied by an antiferromagnetic
pseudogap in the as-grown samples changes to a metallic and superconducting
state 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 of
excess oxygen in the reduced superconducting samples suggests that the
T'-cuprates exhibiting the undoped superconductivity in the parent compounds
are regarded as strongly correlated electron systems, as well as the hole-doped
high-Tc cuprates. We show our proposed electronic structure model to understand
the undoped superconductivity. Finally, unsolved future issues of the
T'-cuprates are discussed.
