  High-temperature superconductivity has been discovered in La2-xBaxCuO4 [1], a
compound that derives from the undoped La2CuO4 crystallizing in the perovskite
T-structure. In this structure oxygen octahedra surround the copper ions. It is
common knowledge that charge carriers induced by doping in such an undoped
antiferromagnetic Mott-insulator lead to high-temperature superconductivity [2-
4]. The undoped material La2CuO4 is also the basis of the electron-doped
cuprate superconductors [5] of the form La2-xCexCuO4+y [6,7] which however
crystallize in the so called T-prime-structure, i.e. without apical oxygen
above or below the copper ions of the CuO2-plane. It is well known that for
La2-xCexCuO4+y the undoped T-prime-structure parent compound cannot be prepared
due to the structural phase transition back into the T-structure occuring
around x ~ 0.05. Here, we report that if La is substituted by RE = Y, Lu, Sm,
Eu, Gd, or Tb, which have smaller ionic radii but have the same valence as La,
nominally undoped La2-xRExCuO4 can be synthesized by molecular beam epitaxy in
the T-prime-structure. The second important result is that all these new
T-prime-compounds are superconductors with fairly high critical temperatures up
to 21 K. For this new class of cuprates La2-xRExCuO4, which forms the
T-prime-parent compounds of the La-based electron doped cuprates, we have not
been able to obtain the Mott-insulating ground state for small x before the
structural phase transition into the T-structure takes place.
