New Class of T'-structure Cuprate Superconductors*

High-temperature superconductivity has been discovered in La$_{2-x}$Ba$_{x}$CuO$_{4}$ that derives from the undoped mother compound La$_{2}$CuO$_{4}$ crystallizing in the K$_{2}$NiF$_{4}$ (so-called $T)$ structure with oxygen octahedra surrounding the copper ions. It has been common sense that high-temperature superconductivity develops upon doping such an antiferromagnetic Mott-insulator with charge carriers. La$_{2}$CuO$_{4}$ is also the basis of the electron-doped cuprate superconductors of the form La$_{2-x}$Ce$_{x}$CuO$_{4+y}$, which however crystallize in the Nd$_{2}$CuO$_{4}$ ($T')$ structure without apical oxygen above or below the copper ions of the CuO$_{2}$-plane. Due to the vicinity to the structural phase transition into the $T$-structure the study of the undoped or low doped mother compound with $T'$-structure is difficult. However, using the \textit{iso}valent substituents Y, Sm, Eu, Gd, Tb, or Lu for La, nominally undoped La$_{2}$CuO$_{4}$ can be synthesized by molecular beam epitaxy in the $T'$-structure. The surprising result is that all these nominally \textit{un}doped $T'$-compounds are \textit{superconductors} with fairly high critical temperatures over 20 K. This suggests a phase diagram for this new class of electron doped cuprates, in which the Mott-insulating, antiferromagnetic ground state is not obtained.