A new superconductor of cuprates with unique features.

The mechanism of superconductivity of cuprates remains one of the biggest challenges in condensed matter physics. High-Tc cuprates crystallize into layered perovskite structure typically featured with copper-oxygen octahedra forming corner shared network. Due to strong Jahn-Teller effect the octahedron in high Tc cuprates is elongated, placing the planar Cu3dx2-y2 orbital level at top, almost degenerate with the planar O2px and O2py orbitals, into which holes are doped. This situation is considered to be unique of the cuprates that sustains high Tc. Here we report the discovery of a new cuprate superconductor Ba2CuO4-d with La2CuO4 structure and with critical temperature (Tc) exceeding 70 K.The x-ray absorption measurement gives evidence that this cuprate is doped with a fairly large amount of holes. However, in contrast to the so-far known hole-doped high-Tc cuprates the new cuprate possesses an extraordinarily compressed octahedron with much shorter apical oxygen distance as well as much expanded in-plane Cu-O bond.Nevertheless, Tc is by more than 30 K higher than that for the isostructural hole-doped La2CuO4.In compressed octahedron the Cu3dz2-r2 orbital level is higher than the Cu3dx2-y2 orbital level.This suggests that considerable dz2-r2 orbital character might be mixed up in the states near the Fermi level which has been thought to be harmful for high-Tc superconductivity.The present work sheds renewed light on comprehensive understanding of necessary ingredients for high Tc and stability of pairing state in cuprate superconductors.