The Unconventional Copper Oxide Superconductor with Conventional Constitution

A new Ba2CuO4-y superconductor with critical temperature (T-c) exceeding 70 K was discovered. The X-ray absorption measurement gives evidence that this cuprate resembles La2CuO4 but is doped with a fairly large amount of holes, while in contrast to the so far known hole-doped high-T-c cuprates, the new cuprate possesses a much shorter local apical oxygen distance as well as much expanded in-plane Cu-O bond, leading to unprecedented compressed local octahedron. In compressed local octahedron, the Cu3d3z(2)-r(2) orbital level will be lifted above the Cu3dx(2)-y(2) orbital level with more three-dimensional features, implying that pairing symmetry may carry admixtures from more than one gap, suggesting that Ba2CuO4-y composed of alkaline earth copper oxides that are the essential elements to form cuprate superconductors may belong to a new branch of cuprate superconductors.