Enhanced cuprate superconductivity from elongated to compressed coordination

Abstract

In 1986 Professor Müller and Bednorz discovered the historic superconductor of LaBaCuO (La214) that opens the prelude of unprecedented era for high Tc superconductivity research worldwide. The Jahn Teller effects combining with strong interlayer Coulomb interactions lead to the elongated copper oxygen coordination in La214, giving rise to the orbital order scenario that the Cu3dx2-y2 locates near Fermi surface while the 3dz2 is far below without contributions to the conductivity, hence single band model is favorite. We recently discovered a new type of cuprate superconductor Ba2CuO4-y (Ba214) synthesized at extreme high oxygen pressure that shares a similar crystal structure to the classical La214 but with notably a very rare compressive type octahedron. The compressed octahedron coordination results from the longer in plane CuO bond length than that for apical oxygen bond. The compressed octahedron will lead to a reversed orbital order, i.e. the 3dz2 will be lifted above 3dx2-y2,which is generally considered unfavorable to superconductivity based on the current theories of high Tc cuprates. Ba214 however exhibits bulk superconducting transition with Tc above 70 K at ambient pressure, a surprisingly 80 % enhancement from that for isostructural La214. Here we briefly introduce the unique structure features as well research progress for Ba214, indicating the paradigm shifts roles of 3dz2 orbital in (I) enhancing cuprate superconductivity, (II) favoring multiband scenario; (III) enabling the s wave pairing mechanism.