Electronic structure and two-band superconductivity in unconventional high-Tc cuprates Ba2 CuO3+δ

The recently discovered cuprate superconductor Ba2CuO3+delta exhibits a high T-c similar or equal to 73 K at delta similar or equal to 0.2. The polycrystal grown under high pressure has a structure similar to La2CuO4 but with dramatically different lattice parameters due to the CuO6 octahedron compression. The crystal field in the compressed Ba2CuO4 leads to an inverted Cu 3d e(g) complex with the d(x2-y2) orbital sitting below the d(3z2)-(r2) and an electronic structure highly unusual compared to the conventional cuprates. We construct a two-orbital Hubbard model for the Cu d(9) state at hole doping x = 2 delta and study the orbital-dependent strong correlation and superconductivity. For the undoped case at x = 0, we found that strong correlation drives an orbital-polarized Mott-insulating state with the spin-1/2 moment of the localized d(3z2)-(r)(2 )orbital. In contrast to the single-band cuprates where superconductivity is suppressed in the overdoped regime, hole doping the two-orbital Mott insulator leads to orbital-dependent correlations and the robust spin and orbital exchange interactions produce a high-T, antiphase d-wave superconductor even in the heavily doped regime at x = 0.4. We conjecture that Ba2CuO3+delta realizes mixtures of such heavily hole-doped superconducting Ba2CuO4 and disordered Ba2CuO3 chains in a single-layer or predominately separated bilayer structure. Our findings suggest that unconventional cuprates with liberated orbitals as doped two-band Mott insulators can be a direction for realizing high-T-c, superconductivity with enhanced transition temperature T-c.