Electronic structures of quasi-one-dimensional cuprate superconductors Ba2CuO3+δ

An intact CuO2 plane is widely believed to be a prerequisite for the high-T-c superconductivity in cuprate superconductors. However, an exception may exist in the superconducting Ba2CuO3+delta materials where CuO chains play a more important role. From first-principles density functional theory calculations, we have studied the electronic and magnetic structures of Ba2CuO3+delta. The stoichiometric Ba2CuO3 and Ba2CuO4 contain quasi-one-dimensional CuO chains and intact two-dimensional CuO2 planes, respectively. In comparison with the nonmagnetic metal Ba2CuO4, Ba2CuO3 is found to be an antiferromagnetic (AFM) Mott insulator. It possesses a nearest-neighbor intrachain AFM coupling and a weak interchain interaction, and its lowest unoccupied band and highest occupied band are contributed by the Cu 3d(b2-c2) orbital (or d(x2-y2) orbital if we denote the bc plane as the xy plane) and O 2p orbitals, respectively. Total energy calculations indicate that the oxygen vacancies in Ba2CuO3+delta, in reference to conventional cuprate superconductors, prefer to reside in the planar sites rather than the apical oxygens in the CuO chains, in agreement with the experimental observation. Furthermore, we find that moderate charge doping can reduce the energy differences between the low-lying magnetic states of Ba2CuO3+delta, which may induce the spin fluctuations that are beneficial for the appearance of superconductivity. Our results also suggest that the spin fluctuations in Ba2CuO3+delta are anisotropic, which deserves further experimental verification.