Apical oxygen ions and the electronic structure of the high T(sub c) cuprates

A five band extended Hubbard model involving four orbitals on the CuO2 planes of the high temperature superconducting oxides and the 2p(sub Z) orbital(s) for the out of plane apical oxygen ion(s) are analyzed. The strong local repulsion between holes on copper is treated by means of a slave boson approach in mean field approximation. The nearest neighbor Cu-O coulombic repulsion is studied within a Hartree decoupling scheme. The variation of the resulting band structure with doping and with varying model parameters is studied and its stability with respect to lattice deformations is examined. The results are compared with experimental data. The effects of the apical oxygen(s) on the electronic structure are analyzed and the amount of holes in states with local a(sub 1) symmetry is identified. An analysis of ten different classes of compounds reveals a correlation between the maximum critical temperature and the excess of a(sub 1) holes. This correlation indicates that the highest critical temperature can be reached in the compounds where the interaction between apical oxygen(s) and CuO2 plane is weakest. The implication of these results is discussed in the light of various theoretical models.