Symmetry of hole states in superconducting oxides: correlation with Tc

Despite their small occupancy, the apical oxygen pz orbitals have the indirect effect of modifying the relative occupancy of holes in states with different local symmetries in the CuO2 planes. In particular it is modified the relative occupancy of the combination of oxygen p orbitals with b1 symmetry leading to the singlets and the one with a1 symmetry which couples with the copper Dsub(3z2-r2) orbital. A well defined correlation is found between the maximum critical temperature Tcmax of various cuprates and the excess of holes in all the states with local a1 symmetry with respect to the doping δmax at which Tcmax is obtained for each compound. Implications for superconductivity models are discussed. (orig.)