Apex oxygen and critical temperature in copper oxide superconductors: Universal correlation with the stability of local singlets

Electronic-energy-level structures of the layered Cu oxide compounds are drawn within the ionic model. Correlations between the level separations obtained and the superconducting critical temperature Tc are examined for all families of the Cu oxide superconductors. It is proposed that the position of the energy level of the apex oxygen atoms is of primary importance for the electronic states of the CuO₂ plane and governs the optimum Tc's of all families of hole-carrier suprconductors. The pressure effect of Tc is discussed within this framework. Based on cluster-model calculations, where the covalency neglected in the ionic model is fully taken into account, we argue that the role of the apex oxygen atoms is to prescribe the stability of local singlet states made of two holes in the Cu 3dₓ²-y² and O 2p_σ orbitals of the CuO₂ plane, and thereby to control the maximum Tc of each family.