Electronic states of cuprate superconductors with apical halogen

The electronic states of newly developed cuprate superconductors with apical halogen are examined by means of the ionic and cluster model approaches developed in a previous study for other cuprates [Y. Ohta, T. Tohyama, and S. Maekawa, Phys. Rev. B 43 (1991) 2968]. We find two features that differ from those of other cuprates: (i) small dielectric constant and (ii) large hopping energy of electrons in the CuO2 plane. Both features are of primary importance for understanding the dependence of the energy gap and the superexchange interaction on the material of the parent compounds. As in other cuprates, the superconducting critical temperature T-c is found to be well correlated with the stability of the Zhang-Rice local singlet in the CuO2 plane.