Bonding and electronic structure in high-Tcsuperconducting oxides: A case for the importance of the cations

It is suggested that the chemical bonding in the high-${\mathit{T}}_{\mathit{c}}$ superconducting oxides is strongly influenced by the large fields induced by the very ionic cations (e.g., barium) and that the presence of such cations is a necessary requirement for superconductivity. The electronic effect of this field is thus to modify the interaction between the oxygen and copper atoms in the cuprate anion of the superconductor. The experimental evidence for these ideas come from a comparison of core- and valence-level photoemission spectra of the superconducting oxides with those of typical transition-metal oxides on the one hand and the very ionic oxides (e.g., BaO) on the other.