A simple way to understand the electronic structures of the Tl-Ba-Ca-Cu-O high Tc superconductors

This paper examines the extended Huckel tight-binding method used to analyze the electronic structures of two families of superconductors, TlBa2Can-1CunO2n+3 and T12Ba2Can-1CunO2n+4. All of them appear to be conductors with o antibonding half-filled bands crossing the Fermi levels. The band structures suggest that the oxidation states of Cu and T1 are around +2 and +3, respectively. The calcium and barium atoms, apart from acting as electron donors, have little effect on the dispersion of the bands near the Fermi level. However, the calcium atoms do produce a small splitting of the half-filled bands. One of the proposed superconducting mechanisms, involving oxidation of Cu2+ by T13+ is discussed. So is the role of vacancies. It there is an oxygen vacancy in TlBa2Ca3Cu4O11, it should prefer the equatorial or basal position in the Cu-O layer. The oxygen-oxygen repulsions may play a crucial role in determining the unit cell parameters