Bonds, bands, charge-transfer excitations, and high-temperature superconductivity

Results of highly precise all-electron local density calculations of the electronic structure for the high T_c superonductors (La_2-xM_xCuO₄, YBa₂Cu₃O_7-δ, Bi₂Sr₂CaCu₂O₈, Tl₂Ba₂CaCu₂O₈ and Tl₂Ba₂Ca₂Cu₃O₁₀), as determined with the full potential linearized augmented plane wave (FLAPW) method are presented. In all these high T_c materials, the Cu-O dpσ (anti-bonding) bands arising from the CuO₂ planes cross E_F and are strongly two-dimensional. The Y, Ba, Sr, and Ca atoms are highly ionic with the Ca²⁺ ions serving to insulate the Cu-O planes. In both Bi₂Sr₂CaCu₂O₈ and the Tl compounds, the Bi-O and Ti-O planes contribute to the density of states at E_F and hence to the transport properties. The similarity and difference between these systems are described in detail. The results obtained demonstrate the close relation of the band structure to the structural arrangements of the constituent atoms and provide an integrated chemical and physical picture of their interactions and their possible relation to the origin of their high T_c superconductivity. Our calculations indicate the inadequacy of a purely electron-phonon mechanism in explaining the high T_c. An excitonic mechanism of superconductivity is discussed.