Electronic structure and properties of Bi₂Sr₂CaCu₂O₈, the third high-T_c superconductor

Results of a highly precise local density determination of the electronic structure (energy bands, densities of states, Fermi surface and charge densities) of the new high-T_c superconductor Bi₂Sr₂CaCu₂O₈ are presented. The calculations employed the full-potential linearized augmented plane wave (FLAPW) method and the subcell structure parameters given by the work of Hazen et al. and Sunshine et al. As in the case of the other Cu-O superconductors, we find a relatively simple band structure at E_F and strongly anisotropic highly 2D properties. The Sr and Ca atoms are highly ionic, with the Ca²⁺ ions serving to insulate the Cu-O planes. The Bi-O planes contribute substantially to N(E_F), the density of states at E_F, and to the transport properties whereas neither the Sr or Ca do. The N(E_F) contribution from the Cu-O planes (∼1.0 states/eV-Cu atom) is lower (per Cu atom) than that found previously for the La-Sr-Cu-O and Y-Ba-Cu-O superconductors. The highly 2D Fermi surface shows regions of strong Cu-O and Bi-O hybridization from which highly anisotropic superconducting energy gaps are predicted. Here, too, an electron-phonon calculation of T_c (using the crude rigid ion approximation) is found to be inadequate. As in the case of YBa₂Cu₃O_7-δ, strongly localized states at E_F (arising from Bi-O and Cu-O) may be responsible for charge transfer excitations (excitons) which cause the high T_c.