Electronic Structure Theory of the High T(c) Copper - Superconductors.

The discovery of high T_{ rm c} Cu-oxide superconductors has generated experimental and theoretical excitement on an unprecedented scale. To understand the mechanism giving rise to their high T_{rm c} superconductivity, a number of important questions become imminent as to the electronic structure and properties of these materials. Results of a highly precise all-electron local density determination of the electronic structure of the known high T_{rm c} Cu-oxide superconductors, La_{rm 2-x}M_{rm x} CuO_4, YBa_2 Cu_3O_{7 -delta}, GdBa_2Cu _3O_{7-delta }, Bi_2Sr_2 CaCu_2O_8 , Tl_2Ba_2 CaCu_2O_8 , and Tl_2Ba_2 Ca_2Cu_3O _{10} are presented and analyzed in detail. The calculations employed the full potential linearized augmented plane wave (FLAPW) method and used the crystal structural parameters found experimentally. 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 superconductivity. We find that the two-dimensional Cu-O dpsigma anti-bonding bands, arising from CuO_2 planes, dominate the electronic structure near E_{rm F} of all the Cu-oxide superconductors and result in a fairly low density of states at E_ {rm F}, ~1 states/(eV-Cu atom). In addition to the 2D Cu-O bands, the intercalcated layers of CuO chains, Bi_2 O_2 and Tl_2 O_2 layers provide a two level electronic structure near E_{ rm F}. The other elements (La, Y, Ba/Sr, and Ca) are found to be strongly ionic and do not contribute to the transport properties. The magnetic isolation of the Gd ions is found to explain the existence of high T _{rm c} superconductivity of the (RE)Ba_2Cu_3 O_{7-delta} compounds (where RE = heavy magnetic lanthanides). Strong indications are demonstrated (based on rigid ion calculations) for the inadequacy of a conventional phonon mechanism for obtaining the observed high T _{rm c}. Through a comparison of the electronic structures of the known high T _{rm c} Cu-oxide superconductors, a possible important role played by the two level structure within the intercalated layers is noted, in which a virtual transition from occupied states to empty states may lead to charge transfer excitations or covalent bond polarizations mediating the pairing of conduction electrons in the metallic CuO_2 planes and serving to enhance their superconductivity.