Multiband superconductivity of doped cuprates

Numerous indications point to the multiband nature of the superconductivity in doped cuprates. The electronic structure of these compounds evolves with doping. Based on experimental findings, a simple model for the description of cuprate superconductor characteristics on the doping is developed. The leading interband pairing channel between an itinerant band and defect states created by doping is postulated. Bare gaps between them are supposed and to be closed by extended doping. Band overlap conditions determine the phase diagram critical points. The supercarrier density varies with the disposition of the chemical potential and bands. Nodal and antinodal momentum regions are distinguished. In the case of hole doping the relevant regime lies near the top of the oxygen band. Illustrative calculations have been made using a mean-field multiband Hamlitonian. Manifestations of various gaps and pairing strength characteristics on the doping level are discussed. A nonmonotonic dependence of the critical coherence length is calculated. The coherence of noncritical mode is also investigated. The supercarrier effective mass diminishes moderately out of underdoping. The penetration depth curve is of the observed type. The isotope effect on supercarrier density is predicted. In the case of electron doping the events concentrate near the bottom of the upper Hubbard band. The gross features of the phase diagram with two pseudogaps remind the hole doping case. The pairing strength and the phase coherence develop simultaneously. The agreement of the results of our model calculations with experimental findings suggests that multiband pairing is an essential aspect of cuprate superconductivity.