Spin-polaron theory of high-{Tc} superconductivity: 3, Gap function and critical temperature

Results from previous papers in this series are used to derive approximate expressions for the gap and {Tc} within the framework of a Cooper-pairing approach. The possible symmetry types of the gap are discussed. It is shown how the proximity of the Fermi level to the Mott-Hubbard band edge and the interplay of O 2p{sigma} and 2p{pi} bands and/or localization effects can provide good fits to the variation of {Tc} with x in La{sub 2-x}Sr{sub x}CuO{sub 4} and YBa{sub 2}Cu{sub 3}O{sub 7-x}. It is concluded that the in-plane gap is either s- or d-like but anisotropic in either case. Other aspects and implications of the model and of the calculations are given and comparisons with Mott`s spin-bipolaron model are made.