Superconducting gap anisotropy within the framework of a simple exchange model for layered cuprates - The theory of HTSC

The oxygen O2p-sigma and copper Cu4s and Cu3d(x-squared)-(y-squared) orbitals are involved in a simple LCAO model for determination of the conduction band, and the oxygen-oxygen hopping is considered as a small parameter with respect to the transition amplitude between nearest neighbors. The traditional Cooper pairing is obtained by taking into account the double-electron excbange between the nearest neighbors within the 2D CuO2 plane. The equation for the superconducting gap is obtained as a result of the standard BCS treatment. It is shown that the order parameter could have either s-type or d-type symmetry depending on the ratio between the transition amplitudes. This model elucidates experiments reporting a pi-shift of the Josephson phase indicative of a d-type gap symmetry, as well as the observed s-type in the case of strongly irradiated samples. (Author)