Pairing Mechanism for the High-TC Superconductivity: Symmetries and Thermodynamic Properties

The pairing mechanism for the high-T-C superconductors based on the electron-phonon (EPH) and electron-electron-phonon (EEPH) interactions has been presented. On the fold mean-field level, it has been proven, that the obtained s-wave model supplements the predictions based on the BCS van Hove scenario. In particular: (i) For strong EEPH coupling and T<TC the energy gap (Delta(tot)) is very weak temperature dependent; up to the critical temperature Delta(tot) extends into the anomalous normal state to the Nernst temperature. (ii) The model explains well the experimental dependence of the ratio R-1 equivalent to 2 Delta((0))(tot)/k(B)T(C) on doping for the reported superconductors in the terms of the few fundamental parameters. In the presented paper, the properties of the d-wave superconducting state in the two-dimensional system have been also studied. The obtained results, like for s-wave, have shown the energy gap amplitude crossover from the BCS to non-BCS behavior, as the value of the EEPH potential increases. However, for T>T-C the energy gap amplitude extends into the anomalous normal state to the pseudogap temperature. Finally, it has been presented that the anisotropic model explains the dependence of the ratio R-1 on doping for the considered superconductors.