Pairing mechanisms viewed from physics and chemistry

Through inspecting the limitations of BCS theory we give a brief review on the microscopic theories of superconductivity. Three basic pairing models have been extracted from the reviewed theories. The flat/steep band model developed from a viewpoint of chemical pairwise interactions is described and explained. The conditions for the occurrence of a flat electronic band within the framework of band theory is discussed together with examples. Characteristic peak-like structures of the electron-phonon coupling, as expected from the model, are shown to occur for conventional superconductors. The functional Psib(Psi) is introduced to relate chemical pairwise interaction with superconductivity. Investigations on experimental and theoretical low-energy electronic structures of superconductors, from simple elements to complicated cuprates, support our flat/steep band condition.