Theory of Multiband Superconductivity

Abstract

Superconductivity arising from coulomb repulsion between electrons has been studied for a multiband system. We consider on-site and inter-site coulomb integrals, inter-site exchange integrals and inter-site exchange-like integrals [K of eq. (42)]. We do not take account of the correlation effect caused by the interaction, such as spin fluctuation etc., and calculate the average of the interaction energy with the BCS wave function. We find that when K is so large that satisfies eq. (50), an s-type superconducting state is stable when there are two fermi surfaces. The gap functions on these fermi surfaces have different signs and so the pair transition between them gives us a negative energy for a positive K, leading to superconductivity.