A PERTURBATIVE APPROACH TO HIGH-T-C SUPERCONDUCTIVITY

Compared to the conventional method of using Hamiltonian in dealing wi th superconductivity, it seems to be more adequate to adopt the pertur bative approach in terms of the Feynman diagrams to the strong couplin g in high-T-c superconductivity. A graphic functional derivative techn ique developed by the authors make it possible to correctly consider t he vertex corrections to the self-energy even in the case of strong co upling. It is found that the first step of perturbation beyond RPA is good enough to understand behaviors of high-T-c superconductivity.