Study of superconducting fluctuations in superconductors with strong fluctuations

The effect of superconducting fluctuations is studied in an approximation beyond the Hartree-Fock approximation for the one-particle Green function and the random-phase approximation for the propagator of the superconducting fluctuations (HF-RPA) in the clean limit. The pair susceptibility is obtained in a form consistent with the self-energy correction in the sense that the phase invariance of the system is not violated. This is done by the systematic use of the Ward-Takahashi relations derived from the phase invariance. Superconducting fluctuations greatly reduce the transition temperature from the mean-field value in a superconductor with a small Fermi energy and a large BCS coupling constant.