CALCULATIONS OF THE SUPERCONDUCTING CRITICAL-TEMPERATURE WITH VERTEX CORRECTIONS

The calculations of the superconducting transition temperature, includ ing the lowest-order vertex corrections, are carried out for three mod els: the BCS-type instantaneous interaction, a retarded isotropic inte raction described by the Eliashberg spectral function alpha(2)F(Omega) and a spin-fluctuation interaction with four delta-function peaks in the corners of a square Brillouin zone. In the isotropic case the effe ct of higher-order self-energy diagrams is to suppress the critical te mperature calculated within the mean-field approximation when the char acteristic energy of the boson responsible for the pairing is not smal l compared to the Fermi energy. In the case of the model with sharp pe aks in the momentum space the vertex correction can lead to an increas e of T-c.