Eliashberg Equations and Coulomb Pseudopotential for Superconductivity via Sign-Opposite Interactions

A new type of attractive interactions, the second retarded ones, is investigated with the Eliaehberg treatment. Several mechanisms, leading to the pairing in the high frequency window, belong to this type. The equations for gap parameter are derived. From these a formula of the Coulomb pseudopotential is obtained. It is found that the Coulomb repulsion against the pairing is completely suppressed by the pseudopotential effect in the unscreened case. Its significance for the high T_c superconductivity is discussed.