Superconductivity from nonphonon interactions

The claims that the electron-electron interaction via exchange of bosons with energies comparable to the Fermi energy yields a high superconducting transition temperature are studied here by solving the Eliashberg equation containing such interactions. A model for the boson exchange within the random-phase approximation yields an unrealistically high Tc. The inclusion of self-energy, vertex, and spin-fluctuation corrections in the electron-electron interaction lowers Tc drastically. In our final estimates for a typical metal of Fermi temperature 10⁵ K, Tc peaks at 60 K when the boson energy is increased to one-tenth of the Fermi energy. The Coulomb interaction including plasmon effects in the electron gas is estimated to yield net repulsive values of μ* between 0.1 and 0.2 for rₛ between 2 and 6.