Calculation of the Superconducting State Parameters with Retarded Electron-Phonon Interaction

TLDR

The study calculates the superconducting energy gap and related parameters using BCS theory in Gor'kov‑Eliashberg form with a realistic retarded electron‑phonon interaction and Coulomb repulsion. The calculation exploits that only local, short‑wavelength phonon interactions matter, approximating the phonon spectrum by an Einstein model, and solves the resulting equation via an approximate iterative procedure. The results agree in magnitude with observed transition temperatures, predict a reduced isotope effect (~15 % less than the −½ exponent), and imply that all metals should be superconductors, albeit with very low transition temperatures for those not yet observed.

Abstract

The energy gap and other parameters of the superconducting state are calculated from the Bardeen-Cooper-Schrieffer theory in Gor'kov-Eliashberg form, using a realistic retarded electron-electron interaction via phonons and including the Coulomb repulsion. The solution is facilitated by observing that only the local phonon interaction, mediated entirely by short-wavelength phonons, is important, and that a good approximation for the phonon spectrum is therefore an Einstein model rather than Debye model. The resulting equation is solved by an approximate iteration procedure. The results are similar to earlier gap equations but the derivation gives a precise meaning to the interaction and cutoff parameters of earlier theories. The numerical results are in good order-of-magnitude agreement with the observed transition temperatures but lead to an isotope effect at least 15% less than the accepted -\textonehalf{} exponent (${T}_{c}$ proportional to ${M}^{\ensuremath{-}\frac{1}{2}}$). Also, the present theory predicts that all metals should be superconductors, although those not observed to do so would have remarkably low transition temperatures.