Conventional and practical metallic superconductivity arising from repulsive Coulomb coupling

Abstract

A concrete question is discussed: Can there be conventional s-wave superconductivity in regular 3D (or 2D) metals, i.e., electrons in a jellium background, interacting via the standard Coulomb coupling? We are interested in 'practical' SC that can in principle be observed in experiments, so the $T=0$ ground state being SC is not of interest, or for that matter a $T_c$ which is exponentially small and therefore 'impractical' is also not of interest in the current work. We discuss both 2D and 3D cases, focusing mostly on the 3D case. We find that almost any theory based on the BCS-Migdal-Eliashberg paradigm, with some form of screened Coulomb coupling replacing the electron-phonon coupling in the BCS or Eliashberg theory, would uncritically predict absurdly high $T_c\sim100$ K for s-wave SC in all metals (including the alkali metals, which are well-described by the jellium model) arising from the unavoidable fact that the Fermi, plasmon, and Coulomb potential energy scales are all $>10^4$ K. Therefore, we conclude, based on reduction ad absurdum, that the violation of the venerable Migdal theorem in this problem is sufficiently disruptive that no significance can be attached to numerous existing theoretical publications in the literature claiming plasmon-induced (or other similar Coulomb coupling-induced) practical SC. Using a careful analysis of the Eliashberg gap equations we find that the $T_c$ of the 3D (or 2D) electron gas can be reduced well below $\sim1$ K depending on choices of frequency cut-off parameters that are introduced to satisfy Migdall's theorem but are apriori unknown. The only believable result is the one discovered 60 years ago by Kohn and Luttinger predicting non-s-wave SC arising from Friedel oscillations with exponentially low $T_c$. We provide several theoretical approaches using both BCS and Eliashberg theories and different screening models to make our point.