Superconductivity beyond Migdal's Theorem and High-Tc Phenomenology

Many properties of the superconducting state of high-Tc materials have been interpreted within the Eliashberg framework with a very large coupling (λ ≥ 3). Here we point out that the inclusion of non-adiabatic corrections acts as an effective coupling for various properties that can, therefore, be reproduced with a much smaller coupling (λ ≈ 1) within the generalized theory. This can provide a new perspective for the analysis of superconductivity data with moderate couplings but within a theory that goes beyond Migdal's theorem.