Breakdown of Migdal’s theorem and intensity of electron-phonon coupling in high-Tc superconductors

Abstract

In this article we quantify the possible effects of the breakdown of Migdal’s theorem on the electron-phonon coupling constant λ, on the critical temperature Tc, and on the superconducting gap Δ by examining different kinds of superconducting materials either with low or high critical temperature. We use the theoretical approach developed by Pietronero and co-workers [Phys. Rev. B 52, 10 516 (1995); 52, 10 530 (1995)] on experimental data taken both from literature and from our recent break-junction tunneling experiments in Bi2Sr2CaCu2O8+x. The results show that a large violation of Migdal’s theorem (as in Bi2Sr2CaCu2O8+x) yields a large increase of λ and, to a first approximation, a large but different increase of Tc and Δ. The same theory gives no modifications when applied to low-Tc conventional superconductors.

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