BREAKDOWN OF MIGDALS THEOREM AND INTENSITY OF ELECTRON-PHONON COUPLING IN HIGH-T-C SUPERCONDUCTORS

In this article we quantify the possible effects of the breakdown of M igdal's theorem on the electron-phonon coupling constant lambda, on th e critical temperature T-c, and on the superconducting gap Delta by ex amining different kinds of superconducting materials either with low o r 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 o ur recent break-junction tunneling experiments in Bi2Sr2CaCu2O8+x. The results show that a large violation of Migdal's theorem (as in Bi2Sr2 CaCu2O8+x) yields a large increase of lambda and, to a first approxima tion, a large but different increase of T-c and Delta. The same theory gives no modifications when applied to low-T-c conventional supercond uctors.