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In this article we quantify the possible effects of the breakdown of Migdal'stheorem on the electron-phonon (e-p) coupling constant, on the criticaltemperature and on the superconducting gap by examining different kinds ofsuperconducting materials either with low and high critical temperature. We usethe theoretical approach developed by Grimaldi, Pietronero and Strassler [PRB52, 10516 & 10530 (1995)] on experimental data taken both from literature andfrom our recent break-junction tunneling experiments in Bi_2Sr_2CaCu_2O_{8+x}(BSCCO) [Physica C 275, 162 (1997) and cond-mat/9612220]. The results show thata large violation of the Migdal's theorem (as in BSCCO) yields to a largeincrease of the observed e-p coupling constant and, in a first approximationand by direct solution of the Eliashberg equations, to large but differentincreases of Tc and gap. The same theory gives no modifications when applied tolow-Tc conventional superconductors. In BSCCO, the renormalized values of thee-p coupling constant and of the ratio 2Delta/kTc (1.85 and 4.98, respectively)are compatible with a strong-coupling electron-phonon origin ofsuperconductivity.Comment: 11 pages REVTeX file, 3 PostScript figures, to be published in Phys.  Rev. B (Rapid Commun.

physical review. b, condensed matter

Breakdown of Migdal’s theorem and intensity of electron-phonon coupling in high-<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi>T</mml:mi></mml:mrow><mml:mrow><mml:mi>c</mml:mi></mml:mrow></mml:msub></mml:mrow></mml:math>superconductors

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