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Keywords:

  • Superconductivity;
  • Eliashberg equations;
  • High-Tc cuprates;
  • Organic superconductors

Abstract

We solve the Eliashberg equations for the case of an explicit k dependence of the interactions, and of the resulting self-energies Σ1(k,ω), Σ2(k,ω). We consider a strong energy-dependence of the electron-electron scattering-rate τmath image, which is associated with a strong energy-dependence of the electron-phonon matrix element g(k,k′). We characterize this energy-dependence by a cutoff ζ1, which is of the order of the phonon frequency ωph. We find that we can account for a large number of unexpected features of the superconductivity of the cuprates by the BCS electron-phonon theory, if we consider very large values of the McMillan coupling constant λph, and small values of the cutoff ζ1. Specifically, the Coulomb interaction is found not to depress Tc; the isotope effect is strongly reduced when ζ1 < ωph. We find solutions in which the gap function Δ(k, ω) has extended s-wave symmetry but is very anisotropic. These large anisotropies are in good agreement with various experiments. We suggest that the underlying cause of the strong energy-dependence is a very small electronic screening parameter at the Fermi surface; the electron-phonon matrix element g is abnormally large, and this accounts for the high transition temperatures of the cuprates. An order of magnitude estimate suggests that the electron-phonon mechanism can account for transition temperatures up to about 200 K. We thus propose a very-strong-coupling theory, in which the renormalization functions, in particular the energy-renormalization X, depend very strongly on the superconducting gap Δ, and thus display a very strong temperature-dependence between Tc and T = 0. An experimental manifestation of the very strong coupling with a small cutoff is a zero bias anomaly sometimes observed in tunneling experiments.