High-temperature superconductivity: the role of dielectricity

The large ionic dielectric constant observed in the cuprates and in organic superconductors has drastic effects on both superconducting and normal-sta te properties. Specifically, the electron-phonon coupling is enhanced to an extent that it can account for high-temperature superconductivity via a ph onon-mediated interaction. The electron-phonon scattering becomes very larg e for forward scattering accounting for the d-wave symmetry of the gap para meter. We generalized the Eliashberg equations for the case of a renormaliz ed, frequency-dependent coupling constant arising from this huge ionic pola rizability. The solution of the generalized Eliashberg equations along the imaginary-omega axis is straightforward. Extension of the solutions to the real-omega axis requires an elaborate algorithm. We worked it out, and find highly unusual spectroscopic properties, accounting for the experimentally observed extended Van Hove singularity, overdamping of electronic states, and the unusually large values of 2 Delta(0)/T-c. Thus, electron-phonon cou pling can account for the various unusual properties of 'exotic' supercondu ctors, when the proximity to a ferroelectric transition, characteristic of the perovskites, is taken into account. (C) 1999 Elsevier Science B.V. All rights reserved.