Interplay of electron-phonon interaction and strong correlations: the possible way to high-temperature superconductivity

The pairing mechanism in high-T-c-superconductors (HTS) is still, 13 years after the discovery of HTS, under dispute. However, there are experimental evidences that the electron-phonon (E-P) interaction together with strong electronic correlations plays a decisive role in the formation of the normal state and superconductivity. Tunneling spectroscopy shows clear phonon features in the conductance and together with infrared and Raman optic measurements give strong support for the electron-phonon interaction as the pairing mechanism in HTS oxides. The tunneling experiments show also that almost all phonons contribute to the pairing interaction and the E-P interaction is sufficiently large to produce T-c similar to 100 K. The strong E-P interaction is due to (a) the layered and almost ionic-metallic structure of HTS oxides; (b) the almost two-dimensional motion of conduction carriers, which give rise to large contribution of the Madelung energy in the E-P interaction, especially for axial phonons. On the other hand, a variety of phase-sensitive measurements give support for d-wave pairing in HTS oxides, which has been usually interpreted to be due to the spin-fluctuation mechanism. We argue in this review that contrary to low-T-c-superconductors (LTS), where the phonon mechanism leads to a-wave pairing, strong electronic correlations in HTS oxides renormalize the electron-phonon (E-P) interaction, as well as other electron-boson scattering processes related to charge fluctuations, in such a way that the forward scattering peak (FSP) appears, while the backward scattering is suppressed. The FSP mechanism is also supported by the long-range Madelung E-P interaction and the former is pronounced for smaller hole doping delta << 1. The renormalization of the E-P interaction and other charge scattering processes (like impurity scattering) by strong correlations gives rise to (i) a significant (relative) increase of the coupling constant for d-wave pairing lambda(d) making lambda(d) approximate to lambda(s) for delta less than or equal to 0.2, where lambda(s) is the coupling for a-wave pairing. The residual Coulomb repulsion between quasiparticles (or the interaction via spin fluctuations, which is peaked in the "backward" scattering at Q approximate to (pi, pi)) triggers the system to d-wave pairing, while T-c is dominantly due to the E-P interaction; (ii) a reduction (with respect to the pairing coupling constant lambda) of the transport E-P coupling constant lambda(tr)(less than or similar to lambda/3), i.e. to the quenching of the resistivity rho(T) where rho similar to lambda(tr)T for T > Theta(D)/5; (iii) a suppression of the residual quasiparticle scattering on nonmagnetic impurities; (iv) robustness of d-wave pairing in the presence of nonmagnetic impurities and (v) nonadiabatic corrections to the E-P interaction and accordingly to a possible increase of T-c in systems with omega(D) less than or similar to E-F Furthermore, the development of the forward scattering peak in the E-P interaction of the optimally hole-doped HTS oxides gives rise, besides the d-wave superconductivity, also to (a) the small isotope effect; and (b) the strong temperature dependence of the gap anisotropy. In the overdoped oxides the FSP mechanism and spin fluctuations are suppressed which leads to (a) anisotropic a-wave pairing with moderate gap anisotropy, and (b) an increase of the isotope effect. (C) 2000 Elsevier Science B.V. All rights reserved.