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 e lectronic correlations plays a decisive role in the formation of the normal state and superconductivity. Tunneling spectroscopy shows clear phonon fea tures in the conductance and together with infrared and Raman optic measure ments give strong support for the electron-phonon interaction as the pairin g mechanism in HTS oxides. The tunneling experiments show also that almost all phonons contribute to the pairing interaction and the E-P interaction i s sufficiently large to produce T-c similar to 100 K. The strong E-P intera ction 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 interact ion, especially for axial phonons. On the other hand, a variety of phase-sensitive measurements give support f or d-wave pairing in HTS oxides, which has been usually interpreted to be d ue to the spin-fluctuation mechanism. We argue in this review that contrary to low-T-c-superconductors (LTS), whe re the phonon mechanism leads to a-wave pairing, strong electronic correlat ions in HTS oxides renormalize the electron-phonon (E-P) interaction, as we ll as other electron-boson scattering processes related to charge fluctuati ons, in such a way that the forward scattering peak (FSP) appears, while th e backward scattering is suppressed. The FSP mechanism is also supported by the long-range Madelung E-P interaction and the former is pronounced for s maller hole doping delta << 1. The renormalization of the E-P interaction and other charge scattering proc esses (like impurity scattering) by strong correlations gives rise to (i) a significant (relative) increase of the coupling constant for d-wave pairin g lambda(d) making lambda(d) approximate to lambda(s) for delta less than o r equal to 0.2, where lambda(s) is the coupling for a-wave pairing. The res idual Coulomb repulsion between quasiparticles (or the interaction via spin fluctuations, which is peaked in the "backward" scattering at Q approximat e to (pi, pi)) triggers the system to d-wave pairing, while T-c is dominant ly due to the E-P interaction; (ii) a reduction (with respect to the pairin g coupling constant lambda) of the transport E-P coupling constant lambda(t r)(less than or similar to lambda/3), i.e. to the quenching of the resistiv ity rho(T) where rho similar to lambda(tr)T for T > Theta(D)/5; (iii) a sup pression of the residual quasiparticle scattering on nonmagnetic impurities ; (iv) robustness of d-wave pairing in the presence of nonmagnetic impuriti es 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 inte raction of the optimally hole-doped HTS oxides gives rise, besides the d-wa ve superconductivity, also to (a) the small isotope effect; and (b) the str ong temperature dependence of the gap anisotropy. In the overdoped oxides the FSP mechanism and spin fluctuations are suppres sed which leads to (a) anisotropic a-wave pairing with moderate gap anisotr opy, and (b) an increase of the isotope effect. (C) 2000 Elsevier Science B .V. All rights reserved.