Electron-phonon coupling in high-Tc superconductors

For a long time the majority opinion on the role of phonons in the mechanism of high-temperature superconductivity has been that it is either irrelevant or even harmful, in spite of a large amount of experimental observations on the lattice effects. However, recent more direct experimental evidence of strong electron-phonon coupling and isotope effects suggest that we should not dismiss its role so easily. We argue that the electron-phonon coupling in strongly correlated electron systems is unconventional and substantially different from that in simple metals. In the cuprates certain phonons induce strong inter-site charge transfer, which can be spin dependent. This could lead to a strong spin-charge-phonon coupling, and to formation of the vibronic state in the presence of nano-scale phase separation. It is most likely that the unconventional electron-phonon coupling is the critical element of the mechanism of superconductivity in the cuprates.