Key Pairing Interaction in Cuprate Superconductors

It has been now over 20 years since the discovery of the first high temperature superconductor by Georg Bednorz and Alex Müller and yet, despite intensive effort, no universally accepted theory exists about the origin of superconductivity in cuprates. A controversial issue on weather the electron–phonon interaction (EPI) is crucial for high-temperature superconductivity or weak and inessential has been one of the most challenging problems of contemporary condensed matter physics. Here, it is suggested that the true origin of high-temperature superconductivity is found in a proper combination of strong electron–electron correlations with a significant finite-range (Fröhlich) EPI so that low energy quasi-particles are small mobile polarons and bipolarons in cuprate superconductors. Our recent development of the bipolaron theory of tunnelling accounting for two energy scales, their temperature and doping dependencies, asymmetry and inhomogeneity of tunnelling spectra of cuprate superconductors is briefly discussed.