High-temperature superconductors: results and relations

Recent experiments and theories have advanced the fundamental understanding of the copper-oxide superconductors considerably. As an introduction, results from a number of groups will be reviewed. Data from inelastic neutron scattering and photoemission underline the basic vibronic character of the CuO2 planar system. Furthermore, inelastic neutron scattering has revealed huge oxygen- and copper-isotope effects at the stripe-formation temperature T*, which are understood by the same vibronic concept. A model for Jahn–Teller bipolarons properly reflects the phonon anomalies observed by inelastic neutron scattering, extended X-ray absorption fine structure, electron paramagnetic resonance (EPR), and other important experimental findings regarding the superconducting transition temperature Tc. Another quantitative model, in which the planar and out of plane electronic bands are coupled via rotational tilting modes, yields the isotope effects on T* and its doping dependence, and is supported by the quantitative understanding of the fast EPR relaxation observed.