Abstract
An undoped cuprate with apical fluorine and inner () and outer () layers is a 60 K superconductor whose Fermi surface has large - and -doped sheets with the superconducting gap on the sheet twice that on the sheet. The Fermi surface is not reproduced by the local density approximation, but the screening must be substantially reduced due to electronic correlations, and oxygen in the layers must be allowed to dimple outwards. This charges the layers by , causes a 0.4 eV Madelung-potential difference between the and layers, quenches the hopping, and localizes the sheets onto the layers, thus protecting their -wave pairs from being broken by scattering on impurities in the BaF layers. The correlation-reduced screening strengthens the coupling to -axis phonons.
- Received 7 July 2006
DOI:https://doi.org/10.1103/PhysRevLett.98.047001
©2007 American Physical Society