Xie Wenhui, Jepsen O, Andersen O K, Chen Yulin, Shen Z-X
Max-Planck Institut für Festkörperforschung, Heisenbergstrasse 1, D-70569 Stuttgart, Germany.
Phys Rev Lett. 2007 Jan 26;98(4):047001. doi: 10.1103/PhysRevLett.98.047001. Epub 2007 Jan 22.
An undoped cuprate with apical fluorine and inner (i) and outer (o) CuO(2) layers is a 60 K superconductor whose Fermi surface has large n- and p-doped sheets with the superconducting gap on the n sheet twice that on the p 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 o layers must be allowed to dimple outwards. This charges the i layers by 0.01|e|, causes a 0.4 eV Madelung-potential difference between the i and o layers, quenches the i-o hopping, and localizes the n sheets onto the i layers, thus protecting their d-wave pairs from being broken by scattering on impurities in the BaF layers. The correlation-reduced screening strengthens the coupling to z-axis phonons.
