Photoemission studies of the normal and high-temperature superconducting states

Photoemission has become a most powerful electronic probe of high-T-C superconductors. The main present emphasis is on the BCSCO-2212 gap symmetry. After much controversy a consensus emerges: (1) no symmetric s-waves; (2) the symmetry depends on temperature and doping; (3) the optimal doping data are consistent with d(x)2(y)2 waves; (4) for other dopings, the symmetry is consistent with mixed s and d-waves. The data, though, cannot rule out other hypotheses, specifically asymmetric s-waves. Photoemission also revealed unexpected phenomena, notably for Co-doped BCSCO-2212. By fine-tuning the doping one can replace the room temperature itinerant electronic states with localized states; Te changes too, but there is still high-T-C superconductivity. This indicates that Anderson localization and high-T-C superconductivity can coexist.