| Unusual renormalization of the band dispersion and the nonmonotonic superconducting gap are believed to be intimate signatures of HTSC. However, there are known difficulties with experimental observation of these effects in photoemission studies of YBCO-123. The solution to this puzzle is that the photoelectron spectrum of YBCO-123 generally includes two components: one corresponding to a highly overdoped near-surface region and the other still retaining the superconducting properties. Choosing an appropriate excitation energy and polarization one can clearly identify the low energy bands as well as partially suppress the intensity of the overdoped component and reveal the true superconducting one. The circular polarised light can be used as an alternative tool to distinguish between the overdoped and superconducting components, as well as to ascertain surface and bulk localization of the two components. The superconducting component, thus observed, exhibits strong band renormalization which disappears above Tc and has pronounced momentum dependence, being strongest at the nodal point and decreasing when moving away. For Ca substituted samples the superconducting gap was clearly observed, which again suggests that superconductivity affects the electronic structure in way a similar to Bi-2212 case. |
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