Electronic Structure of YBa 2 Cu 3 O 7- x by Positron Annihilation

We report the first 2 D -ACPAR measurement performed on single crystals of the high- T c superconductor YBa 2 Cu 3 O 7- x . The main features of the electronic structure have been experimentally determined. The electronic occupation number in the Brillouin zone supports the results of band structure calculations. Occupation numbers do not change drastically at T c . The 2 D shape parameter is anisotropic. Its temperature dependence exhibits an anomaly around the superconducting transition and has a larger slope in the normal state.