POSITRON-ANNIHILATION IN YBA2CU3O7-DELTA - A CLEARER PICTURE

We obtain remarkable agreement between 2D-ACAR measurements in YBa2Cu3 O7-delta and calculations by the FLAPW method: order of magnitude for the amplitude of the (3.5 +/- 1 %) 2D-ACAR anisotropy. Theoretical pre dictions of the two-photons momentum distribution were made using both the FLAPW and the LMTO technique; the FLAPW results came out to be cl oser to measurement. The sharpness of the Fermi surface is comparable with theoretical prediction. Estimations give a full width of the ridg e of 1.6 +/- 0.2 mrad which is the value given by our LDA band structu re calculations. Substitution of Cu with Al (T(c) = 77 K) yield a near ly tetragonal signal with much less ridge contribution. Substitution w ith comparable amounts of Ni and Zn had much less influence on the 2D- ACAR picture: this seems Lo be due to the fact that Al goes into the c hains, where positron density is large, while Ni and Zn are located in to the layers.