POSITRON-ANNIHILATION ON SINGLE-CRYSTALS OF YBA2CU3O7-X

We present lifetime and two-dimensional angular correlation of (positron) annihilation radiation (2D-ACAR) studies on YBa2Cu3O7-x single crystals, both on metallic and on insulating samples, in the hope of discriminating between band-structure and strong-interaction theories. Our measurements are discussed in the light of a large number of positron-annihilation investigations also made by other groups. Self-consistent band-structure calculations (linear muffin-tin orbitals and full-potential linear augmented plane waves) show that in the k-space-folded (001) projection we should see breaks from the cylindrical Fermi surface (FS) in the case of an ideal sample. Our experimental distributions suggest a smooth modulation (only to some extent described by the calculations) without sharp breaks, compromising a direct interpretation of the distributions in terms of FS topology. The 2D-ACAR technique cannot exclude the existence of the FS, since the trapping of positrons may occur in nonstoichiometric YBa2Cu3O7-x samples. However, it is to be noted that different samples measured by different groups are found to have similar momentum densities.