POSITRON TRAPPING IN OXIDE SUPERCONDUCTORS

Open-volume atomistic defects trap positrons in materials, which has a harmful effect on studies aiming at bulk properties, such as the electron momentum density (Fermi surface) investigations. The role of different types of defects in oxide superconductors is discussed fro the point of view of positron annihilation. It is argued that metal vacancies are usually strong traps and isolated oxygen vacancies weak ones. In inhomogeneous samples, positrons will show a preference to oxygen-deficient zones. As the defects can appear at different charge states (with different local relaxations) controlled by temperature and composition, changes in the trap population and detrapping make the overall temperature dependence complex. Also shallow traps as those associated with twin boundaries have a detrimental effect on momentum density measurements at low temperatures.