A study of positron distribution and annihilation characteristics in YBa₂Cu₃O_7-x

The positron distribution and lifetimes have been computed for the model structures of orthorhombic YBa₂Cu₃O₇, tetragonal YBa₂Cu₃O_6.5 and tetragonal YBa₂Cu₃O₆, which consist of different ordered arrangements of oxygen vacancies. The positron potential was obtained as a sum of the full lattice electrostatic potential and the correlation potential in the local density approximation. The positron wavefunction and the eigenvalue of the lowest-lying itinerant positron state was obtained by solving the Schrodinger equation in the finite-difference scheme. The positron wavefunction displays maxima at the oxygen vacancy positions and it is seen that the positron probes predominantly the oxygen vacancies in the basal plane. By computing the overlap of the positron density with the core and valence electron densities, the positron lifetimes for YBa₂Cu₃O₇, YBa₂Cu₃O_6.5 and YBa₂Cu₃O₆ are estimated to be 190 ps, 222 ps and 234 ps, respectively. Calculation of the positron wavefunction and lifetime has also been carried out for a positron localised at an isolated oxygen vacancy along the chain in orthorhombic YBa₂Cu₃O₇. The calculated values of lifetimes for different arrangements of vacancies have been compared with the experimental values of positron lifetimes in YBa₂Cu₃O_7-x measured after various heat treatments.