Positron Annihilation and Correlation in High-t, Oxides

Spectra of positron annihilation in YEa.$u&+ and Nd$uO, are calculated using self-consistent LM’IC calculations and compared with various experimental data. Methods for including effects of correlation and enhancement arc presented and applied in addition to the independent panicle model. Such effects are not crucial for the question of Fermi surface breaks in these materials. However from comparison of lifetimes between theory and experiment in the oxides and other materials we conclude that lattice imperfections can mask the Fermi surface which has been observed in recent experiments. Keywordr: positron annihilation spectroscopy, positron states, superconductivity