POSITRON-ANNIHILATION STUDIES IN THE ND-CE-CU-O SUPERCONDUCTOR

In the superconducting Nd1.85Ce0.15CuO3.98, the positron lifetime is observed to decrease from 211 to 205 ps in the temperature range of 150 50 K, whereas in the nonsuperconducting Nd1.85Ce0.15CuO4, having a lifetime value of 231 ps, no significant temperature dependence of lifetime is observed. The difference in the lifetimes of the superconducting and nonsuperconducting samples and their temperature dependencies are understood in terms of positron interaction with the vacancies in the system. Doppler-broadened line shapes of energy spectra are found to show similar results as lifetime measurements. Theoretical calculations are used to show that the oxygen vacancies are weaker traps compared with the vacancies at the Cu and Nd sites. The observed decrease in lifetime in the superconducting sample is interpreted in terms of an increase in the fraction of positrons trapped at the oxygen vacancies as the temperature is lowered. Plausible reasons for the temperature independence of lifetime across Tc in the superconducting sample are discussed. © 1990 The American Physical Society.