POSITRON ANNIHILATION STUDY ON HIGH T_C SUPERCONDUCTORS Y-Ba-CU-O

The Doppler broadening lineshape of 5llkeV gamma rays of positron annihilation radiation in the high T_c superconductors Y-Ba-Cu-O has been investigated. The lineshape parameters as a function of temperature were measured from 85K to 300K. Anomalous dependence of lineshape parameters on temperature was first observed in polyphase samples, i. e., the lineshape parameters occurred catastrophe for some narrow temperature regions (90K. 107K, 115K and 180K). This anomaly is probably due to the migration and redistribution of oxygen vacancies in the samples during superconducting transition.