POSITRON LIFETIME STUDIES IN BOROCARBIDES

Positron lifetime measurements have been carried out in RNi(2)B(2)C, w ith R = Y, Lu, Ho, Er,Tb and Ce, at room temperature. A systematic inc rease in the bulk lifetime from 130 ps to 145 ps is observed with incr ease in unit cell volume. Theoretical computations of the positron lif etime, also carried out on these systems show a trend in agreement wit h the experiment. Positron lifetime measurements in YNi2B2C, done in t he temperature range of 300-5 K, across the superconducting transition , shows a monotonic decrease in lifetime with the lowering of temperat ure. This variation in lifetime is related to the trapping behaviour o f positrons at carbon vacancies and their clusters.