Positron lifetime studies in borocarbides

Positron lifetime measurements have been carried out in RNi<SUB>2</SUB>B<SUB>2</SUB>C, with R = Y, Lu, Ho, Er, Tb and Ce, at room temperature. A systematic increase in the bulk lifetime from 130 ps to 145 ps is observed with increase in unit cell volume. Theoretical computations of the positron lifetime, also carried out on these systems show a trend in agreement with the experiment. Positron lifetime measurements in YNi<SUB>2</SUB>B<SUB>2</SUB>C, done in the temperature range of 300-5 K, across the superconducting transition, shows a monotonic decrease in lifetime with the lowering of temperature. This variation in lifetime is related to the trapping behaviour of positrons at carbon vacancies and their clusters