POSITRON-LIFETIME STUDIES IN YNI2B2C

Positron-lifetime measurements have been carried out in the temperatur e range of 300 to 5 K in the borocarbide superconductor, YNi2B2C. The positron lifetime 189 ps at 300 K is observed to decrease to 179 ps at 5 K. No change in lifetime is seen across the superconducting transit ion at similar to 16 K. The observed magnitude of lifetime and its tem perature variation are discussed with support from theoretical computa tions of positron density distribution and annihilation characteristic s for a perfect crystal of tetragonal YNi2B2C and for various plausibl e defect configurations. On the basis of these calculations, it is inf erred that the lifetime and its temperature dependence are governed by positron annihilation from carbon vacancies.