Positron-lifetime studies in Y${\mathrm{Ni}}_2$${\mathrm{B}}_2$C

Positron-lifetime measurements have been carried out in the temperature range of 300 to 5 K in the borocarbide superconductor, Y${\mathrm{Ni}}_2$${\mathrm{B}}_2$C. The positron lifetime 189 ps at 300 K is observed to decrease to 179 ps at 5 K. No change in lifetime is seen across the superconducting transition at ∼ 16 K. The observed magnitude of lifetime and its temperature variation are discussed with support from theoretical computations of positron density distribution and annihilation characteristics for a perfect crystal of tetragonal Y${\mathrm{Ni}}_2$${\mathrm{B}}_2$C and for various plausible defect configurations. On the basis of these calculations, it is inferred that the lifetime and its temperature dependence are governed by positron annihilation from carbon vacancies.