Positron lifetime in oxide superconductors YBa₂(Cu_1-xM_x)O_7-y (M=Fe, Ni, Zn)

The authors present the temperature dependence of positron lifetime parameters measured on a series of specimens of YBa₂(Cu_1-xM_x)₃O_7-y (M=Fe, Ni, Zn) in the temperature range 20-300 K. In a non-doped specimen, the defect-related component tau ₂ showed a step-like increase in the temperature range 40-100 K. Above 180 K, the mean lifetime tau _mean decreased with increasing temperature. These variations in tau ₂ or tau _mean are explained in terms of a thermally activated escape of positrons from the two kinds of trapping centres. The bulk lifetime tau _bulk showed a slight decrease in the region 100-300 K. The iron doping moved the shift of the transition region of tau ₂ towards a higher temperature and removed the decrease of tau _mean out of the temperature range in which experiments were performed. These phenomena are ascribed to the enhancement of the positron-defect binding energies, which is theoretically predicted. On the other hand, nickel and zinc did not much alter the behaviour of tau ₂, but affected that of tau _bulk. A direct correlation between the superconducting critical temperature (Tc) and the lifetime-temperature curve was not found.