POSITRON LIFETIME IN OXIDE SUPERCONDUCTORS YBA2(CU1-XMX)3O7-Y (M = FE, NI, ZN)

We present the temperature dependence of positron lifetime parameters measured on a series of specimens of YBa2(Cu1-xMx)3O7-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.