Anomalous Up-and-Down Variation of Positron Lifetime Near Tc in High-temperature Superconductors

The temperature dependence of the positron lifetimes in the polycrystalline high T_c superconducting Y Ba Cu O and Bi Sr Ca Cu O and the non-superconducting Y Ba Cu O have been investigated by using the position lifetime technique. It is found from the experiments that there is an evident temperature dependence for the two kinds of superconducting samples but no temperature dependence for non-superconducting sample. It is important that the anomalous up-and-down variation of positron lifetime near T_c has been observed in the measured superconducting samples. We suggest that this strange temperature dependence of the positron lifetimes near T_C may result from some changes induced by the phase transition.