TEMPERATURE-DEPENDENCE OF THE POSITRON-ANNIHILATION PARAMETERS IN BI2-XPBXCA2SR2CU3OY AND BI2CASR2CU2OY SUPERCONDUCTORS

We report the results of temperature dependence of S-parameter and lif etime (tau) measurements on the normal and superconducting states of B i2CaSr2Cu2Oy (Bi-2:1:2:2) and Bi2-xPbxCa2Sr2Cu3Oy (Bi-2:2:2:3) superco nductors. A decrease of the S parameter as well as tau below T(c) is s een for both compounds. These observations are interpreted qualitative ly considering a reported positron-density-distribution calculation an d invoking the local charge-transfer process from CuO2 to Bi-O planes at the onset of the superconducting transition. The possible influence of sample preparation conditions and structural changes on the temper ature dependencies of the positron annihilation parameters across T(c) is also discussed.