POSITRON LIFETIME STUDIES OF Y_1-xCa_xBa₂Cu₃O_7-\delta AND Y_1-xCa_xBa₂Cu_3-xFe_xO_7-\delta

In this paper it is reported that the measurement of the bulk positron lifetime as a function of substitution content x in the temperature range from 70 to 220K was performed in two high-T_c superconducting systems, Y_1-xCa_xBa₂Cu_3-xFe_xO_7-\delta and Y_1-xCa_xBa₂Cu₃O_7-\delta. It was found that \tau_B of both systems decreases significantly with x and the temperature dependence of \tau_B is very weak in normal state. In lower temperature region (Tc), a dramatic x-dependent temperature variation of \tau_B was observed in the Ca-substituted system: from a decrease of \tau_B with decreasing T to an increase of \tau_B. With increasing x, the temperature dependence of \tau_B remains weak in the Ca- and Fe- substituted systems. Compared with the experimental data of positron lifetime in other substituted systems and the calculation of the positron density distribution, the authors suggest that positron bulk lifetime spectra behaviour can be interpreted by the physical model based on the transfer of electron density between the CuO₂ planes and Cu-O chains. Therefore, the study of positron lifetime spectra provides a useful means to detect the local charge density and to study the correlation between the electronic structure and the high-T_c superconductivity.