a Department of Physics, Zhejiang University, Hangzhou 310027, China; b International Center of Materials Physics, Academia Sinica, Shengyang 110015; Department of Modern Physics, University of Science and Technology of China, Hefei 230026, China
Abstract 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-Tc superconducting systems, Y1-xCaxBa2Cu3-xFexO7-$\delta$ and Y1-xCaxBa2Cu3O7-$\delta$. 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-Tc superconductivity.
Received: 30 November 1992
Accepted manuscript online:
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