Structural characteristics of Bi-based superconductors studied by positron annihilation

Abstract

We have measured the positron lifetime spectra as a function of temperature from 80 to 300 K and Doppler-broadening annihilation radiation (DBAR) spectra in normal and superconducting states for three kinds of Bi-based superconductors: Bi2212, Pb- and F-doped Bi2223. For Pb-doped Bi2223 superconductor, anomalies in the temperature dependence of positron lifetimes indicate the structural instabilities. However, a continuous increasing in the mean lifetime is observed for F-doped Bi2223 system and interpreted in terms of a simple thermal expansion with an average linear expansion coefficient alpha is similar to 35*10(-6). The difference spectra of DBAR spectra after deconvolution between two states show an evidence for charge transfer between the Cu-O and Bi-O planes in Bi-based superconductors. The role of oxygen defects in charge transfer was discussed.