Positron Lifetime Studies of Defect Structures in Ba_l-xK_xBiO₃

Abstract

Temperature-dependent positron lifetime experiments have been performed from room temperature to cryogenic temperatures on Ba1−xKxBiO3. for x=0.4 and 0.5. From the temperature dependence of the positron lifetime in the normal state, we observe a clear signature of competition between separate defect populations to trap the positron. Theoretical calculations of lifetimes of free or trapped positrons have been performed on Ba1−xKxBiO3, to help identify these defects. Lifetime measurements separated by long times have been performed and evidence of aging effects in the sample defect populations is seen in these materials.

Defect Dependence of Positron Lifetimes in Oxide Superconductors

“…First principles calculations of the positron lifetime in pure and defected BKBO have been used to analyse the experimental lifetime data of Ref. [3]. We find that the lifetime is dominated by annihilation at defect sites in ali cases.…”

“…Details of the experiment are presented in an accompanying paper. [3] We use first " principles calculations of the annihilation rate to deduce what the positron lifetime should be for bulk, defect-free BKBO. We also compute the range of lifetimes expected for some simple monovacancies, and use these calculations to interpret the experimental data.…”

“…If the temperature dependence were due to competition between just two defects, each with its own characteristic lifetime, the experimental analysis of the data would have obtained a fit with two distinct sample components to the lifetime. In fact, the best fit in all cases came with a single sample lifetime component [3].…”