Defect Dependence of Positron Lifetimes in Oxide Superconductors
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DEFECT DEPENDENCE OF POSITRON LIFETIMES IN OXIDE SUPERCONDUCTORS P.A. STERNE-, J.C. O'BRIEN-, R.H. HOWELL" and J.H. KAISERt *Lawrence Livermore National Laboratory, Livermore, CA 94550 t Department of Physics, University of Texas at Arlington, TX 76019
ABSTRACT First principles calculations of the positron lifetimes are used to interpret experimental lifetime data for Ba1_...KBiO3. The observed lifetimes are found to be very sensitive to the presence of defects in the sample. The temperature dependence of the lifetime and the change in lifetime with sample aging are both attributed to defects. The qualitative differences between metal vacancy defects and oxygen vacancy defects in metal oxides are also discussed.
INTRODUCTION Positron lifetime experiments provide a very sensitive probe of the defect structure in materials,[1, 2] since the positively charged positrons are strongly repelled by the atomic nuclei, and so tend to favor regions with vacancies and other defects. Positrons are trapped at these defects and annihilate with electrons there. The annihilation rate, the inverse of the lifetime, depends on the electron charge density sampled by the positron, and so different defects have their own characteristic lifetimes. In some cases, the experimental spectra provide clear evidence of the influence of defect trapping on the positrons, such as a strong temperature dependence in the measured lifetime. In other cases, the spectrum may be almost temperature independent, and apparently characteristic of the bulk, defect-free lifetime, when, in fact, the annihilation rate is dominated by trapping at defects. One of the major problems with this experimental technique is determining when the measured lifetimes are defect rather than bulk related, and which defects are responsible for the observed lifetime, or range of lifetimes. In this paper we present a theoretical study of positron lifetimes in on Bal-.K.KlBiOa (BKBO) and compare our calculations with experimental data for samples with x = 0.4 and 0.5. 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. The calculations indicate that values of lifetimes consistent with experiment are only obtained when we consider positron lifetimes at defect sites. This is consistent with the observed lifetime dependence on sample aging, which is presumed to be due to a rearrangement of these defects over time. The different effects of metal and oxygen vacancies are also discussed.
METHOD The theoretical calculations have been performed using the method described in detail in Ref. [4]. This is a first principles approach for calculating the positron lifetime in solids, based on the Linear Muffin Tin Orbital method.[5] Electron-electron and electron-positron interactions are included throu
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