Positron localization in Li-Mg alloys

Abstract

Positron-annihilation experiments have been performed on a number of disordered Li-Mg alloys, and information on the spatial distribution of annihilating positrons has been deduced. Measurements of the angular correlation of the two annihilation $\ensuremath{\gamma}$ rays and of the positron lifetime have been made and the results used to obtain core-annihilation rates. The core rate is sensitive to the distribution of the annihilating positrons between the two different sorts of sites in the alloy, and the results indicate that the positrons are sampling a much greater proportion of Li atoms in the alloy than the mean Li concentration would indicate. First-principles calculations of the positron distribution based on the positron pseudopotential are in qualitative agreement with the experimental results but do not include effects of localization. Smearing of the Fermi cutoff region of the angular correlation curve can be a measure of positron localization. Experimental results for the smearing for the dilute Li alloys indicate that the annihilating positron is localized in a region extending roughly 20 \AA{}. It is concluded that the positron is annihilating from bound states localized to Li-rich regions of the alloy. These results are important in the interpretation of positron-annihilation experiments in disordered alloys and also in the understanding of low-lying electron states in disordered alloys.