Diffusion and trapping of muons in aluminum: New experiments and comparison with Kondo theory

Abstract

The diffusion and trapping of positive muons in aluminum has been studied using the method of muon spin rotation. New measurements have been performed on Al samples doped with Mg, Si, Ga, and Ge impurities (which trap the muons), and for comparison also on an Ag sample doped with Er. Trapping rates and trapping site information were obtained for temperatures between 0.05 and 50 K. A global fit to these and earlier published data for trapping by other impurities (Li, Mn, Ag) and vacancies has made it possible to deduce the temperature dependence of the intrinsic diffusion in Al for the range 0.05–200 K and to compare it with the recently developed theory by Kondo for light interstitial diffusion in metals. In the low-temperature range (0.05–2 K) it shows a $T^{\mathrm{-}0.7}$ dependence, followed by an approximately linear T dependence (2–20 K) and an exponential (activated) behavior at higher temperatures. The theory for the low-T diffusion, which is based on tunneling with energy dissipation through the screening electrons, describes the experimental data with reasonable parameters. For the intermediate range there are strong indications of one-phonon-assisted diffusion. The trapping sites close to the impurities are discussed with reference to the elastic distortions and the electronic density modifications introduced by the different impurities.