The diffusion of positive muons was studied in the normal and superconducting states of aluminum. Large differences were observed, indicating that the diffusion mechanism depends sensitively on the interaction between the muon and the conduction electrons. The samples were high-purity aluminum doped with controlled amounts of lithium. The lithium impurities act as traps for the muons, leading to a loss of muon polarization. However, the time and temperature dependencies of the muon depolarization function G(t) could not be satisfactorily explained using a simple diffusion limited trapping model. In particular, in the superconducting state the measured G(t) requires a more complex model for a reasonable fit. These data can be described, in a first approximation, with two fractions, one corresponding to muons stopped in the defect potential created by the doping element and the other representing muons which diffuse freely without being trapped. In this model, which supports a microscopic theory by Kagan and Prokof’ev taking into account static as well as dynamic effects on the tunneling of positive particles in a conducting medium, a large fraction of the muons is practically immobile below 0.3 K in doped samples. The data can also be fitted by assuming that the spatial distribution of doping elements leads to a distribution in trapping times and that the fraction of diffusing muons is described by a so-called stretched exponential time dependence. The relative merits of these two types of interpretation are discussed.

• Received 1 February 1995

DOI:https://doi.org/10.1103/PhysRevB.52.6417