Muonium states in semiconductors

Positive muons implanted into diamond and zincblende-structured semiconductors often form hydrogen-like paramagnetic muonium (${\ensuremath{\mu}}^{+}\ensuremath{-}{e}^{\ensuremath{-}}$) states whose characteristics can be investigated with the "muon spin rotation" ($\ensuremath{\mu}\mathrm{SR}$) technique. In contrast to the case of hydrogen, which is not known to form a paramagnetic state in semiconductors, two coexisting types of muonium states are seen. "Mu" with a large isotropic hyperfine interaction, and "${\mathrm{Mu}}^{*}$" with a small [111]-axially symmetric hyperfine interaction. Both "spectroscopic" properties of these states, such as the electronic $g$ factors and the nuclear hyperfine interactions, and "dynamic" properties, such as their diffusion rates and their rates of interconversion, are accessible with $\ensuremath{\mu}\mathrm{SR}$. Direct information about the site of the muonium states is available using the channeling effect of the positron from muon decay in a crystalline host. The techniques for probing semiconductors with positive muons are described in this review, and the results they have provided to date are critically discussed. The considerable amount of theoretical work that has been invested in microscopic models of Mu and ${\mathrm{Mu}}^{*}$ is also summarized.