Identification of neutral bond-centered muonium in n-type semiconductors by longitudinal muon-spin relaxation

We report measurements of the longitudinal field-dependent muon-spin relaxation rate (${\mathit{T}}_1^{\mathrm{-}1}$) in n-type Si and GaAs under conditions where coherent spin precession of muonium is unobservable. A peak in ${\mathit{T}}_1^{\mathrm{-}1}$ as a function of magnetic field is observed and shown to be characteristic of neutral bond-centered muonium (${\mathrm{Mu}}_{\mathrm{BC}}^0$) experiencing spin-exchange scattering with free carriers. These results establish that neutral ${\mathrm{Mu}}_{\mathrm{BC}}^0$ does not convert to negatively charged muonium in n-type Si below approximately 200 K and is present in metallic n-type GaAs:Si at 5.5 K.