The formation of an electronic polaron is believed to be the limiting mechanism for electron transport in sapphire (Al${}_2$O${}_3)$. Measurements of the spin depolarization of positive muons $\left({\mu }^{+}\right)$ implanted in a sapphire crystal in electric fields up to 100 kV/cm show that the muonium atom (Mu=${\mu }^{+}{e}^{-})$ is formed via transport to the ${\mu }^{+}$ of a radiolysis electron created in the muon’s track. At high temperature, electron transport is consistent with a two-phonon mechanism of quantum diffusion, while at low temperature it is temperature independent due to a crossover to a one-phonon mechanism.

• Received 31 March 1997

DOI:https://doi.org/10.1103/PhysRevB.56.55