Field dependence of the longitudinal muon polarization for anomalous muonium

An unusual longitudinal-magnetic-field dependence of the muon polarization for anomalous muonium (Mu[sup *]) in polycrystalline semiconductor targets is predicted, and has been observed for Si. As a function of field, the experimentally measured muon polarization at 53 K exhibits a cusp, i.e., a discontinuous jump in the slope from negative to positive, at 340(5) mT, in good agreement with theory. This occurs because the effective field on the [mu][sup +] vanishes at this external field as the angle between it and the Mu[sup *] symmetry axis approaches 90[degree]. At 200 K no cusp is seen because Mu[sup *] has been thermally annihilated. Fractional populations of the different muonium species are derived from the 53-K data. All Mu[sup *] and other anisotropic centers, including molecular radicals, should exhibit such a cusp, allowing the possibility of observing such species even in disordered or amorphous solids where spectroscopic detection might be impossible. Integral counting (and therefore very high data rates) can be used for these observations.