A new type of muonium defect center was observed in undoped material below 20K. The hyperfine interaction amounted to only some 0.01% of the vacuum value, and was shown to have an axial symmetry along the Cd-S bond direction. The results suggested that the muon was close to the S anti-bonding site, and that the paramagnetic electron density was distributed over a large volume. In contrast to the behavior in other semiconductors, the muonium here formed a shallow center. It was suggested that analogous isolated H impurity atoms would act as electrically active shallow-level defect centers in CdS.

Novel Muonium State in CdS J.M.Gil, H.V.Alberto, R.C.Vilão, J.P.Duarte, P.J.Mendes, L.P.Ferreira, N.Ayres de Campos, A.Weidinger, J.Krauser, C.Niedermayer, S.F.J.Cox: Physical Review Letters, 1999, 83[25], 5294-7