Novel muonium state in CdS

A new type of muonium defect center has been observed in undoped CdS below 20 K. The hyperfine interaction amounts only to approximately 10(-4) of the vacuum value, and is shown to have axial symmetry along the Cd-S bond dire ction. Results suggest that the muon is close to the sulfur antibonding sit e and the paramagnetic electron density is distributed over a large volume. In contrast to the behavior in other semiconductors, muonium forms a shall ow center in CdS. By implication, analog isolated hydrogen impurity atoms a ct as electrically active shallow-level defect centers in CdS.