Shallow-level muonium centre in CdS

Abstract

A new type of muonium defect centre has been observed in undoped CdS at low temperatures (T<20Â K). The hyperfine parameters are A[short parallel]=335 (7)Â kHz and A[perpendicular]=199 (6)Â kHz (approximately 10-4 of the vacuum value) with the symmetry axis along the Cd-S bond direction. The disappearance of the centre at around 20Â K is in agreement with the binding energy of a shallow centre. The extremely weak hyperfine interaction implies that the electron is only weakly bound to the muon and is distributed over a large complex of atoms in the neighborhood of the muon resembling the dilated hydrogen-like wave functions of shallow centres in a dielectric medium. Comparing the hyperfine interaction with that of free muonium, a Bohr radius of ad=26a0=1.4Â nm is calculated for this defect, with a0 being the Bohr radius for the free muonium.