Shallow-level muonium centre in CdS

Abstract

A new type of muonium defect centre has been observed in undoped CdS at low temperatures ( T<20 K). The hyperfine parameters are A ∥=335 (7) kHz and A ⊥=199 (6) 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 20 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 a d=26 a 0=1.4 nm is calculated for this defect, with a 0 being the Bohr radius for the free muonium.