Photo-muSR study of the shallow muonium state in CdS

In cadmium sulfide, shallow donor muonium (Mu) centers can be formed and stabilized in low temperatures. They are characterized by hyperfine (HF) constants typically on the order of 0.1~MHz. This is significantly smaller than the HF constants of Mu in other semiconductor systems. Hence, electron orbitals of shallow Mu are quite dilated, with their Bohr radius in the range of a few nm. This size is significantly larger than other Mu species studied in traditional semiconductors. Hence, it is readily anticipated that their interaction with charge carriers is also different, presumably having a greater sensitivity due to the physical size. This is an intriguing problem in Mu studies that have never been addressed. In this proposal, we aim to observe Mu in CdS undergoing spin and charge exchange interaction with optically injected free carriers.