A paramagnetic muonium (Mu) state with an extremely small hyperfine parameter was observed for the first time in single-crystalline GaN below 25 K. It has a highly anisotropic hyperfine structure with axial symmetry along the [0001] direction, suggesting that it is located either at a nitrogen-antibonding or a bond-centered site oriented parallel to the c-axis. Its small ionization energy (=< 14 meV) and small hyperfine parameter (--10^{-4} times the vacuum value) indicate that muonium in one of its possible sites produces a shallow state, raising the possibility that the analogous hydrogen center could be a source of n-type conductivity in as-grown GaN.
@article{arxiv.cond-mat/0402155,
title = {Muonium as a shallow center in GaN},
author = {K. Shimomura and R. Kadono and K. Ohishi and M. Mizuta and M. Saito and K. H. Chow and B. Hitti and R. L. Lichti},
journal= {arXiv preprint arXiv:cond-mat/0402155},
year = {2009}
}