Muonium in Sulphur: μSR's Oldest Puzzle

Abstract

New level–crossing resonance data shows the muonium defect centre in solid sulphur to have axial symmetry with huge dipolar anisotropy at low cryogenic temperatures. Above 100K,the two principal values of the hyperfine tensor fall dramaticallyand pro rata, both apparently collapsingto zeroatthe melting point. Thefallis accompaniedby the onset of muon spin–lattice relaxation, visible on the microsecond timescale with low–field rates peaking around room temperature. In conjuction with old zero–field data, the low–T hyperfine parameters are determined accurately. New supercell density–functional calculations suggest their assignment to muonium at a bond–centre (BC) site in a closed–ring S8Mu BC complex. The striking decrease of time–average parameters and the appearance of fluctuations causing relaxation are attributed to a dynamic equilibrium or chemical exchange with neutral configurations having much lower hyperfine coupling, accessed by small cyclic displacements from the BC site. Time–average occupancy of this site falls with temperature and vanishes at the melting point.