Diffusion of interstitial muonium, MuT, in a 13C diamond

We report on the motional behavior of the tetrahedral interstitial muonium (MuT) the chemical analogue, and light isotope of neutral atomic hydrogen, in a 90% enriched 13C diamond. Muonium forms after implantation of positive muons into diamond. In the 13C diamond, the muon spin relaxes due to the motion of MuT against the background of randomly oriented spins of the host nuclei. Spin-lattice relaxation of the initially spin-polarized muons, in their capacity as nuclei of the MuT atoms, was studied as a function of magnetic field and temperature in the ranges 20 mT-0.4 T and 10-400 K, respectively. The extracted hop rate is large, τc−1∼1011 Hz, indicating fast diffusion of MuT, as expected for such a light interstitial atom. Its approximately constant value over the entire measured temperature range suggests weakly scattered coherent tunneling rather than incoherent or phonon-assisted tunneling, consistent with the high Debye temperature of diamond and the shallow interstitial potential for neutral hydrogen or muonium. There are indications of increasing localization of the wave-packet, i.e., of an incipient change to incoherent tunneling, towards the higher temperatures. Thermal conversion of the mobile MuT state to the stable bond-centered muonium (MuBC) state prevents the diffusion of MuT being explored beyond 400 K.