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

We report on the motional behavior of the tetrahedral interstitial muonium (Mu_T) 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 Mu_T 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 Mu _T 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, τ_c^-1∼10¹¹ Hz, indicating fast diffusion of Mu_T, 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 Mu_T state to the stable bond-centered muonium (Mu_BC) state prevents the diffusion of Mu _T being explored beyond 400 K.