Quantum diffusion of positive muons in iron

Abstract

The longitudinal muon-spin relaxation rate ${\ensuremath{\Gamma}}_{1}$ of a spherical $\ensuremath{\alpha}$-iron single crystal in an applied magnetic field of 4.5 T parallel to 111> shows a maximum at $T=3.7$ K. Near the maximum ${\ensuremath{\Gamma}}_{1}(T)$ is in agreement with a muon diffusivity ${D}^{{\ensuremath{\mu}}^{+}}$ proportional to the temperature $T$ and hence with one-phonon-assisted tunneling. The results strongly support the view that the stable muon site is the tetrahedral interstice. Below about 3 K evidence for metastable occupancy of a second type of site has been found.