Muon diffusion and spin dynamics in copper

We have studied the quantum diffusion of positive muons in pure copper over the temperature range 12 mK≤T≤150 K using the technique of muon-spin relaxation. The rate of diffusion has been deduced from its effect upon the muon-polarization function. The measurements were made in a weak longitudinal external magnetic field, where the spin relaxation has proved to be the most sensitive to the muon hop rate below 150 K. Our results for the behavior of the muon hop rate are well explained by the recently developed theories for the quantum diffusion of light interstitials in metals by Kondo, Yamada, and others. These theories stress the effects of the conduction electrons in the metal in providing a form of ‘‘friction,’’ retarding the diffusion process. In addition, we have utilized the technique of level-crossing resonance spectroscopy, by measuring the electric-quadrupole interaction strength of the copper nuclei. These results have enabled us to show that the muon occupies the same octahedral site at all the temperatures studied, ruling out the possibility of metastable muon sites contributing to any significant portion of the muon polarization in the temperature range studied.