Quantum diffusion and spin dynamics of muons in copper

We have studied the quantum diffusion of positive muons in pure copper over the temperature range 20 mK ≤ T ≤ 200 K using the technique of muon spin relaxation (μSR). 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 behaviour of the muon hop rate are well explained by the recently developed theories of 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, in its first application to μSR 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. This allows one to rule out the possibility of metastable muon sites contributing to any significant portion of the muon polarization.Science, Faculty ofPhysics and Astronomy, Department ofGraduat