LOW-TEMPERATURE REGIME OF DIFFUSION OF MUON IMPURITIES IN COPPER

We extend the usual small-polaron theory of diffusion to take into acc ount states with finite energy width. The possibility of incoherent ho pping down to T = 0 follows as a direct consequence. Assuming that pos itive muons in copper are in tunneling states involving just two sites and resonating with the crystal modes we calculate the temperature de pendent diffusion coefficient. The results explain quantitatively the experimental data, including: the anomalous regime occurring at temper atures below T = 80 K and the flat region close to T = 0. The calculat ion considers the real vibrational modes of copper, which leads to a r emarkable agreement between theory and experiment.