Depolarisation studies of positive muons in copper, vanadium, niobium and tantalum single crystals

The behaviour of positive muons (μ+) has been studied in Cu, V, Nb and Ta single crystals, using the muon spin rotation (μSR) technique. The measurements were performed in the temperature regime 5-365 K, in transverse external magnetic fields of 50-5500 G. In FCC Cu the μ+ diffusional motion can be described in terms of quantum mechanical jump processes of a small polaron among octahedral interstitial sites. The activation energy, pre-exponential factor and tunnelling matrix element are obtained from an orientation average. The electric field gradient (EFG) at the μ+ nearest-neighbour nuclei has been determined as q = 0.30 A-3. In BCC V. Nb and Ta the μ+ diffusion is trap-limited. As a function of the temperature the μ+ probe different classes of trapping sites associated with host impurities. At low temperature, before decaying, the μ+ always end up in a trapped state which shows that the intrinsic μ+ motion between traps is fast and seems to reflect properties of a band diffusion. The plateau (maximum) relaxation rates and their deviation from rigid lattice values cannot be explained on the basis of a μ+ local lattice expansion alone. Three additional possibilities have to be considered: (i) the μ+ never reach the static limit; (ii) tunnelling states are formed; (iii) the μ+ local environment is distorted. The magnitudes of the EFG at the μ+ nearest-neighbour nuclei have been evaluated. (author)