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

The behaviour of positive muons ( mu + ) has been studied in Cu, V, Nb and Ta single crystals, using the muon spin rotation ( mu SR) technique. The measurements were performed in the temperature regime 5-365K, in transverse external magnetic fields of 50-5500 G. In FCC Cu the mu + diffusional motion can be described in terms of quantum mechanical jump processes of a small polaron among octahedral interstitial sites. An orientation average leads to an activation energy of E=79.2+or-3.1 meV, a pre-exponential factor of D(0)=(1.66+or-0.33)*10 -8 cm 2 s -1 and a tunnelling matrix element of J=(18.3+or-1.8)*10 -6 eV. The mu + total volume expansion was found to be Delta V=3.91 AA 3 . Furthermore, the electric field gradient (EFG) at the mu + nearest-neighbour nuclei has been determined as q=0.30 AA -3 . In BCC V, Nb and Ta the mu + diffusion is trap-limited. As a function of the temperature the mu + probe different classes of trapping sites associated with host metal impurities. At low temperature, before decaying, the mu + always end up in a trapped state which shows that the intrinsic mu + motion between traps is fast and seems to reflect properties of a band diffusion.