No abstract

We have investigated muon diffusion in niobium with controlled amounts of interstitial impurities. The polarization decay was interpreted in terms of a two-state model where the muon is alternatively in a state of free diffusion or in traps, A good fit of all data was obtained, yielding correlation times for the jump motion, as well as capture and release rates of the traps. The muons are found to be mobile in Nb down to 14 K. ^ In this work we investigate the diffusion of positive muons in samples of niobium with controlled amounts of impurities and develop a model which makes it possible to understand and analyze the diffusion of muons in the presence of traps. The influence of defects must be better understood before the elementary diffusion processes in ideal lattices can be discussed. This influence is also a phenomenon of interest in itself. The problem is especially interesting for group-Vb bcc metals, where extensive data on hydrogen diffusion are available for comparison/ In these metals the muon-spin-rotation (/xSR) results of different groups^'^ have so far been inconsistent, probably because of differences in sample preparation and impurity content. It is known from diffusion studies of H in Nb-N samples that immobile interstitials (N) act as traps for the diffusing species (H) such that the diffusion is strongly affected,^'^ When the diffusion of muons in solids is studied by the juSR method,^ the interesting quantity is the decay of the average muon polarization P{t) during the observation of the muon spin precession^ The decay is due to the dephasing of the individual muon precession by the randomly directed dipole fields from surrounding nuclei, and the formula developed for T2 in NMR should apply, We have, in the case of diffusion in a homogeneous crystal,'^ P,U) = exp{-2a/T,nexp(-//T,)-l+//Tj}, (1) where 0/ is the second moment of the frequency distribution due to the internal fields, and r^ is the average time during which correlations between the frequencies exists; r^ is proportional to the mean residence time r of the muons at interstitial sites. For a frozen-in muon we have a^T^^»l and the damping is Gaussian, Pdt) ~exp{-o/t^)y whereas for a diffusing muon with a^2T/«l we expect Piit) =exp{'-2(j/T^t) (Lorentzian damping),Here we will consider the depolarization for muon diffusion in a crystal with randomly distributed traps. We treat this case by introducing a two-state model which describes repeated capture and release processes,^ A muon which diffuses in the undisturbed lattice is caught by a trap on the average after a time r^; a muon in a trap escapes after an average time TQ. The two states have different polarization decay. The polarization decay of the free state P^it) is given by Eq. (1) in the case of infinite lifetime of this state, Tj -°o. The polarization decay of the trapped state is governed by an analogous expression Po(/), where a/ is replaced by a^^, the second moment of the frequency distribution in the traps, and r^ is replaced by r^^, the corre-

No abstract

No abstract

No abstract