Initial or thermally controlled impurity trapping of muons in niobium?

Abstract

We have studied muon depolarization in some very well characterized samples of niobium (pure Nb with <1 ppm impurities and Nb doped with 15 at.ppm N and 53 at.ppm Ta, respectively). This has allowed us to separate the influence of substitutional and interstitial impurities on theμSR linewidthσ. The purest sample shows a low but non-zero linewidth from 0.1 to 70 K. Ta-doping increases the width strongly below 20 K. while N-doping gives a broad maximum between 30–70 Kand a considerable width below 20 K.

Conventional two-trap models cannot explain the occurrence of a linewidth significantly lower that that predicted for staticμ ⁺ and constant over a wide temperature range. A consistent explanation of these three observations can however be obtained from the following model: In pure Nb only a fraction of the muons is self-trapped thermally; the other muons do not form small polarons but remain in a propagating metastable nonlocalized state. Impurities can catalyse further initial polaron formation, decreasing the metastable fraction. This process causes temperature-independent plateaus inσ up to the detrapping temperature. The muons localized at shallow traps (Ta induced) can diffuse at higher temperatures and be trapped again at deeper traps (associated with the N-impurities).