Diffusion and localization of positive muons in niobium single crystals at low temperatures in the presence of impurities measured in zero and high transverse fields

• Published in: Journal of the less-common metals 1984-01, Vol.101, p.335-342
• Main Authors: Gygax, F.N, Hintermann, A, Rüegg, W, Schenck, A, Studer, W, Van Der Wal, A.J, Kaplan, N
• Format: Article
• Language: English
• ISSN: 0022-5088
• DOI: 10.1016/0022-5088(84)90110-3

We studied the trapping and diffusion behaviour of positive muons ( μ +) in niobium single crystals by comparing the measured depolarization rate with model calculations. Zero-field muon spin resonance measurements reveal the temperature dependence of the diffusion coefficient and also clearly indicate that the different trap configurations above and below about 22 K are correlated. The activation energy needed to escape from the low temperature traps was found to be 200 ± 20 K (17 ± 1.5 meV). Subsequently, the angular dependence of the depolarization rate was measured at 14.0 and 36.8 K in a transverse field of 7.47 kG by rotating a niobium single crystal around its 〈110〉 axis, which was kept perpendicular to both the field and the μ + polarization. This allows the site symmetry of the trapped muon to be determined as well as the lattice distortions around it. The trapping site at 36.8 K could be identified as a tetrahedral site next to a tantalum impurity, and possibly also close to an interstitial (nitrogen or oxygen) impurity. A local lattice relaxation ΔR R of 6.7(6)% for nearest neighbours and of −(6 ± 2)% for next-nearest neighbours has been deduced. The low temperature curve shows a much more pronounced angular dependence than the high temperature curve, indicating a completely different μ + environment. A satisfactory explanation for these data has not yet been found.