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

Abstract

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.