Anomalous temperature dependence in the depolarization rate of positive muons in pure niobium

The temperature dependence of the depolarization rate for positive muons implanted into a pure niobium crystal has been measured in an external field of 102 Oe. Depolarization is observed at temperatures below 100 K, with a pronounced minimum occurring in the vicinity of 20 K. A study of the magnetic-field dependence at low temperature indicates that the muons occupy tetrahedral interstitial sites. The data are interpreted in terms of the self-trapping of the muon at low temperature, and trap-limited diffusion at temperatures above 65 K. Comparisons are made with the properties of hydrogen in niobium.