Muon-spin rotation as a tool for investigating the diffusion of positive muons in type-II superconductors

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Abstract
The measurement of the spin precession frequencies of immobile positive muons in the mixed state of uniformly magnetized type-II superconductors gives the distribution function of the magnetic field. Increasing muon diffusivity leads first to the suppression of the van Hove singularity associated with the field-strength maximum in the flux-line centres, then to an enhancement of the damping of the main precession frequency, and finally to a shift and motional narrowing of the precession frequency distribution. These phenomena can be used to determine muon diffusivities in Nb exceeding about 10−12m2s−1. The required information on the magnetic-field distribution function in the mixed state is presented in detail.