Muons in type-II superconductors: μ+ diffusion in ultra-pure niobiumdiffusion in ultra-pure niobium

Abstract The diffusivityD μ of positive muons (μ+) in the mixed state of superconducting high-purity, high-perfection niobium single crystals is investigated by measurements of the relaxation of the transverse muon spin polarization (μ+SR). The method makes use of the strong magnetic field gradients existing in the mixed state of Type-II superconductors and monitorsD μ through the variation of the magnetic field felt by the μ+ during their diffusion through the crystals. For μ+ near the centres of the flux lines inNb it givesD μ(4.6 K)=(8±2)·10−11m2S−1. The positive temperature coefficient ofD μ indicates that at liquid-helium temperatures the diffusivity of μ+ inNb is mainly due to phonon-assisted tunnelling processes.