Experimental evidence for surface pinning by defects in superconducting niobium crystals

The extrapolated critical current density JF is observed to increase strongly as the angle ⊘ between the magnetic field (HC2 ) and a flat surface of a niobium crystal prism is decreased. The data can be represented by the empirical equations, where JF0 and JFi are critical current densities when the Lorentz force on flux lines is out of, and into, the surface respectively. The equation for Jp[sgrave] holds for all values ⊘ but that for JF i holds only for ⊘ greater than some critical value. A 1% tensile deformation increases a by at least an order of magnitude for H<2·0 koe; for each face of the prism the increase in a is proportional, to the number of dislocations threading that face. Annealing decreases a, ageing the prism in air at room temperature increases a. These results demonstrate that pinning by crystal defects near the surface is the principal factor in determining the surface contribution to the critical current in niobium and imply further that crystal defects near the surface are much more effective pinning centres than the same species of defect in the bulk. Since the increase in a, induced by plastic deformation, extrapolates to zero at H c2. the enhanced effectiveness of dislocation pins near the surface cannot be attributed to the presence of the surface sheath. Although the surface flux pinning model outlined in the preceding paper can predict many of these results, there are also important discrepancies.