Critical Current Densities in the Surface Pinning Region of a Superconducting Pb–20%Tl Rod

The internal magnetic-field profile was investigated for a superconducting Pb–20%Tl rod by a modified Campbell method. A large surface current was observed and it can be interpreted as carried by dislocation cells in the surface region. The shear modulus of the fluxoid lattice was estimated directly from the stress-strain relation in the reversible range of the ac field, and it was found agreeable with Labusch's expression derived for the point pins. The pinning mechanism in the present specimen is investigated and an expression of the pinning force density is derived for the existence of dense pinning-centres. The obtained magnetic-field dependence of the pinning force density agrees well with the observed data.