Pinning effects in very reversible superconductors

When the surface barrier is removed from reversible PbTl specimens, it is found that they do not obey the Bean model in a.c. conditions. The response is nearly linear with large penetrations of the a.c. signal. However, a model which takes into account the reversible oscillations of flux lines can explain the results. Critical current densities obtained from this model agree with those measured by other methods. Critical current densities as low as 4 A/sq cm were found and the factors affecting reversibility are discussed.