ON THE FLUX BUNDLE SIZE IN WEAKLY PINNED SUPERCONDUCTORS

Recently it was found that the transverse flux bundle size estimated from the observed irreversibility field in terms of the flux creep theory takes much smaller values than the prediction from the elastic correlation length of the fluxoid lattice. This is more remarkable for the case of larger correlation length, i.e., for superconductors with a weaker flux pinning strength. It is assumed that the flux bundle size is determined so that the critical current density under the flux creep is maximized. This assumption is analogous to the irreversible thermodynamic principle for the linear dissipation systems that the energy dissipation is minimized. The obtained flux bundle size is proportional to the one-third power of the value derived from the elastic correlation length. Qualitative and quantitative agreements are obtained between the prediction and the experiments. The theoretical result also explains the observed finite relaxation rates at ultra-low temperatures which have been believed to be explained only by the quantum tunneling mechanism.