Evidence for collective pinning in high-temperature superconductors

The relaxation behavior of the remnant moments of two thin-wall rings, one closed and the other open, and powders of various sizes of melt-textured YBa2Cu3O7 has been investigated. The activation energies (U0's) obtained are the same at all temperatures in all cases, directly demonstrating that the relaxation in a high-temperature superconductor (HTS) cannot be explained in terms of a distribution of U0's associated with different domains as previously suggested. A temperature-dependent pinning volume of flux bundles is necessary to account for the experimental results. A scaling relationship, U0[1-(T/Tc)2]m/(BJ2)k, is therefore proposed for such a collective pinning to explain the macroscopic properties of HTS's.