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 ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$ has been investigated. The activation energies (${\mathit{U}}_0$’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 ${\mathit{U}}_0$’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, ${\mathit{U}}_0$∝[1-(T/${\mathit{T}}_{\mathit{c}}$ ${)}^2$ ${]}^{\mathit{m}}$/(${\mathit{BJ}}^2$ ${)}^{\mathit{k}}$, is therefore proposed for such a collective pinning to explain the macroscopic properties of HTS’s.