History effects and transition to plastic vortex dynamics in YBa 2Cu 3O 7− δ single crystals

Abstract

Partial magnetization loops have been performed in pure YBa 2Cu 3O 7− δ single crystals with various types and densities of pinning sites. The obtained results have demonstrated that the critical current density in detwinned samples depends strongly on the magnetic history of the superconductor. As shown in a recent study [S. Kokkaliaris, P.A.J. de Groot, S.N. Gordeev, A.A. Zhukov, R. Gagnon, L. Taillefer, Phys. Rev. Lett., 82 (1999) 5116], this effect can be attributed to the presence of metastable topological disorder in the vortex system following the transition from a dislocation-free Bragg glass to a disordered vortex phase. Mapping of this transition in the field–temperature ( B– T) phase diagram has revealed two important features: the transition line is always located in proximity to the onset of the second magnetization peak and shifts to lower fields with increasing oxygen deficiency. The influence of twin boundaries on the Bragg glass phase has also been investigated and found to be negligible for low densities of twins. In contrast, a high concentration of twin planes is shown to destroy it and eliminate the memory effects.