Vortex regime crossover in the Bragg glass phase in a YBa2Cu3O7−δ crystal

We have investigated the behavior of vortex lattice in a weakly pinned YBa2Cu2O7−δ crystal using rotational magnetization measurements. At low applied magnetic fields parallel to the ab plane of the crystal, we have found that the vortex state undergoes a crossover from a relatively strongly pinned vortex regime to a weakly pinned vortex regime in the Bragg glass phase. In the weakly pinned vortex regime of the Bragg glass phase, vortices were described to be completely depinned and aligned parallel to the applied field.