Symmetry of the remanent-state flux distribution in superconducting thin strips: Probing the critical state

The critical-state in a thin strip of YBa2Cu3O7-delta a is studied by magneto-optical imaging. The distribution of magnetic flux density is shown to have a specific symmetry in the remanent state after a large applied field. The symmetry was predicted [Phys. cRev. Lett. 82, 2947 (1999)] for any j(c)(B), and is therefore suggested as simple tool to verify the applicability of the critical-slate model. At large temperatures we find deviations from this symmetry, which demonstrates a departure from the critical-state behavior. The observed deviations can be attributed to an explicit coordinate dependence of j(c) since both a surface barrier and flux creep would break the symmetry in a different way.