PINNING-TO-BARRIER CROSSOVER IN YBA2CU3O7-DELTA SINGLE-CRYSTALS

The behaviour of magnetic flux in high purity untwinned YBa2Cu3O7-delt a (YBCO) single crystals is visualised by means of the magneto-optical technique. It is observed that after zero field-cooling at high tempe ratures near T-c, flux penetrates directly to the sample center over a flux free edge area, in contrast to the usual Bean-like flux penetrat ion from the edges. This fact clearly shows that volume pinning become s negligible compared with the edge barrier. The role of the recently rediscovered geometrical barrier in the crystal magnetisation is discu ssed.