INFLUENCE OF TWIN PLANES IN YBA2CU3O7 ON MAGNETIC-FLUX MOVEMENT AND CURRENT FLOW

The effect of twin planes in YBa2Cu3O7 on the superconducting current flow is investigated. A single crystal containing only two twin planes is studied magneto-optically up to 1 T. It is found that flux enters the crystal very easily along the twin planes, but that it does not le ave the sample preferentially along the twin planes. The flux present in the twin planes reduces the flux penetration in adjacent regions of the sample. During flux penetration, the current density across the t win plane is about half of the current density in the rest of the samp le. Due to this, the shielding current breaks up into several subloops . From a simulation follows that in a thin sample flux penetration is enhanced at inward pointing irregularities of the edge of the sample.