MAGNETOOPTICAL STUDY OF TWIN BOUNDARY PINNING IN YBA2CU3O7-DELTA

Using a high-resolution magneto-optical technique we studied the flux penetration and pinning in YBa2Cu3O7-delta crystals in the presence of isolated twin boundaries. The most important factor determining the b ehavior of the twin boundaries is the direction of flux motion with re spect to the boundary. Up to temperatures close to T-c twin boundaries are effective barriers for flux motion transverse to the boundaries. For flux motion parallel to the boundary the characteristic ''flame''- shaped flux patterns indicative of weak link behavior (i.e. low J(c)) of the boundary are observed.