OBSERVATION OF SYMMETRICAL PATTERNS IN THE FLUX DISTRIBUTION OF THIN YBA2CU3O7-X FILMS

We have investigated the magnetic field distribution in epitaxial YBa2Cu3O7-x films on square SrTiO3 and MgO substrates. In fields between 20 mT and 1T a magnetooptic method was applied, complemented by a scanning micro Hall probe for the low field range. The flux is observed to penetrate in a rather regular way, displaying the fourfold symmetry of our samples. The penetration starts preferentially from the sample edges. A qualitative description of this penetration process is suggested, which relies on the curvature of the shielding current paths.