DETERMINATION OF CURRENT AND FLUX DISTRIBUTION IN SQUARES OF THIN-FILM HIGH-TEMPERATURE SUPERCONDUCTORS

An experimental determination of the flux distribution in square sampl es of thin-film superconductors has been conducted by the use of a sca nning Hall probe and a magneto-optical thin film. Two 10 mm x 10 mm sa mples, one YBa2Cu3O7 and one TlBaCaCuO, have been examined at 77 K wit h a field applied perpendicular to the surface of the film. For both s amples, the current distribution was calculated from the measured fiel d data. At low fields, flux penetrates at the centres of the edges giv ing rise to the distinctive star pattern often noted in magneto-optica l studies and creating a complex current distribution. As the field pe netrates fully, the current distribution pattern evolves into simple c oncentric squares. The evolution of the screening currents was found t o be qualitatively the same in both materials, although the magnitudes of the critical currents differed by a factor of up to 10. For both s amples, in full field penetration, the current is found to follow the outline of the sample. Lines of constant current are to a good approxi mation concentric squares. The remnant magnetization, M(rem), is found to be J(c)D/3, where 2D is the size of the square, in complete agreem ent with the critical-state model.