Experimental study of the geometrical barrier in type-I superconducting strips

We study the energy barrier for flux entry of a geometric origin in type-I superconducting strips using magneto-optical observations of the flux struc ture. The magnetic field is applied perpendicularly to the broad surface an d, optionally, a de current is sent along the strip. As a direct manifestat ion of this barrier, the appearance of flux-free zones at the edges of the strip is observed. The width of these zones, the field for first-flux penet ration, and the critical current originated due to the geometrical barrier are measured. The dependence of these parameters on the geometry of the sam ple, applied field, transport current, and temperature presented here is in good qualitative agreement with a recent theory by Benkraouda and Clem. [S 0163-1829(98)05046-2].