Stability of normal state bubbles in the intermediate state of type I superconductors

The intermediate state in a type I superconducting Indium slab is observed with the high resolution magneto-optical imaging technique. The patterns consist of coexisting cylindrical-shape (bubbles) and elongated-shape (lamellae) normal state domains surrounded by the superconducting phase. We show that the shape cylindrical or elongated of the domains is correlated to their surface area. Those first experimental results indicate that there is a maximum surface area above which bubbles are no more observed. This maximum surface area is found to coincide with the stability limit of the bubble shape.