Patterns close to the critical field in type-I superconductors

We study the patterns formed in the intermediate state of type-I superconducting lead (Pb) slabs close to the transition to the normal state. Magneto-optical images reveal avalanchelike propagation of superconducting stripes in a normal matrix when decreasing the field while they transform into superconducting bubbles when increasing the field. Our results show that the combination of superconducting stripe width and its field dependence is not compatible with any of the present theoretical models. The stability of the structures is studied by modulating the magnetic field periodically in time. We find that close to the transition to the normal state a structure composed of superconducting bubbles is closer to equilibrium than a stripe pattern. We also observe that when the field is subsequently decreased the bubble pattern, as in other systems, transforms into a stripe one by continuous elongation and bending and only rarely by branching. © 2007 The American Physical Society.