Current-driven transformations of the intermediate-state patterns in type-I superconductors

Dynamic structure of the intermediate state was studied in pinning-free thick Pb strips using real-time magneto-optical visualization. It is found that topological hysteresis can be lifted by applying sufficiently large transport current. Namely, laminar structure that appears in a static case when magnetic flux exits the sample is turned into a tubular pattern when sufficiently large transport current is applied. The tubes move under the influence of the Lorentz force and, at higher currents and fields, evolve into a dynamic pattern of equally spaced superconducting walls oriented perpendicular to the current force-free configuration. Magnetic-fieldcurrent phase diagram of the intermediate state is discussed. Our results imply that tubular structure is more favorable than the laminar structure because it is topologically mobile, whereas tubular pattern is topologically constrained with the multitude of degenerate metastable state.