Current-induced magnetic-flux structures in a type-I superconducting constriction

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Abstract
We have studied the current-induced magnetic-flux structures in type-I superconducting constrictions using a high-resolution magneto-optical method. The samples were Pb films 60–100 μm thick with a single constriction of 0.5–1.0 mm width, similar to the Anderson-Dayem bridge geometry. In the resistive state, moving flux-tube trains are nucleated at both sample edges, at first precisely at the narrowest portion of the constriction. With increasing current, the flux-tube trains extend more and more into the outlying regions performing a curved path. From the resistive voltage and our magneto-optical observations the flux-tube nucleation rate for a flux-tube train is estimated using the Josephson relation.