Step Structure Observed in the Superconducting Transition of a Bulk Pb Wire with Very Low Current Densities

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We have developed a SQUID picovoltmeter and precise temperature control ( ∼μK) to measure sharp R(T) superconducting transitions of low impedance samples, with low current densities. We present measurements obtained on a bulk cylinder of lead (Φ0.25 mm) in transverse magnetic field. A distinctive hysteretic step structure is observed. It is washed out on increasing the current, and reinforced in higher field. Classical models for the dissipation mechanisms in the intermediate state are briefly reviewed and used to give a first interpretation of these results, although a complete understanding is not reached yet.

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