Abstract
We have investigated superconducting indium break junctions at large bias current. Most of the contacts showed up to three additional anomalies that can be attributed to the thermodynamic critical field Bth = 29 mT, as well as to the lower and upper critical field Bc1 and Bc2 of the deformed indium at the contact. Bth fits well the expected self-magnetic field for contact diameters derived from the Sharvin formula for ballistic contacts. According to the conventional understanding vortex rings are created at the contact when the self-magnetic field exceeds Bc1. We suggest that this concept can not be applied here because our contacts (1 – 30 nm diameter) are much smaller than the superconducting coherence length (∼ 100 nm) which defines the diameter of the vortex core. We discuss possible beam-narrowing mechanisms.
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