|Abstract:||The nature of the hysteresis in the magnetic superconducting transition of lead reported by Decker,et al. has been studied in some detail. The individual isothermal transitions making up the hysteresis loop are characterized by transition widths approximately equal to that expected on the basis of geometry. The hysteretic transitions are displaced from the reversible transition of a well annealed specimen by approximately equal but opposite field intervals, the superconducting to normal transition occurring at the higher field. The width of the hysteresis loop always increases as temperature is decreased, in extreme cases becoming as large as 40 or 50 gauss at 1.3˚K (where Hc is approximately 780 gauss).
Isothermal resistive measurements of the superconducting transition are also reported. They indicate that some superconducting phase persists in lead to fields as high as three or four hundred gauss above Hc. Increasing the temperature or the measuring current forces the resistive transition back toward Hc. In general, the hysteresis width and persistence of superconductivity to high fields appear to be closely related.
The picture of the lead samples which emerges is similar to the Mendelssohn “sponge” model suggested to explain the behavior of superconducting alloys (although the present effects occur in pure lead). It is thought that there is a connected network of very small filaments with critical field greater than the reversible critical field of the bulk material pervading the entire volume of the specimen. These filaments are believed to be associated with defects in the crystalline lattice. It is shown that most of the observed features of the transitions can be explained in terms of this model.
Two methods have been employed to produce the hysteresis in nearly reversible samples; addition of impurities and, more extensively, low temperature plastic strain. Several measurements are discussed which shed some light on the nature of the lattice defects giving rise to the superconducting filaments in each case.