The interaction distance between flux lines and pinning centres

Fluxon pinning in niobium films

“…Most models of fluxon dynamics [3,4] follow the pioneering work of Bardeen and Stephen [30] which describes the loss mechanism as resulting from a viscous movement of the fluxons. However, the amplitude of the movement of strongly pinned fluxons, as the present study has been dealing with, is expected to be strongly reduced with respect to that of the more commonly studied configurations.…”

“…The value taken by the upper critical field is essential to the understanding of the fluxon structure and of their dynamics [3,4]. Accurate measurements are available for pure niobium [18][19][20] showing that for t ≤ 0.5 the temperature dependence of the upper critical field H c2 and of the thermodynamical critical field H c agrees to within a few percent with the Gorter-Casimir expressions…”

“…To a good approximation, H c2 (0) = 0.40 T and H c (0) = 0.20 T. In the presence of impurities H c2 is expected to increase as l decreases [3,4]. Extensive data [21] are available on the increase of H c2 resulting from the presence in the niobium lattice of interstitial oxygen in various proportions.…”

“…As most models of fluxon dynamics [3,4] predict R fl ≅ R n H ext / H c2 , R n being the value of the surface resistance in the normal conducting state and H c2 the upper critical field, this difference between films and bulk has been blamed [5,6] on a similar difference between the values taken by H c2 . In Reference [1] R fl was measured for a large variety of films spanning a wide range of electron mean free paths and the evolution of R 0 fl anf R 1 fl was tracked for the first time from the dirty limit (films containing more than 10 atomic percent of neon) to the clean limit (deep-drawn bulk niobium subsequently annealed at 1100°C).…”

Fluxoid pinning by vanadium carbide precipitates in superconducting vanadium

“…The specimen is heated above its transition temperature and then cooled through T in the applied magnetic field. This procedure produces an unstrained fluxoid lattice (74). Once the temperature of the specimen returns to the prede te rmined tempera ture , the current is swept and the longitudinal voltage measured across the specimen .…”