Normalizing current of type II superconductors in longitudinal magnetic fields

The current-carrying capacity of semi-reversible type II superconductor, Nb-50 at .% Ta, is studied in the presence of a stationary longitudinal magnetic field H. As the transport current is introduced, the resistivity begins to increase gradually at the critical current and shows a clear jump from the resistive state to the fully normal state at the normalizing current Isub(n). The Isub(n) (H) vs.H curve exhibits a maximum at the field Hsub(m)=0.2Hsub(c2) independent of the temperature in the range examined, 4.2 to 5.5K. The enhancement factor Isub(n)(Hsub(m))/Isub(n)(O) amounts to 1.3 to 1.6. The normalizing current Isub(n) is proportional to the sample radius R, and not to the cross-sectional area. These results are in qualitative agreement with the prediction based on the force free model. However, there is a large discrepancy between theory and experiment in the size of Isub(n): the calculation gives Isub(n) (Hsub(m))/5R.Hsub(c2)=0.58, while the observed values are only 14 to 24% of this ideal value. (author)