Resistive Transition and Current Density Characteristics in Superconducting Niobium Containing Dissolved Gases

Oxygen and nitrogen as interstitial solutes in niobium cause the metal in transverse field to exhibit anomalous resistance minima or critical current maxima (peak effect). The anomaly appears at $H_{c2\mathrm{}}$, the upper critical field of these Type II superconductors. The resistance in the mixed state is decreased by exceeding the solubility limit, by cold working and by strain aging at 170°C, masking out the anomaly. Rapidly quenching from the solution temperature decreases the anomaly. The results suggest that the anomaly is strongest at some intermediate stage of the process of segregation and precipitation at dislocations. Resistance in the mixed state is increased by introducing more solute, by quenching, and by etching. Structural changes brought about by cold working, strain aging, and rapid quenching responsible for large changes in $J_c\mathrm{}\left(H\right)\mathrm{}$ or $R\left(H\right)\mathrm{}$ in the mixed state also cause changes in the region $H_{c2\mathrm{}}<H<\mathrm{}{H}_{c3\mathrm{}}$. In this latter region, field orientation effects and observed $\frac{H_{c3\mathrm{}}}{H_{c2\mathrm{}}}$ ratios are roughly consistent with the concept of a superconducting surface sheath as recently proposed by Saint-James and de Gennes. Pure niobium, however, tends to exhibit consistently higher ratios.