Influence of Controlled Additions of Oxygen on the Superconductivity of Niobium

The critical current density vs applied transverse magnetic field was measured, for a superconducting niobium single crystal of controlled oxygen content, at successively higher levels of concentration; the oxygen content was increased in increments ranging from 2 to 50 ppm, under conditions of constant substructure, from an initial level of approximately 5 ppm to a final level of 360 ppm. The over-all critical current density increased considerably on the addition of oxygen, and assuming γ to be independent of oxygen concentration, the upper critical field increases in accordance with the predictions of the GLAG theory for type II superconductivity. The critical current density above the upper critical field, which has been ascribed to nucleation effects at defects or surfaces, was also very sensitive to the oxygen content.