Concerning the RatioHc3Hc2in Superconducting Nb and Nb-O Solutions

The Saint-James-de Gennes relationship between $H_{c2\mathrm{}}$ and $H_{c3\mathrm{}}$ in type II superconductors has been verified in Nb and Nb-O interstitial solid solutions. The observed ratio $\frac{H_{c3\mathrm{}}}{H_{c2\mathrm{}}}={1.7}_0$ is found to be independent of $\kappa$, the Ginsburg-Landau parameter, for niobium-containing oxygen in concentrations below the solubility limit at the temperature of introduction. Critical-current data $I_c$ versus $H$ above $H_{c2\mathrm{}}$, analyzed in terms of a dimension of the superconducting sheath, (e.g., $J_s=I_c$ per cm width), reveal sharp breaks at $H_{c3\mathrm{}}$ when the material becomes normal. For a cold-worked specimen, two portions of the $J_s\mathrm{}\left(H\right)\mathrm{}$ curve may be identified, meeting at a definite field corresponding to $H_{c3\mathrm{}}$ calculated from theory. The upper portion of the curve first meets the normal state curve at a field $H_n$. The ratio $\frac{H_n}{H_{c2\mathrm{}}}$ is larger for cold-worked niobium than for cold-worked niobium-oxygen alloys.