Surface superconductivity in niobium and niobium-tantalum alloys

The critical field for surface superconductivity, $H_{c3}$, and the upper bulk critical field, $H_{c2}$, have been measured as a function of temperature and mean free path for Nb and Nb(Ta) alloys in an attempt to study the spatial variations of the superconducting interaction constant $N \mathrm{}\left(0\right)\mathrm{} V$ near a vacuum-metal interface. At temperatures below $\frac{T}{T_c}=0.85\mathrm{}$ the pure-Nb sample shows $\frac{H_{c3}}{H_{c2}}$ values well above 1.695, as predicted by Hu and Korenman for the limit of long electronic mean free path. As the mean free path decreases, there is a regular depression of $\frac{H_{c3}}{H_{c2}}$ toward 1.695. At temperatures above $\frac{T}{T_c}=0.85\mathrm{}$, however, there are striking deviations from the theory which may arise because the interaction constant is slightly depressed at the surface. A model calculation by Hu shows that changes in $N \mathrm{}\left(0\right)\mathrm{} V$ at the surface of 0.7% for pure Nb and 1.6% for Nb-1.0-at.%. Ta can account for the for the experimental results.