Inductive transition of niobium and tantalum in the 10-MHz range. I. Zero-field superconducting penetration depth

Abstract

The inductive transition of superconducting Nb and Ta was investigated at 15 MHz. These measurements provide values of ${\lambda }_L\mathrm{}\left(0\right)\mathrm{}$ for both Nb and Ta which are compared with other work. Particular attention has been given to impurities at the metal surfaces, and to how $\lambda$ is affected through a change in the boundary condition on the Ginzburg-Landau order parameter. We emphasize that in accurate measurements the surface must be characterized by a parameter describing the slope of the order parameter at the surface. A peak in the inductive skin depth has been observed in the region very close to $T_c$, where $\lambda >\delta$, and this allows $T_c$ to be determined very precisely.