Critical current of high quality NbTa thin films and heterostructures

We report results from experiments aimed at controlling the pinning properties of a model thin film system. In general, pinning in thin films (even epitaxially grown) is dominated by the surfaces, both through intrinsic surface pinning, and as a result of growth nuclei and defects at the surface. However, by grading the surfaces of NbTa films to pure Ta in a heteroepitaxial system we have eliminated surface pinning, and the grain boundary pinning present in polycrystalline films. Measurements of critical current as a function of magnetic field angle have shown that the surface pinning contribution follows a (sinΦ)1/2 dependence identical to that attributed to intrinsic pinning in high Tc materials. (orig.)