| In this thesis, the synthesis, structure, and superconducting properties of Nb/Ta multilayers are discussed. The multilayers were prepared by magnetron sputtering. X-ray diffraction was used to study the layering and structure of the samples. Longitudinal electrical resistivity measurements were used to obtain the elastic mean free paths in the individual layers. Using the information on structure and mean free path, the results of critical temperature, upper critical field, and critical current measurements were analyzed.; The multilayers are found to grow epitaxially on sapphire substrates, but with substantially less dependence on substrate orientation or deposition temperature than has been previously seen. The system forms a coherent superlattice, with very little strain between the niobium and tantalum layers, as expected. The x-ray diffraction profiles are identical to those of a mosaic crystal, indicating that the multilayers are well ordered. Electrical resistivity measurements indicate that the samples are in the nearly clean to moderately dirty limit, in a superconducting sense.; The results of superconducting critical temperature measurements are analyzed in terms of the current proximity effect theories. For multilayers with a bilayer period, (LAMDA), of less than 100 (ANGSTROM), the effect of interdiffusion at the interface is important, causing a reduction in the measured critical temperature below the predicted Cooper limit. For larger (LAMDA), the current proximity effect theories are only able to qualitatively describe the data, which is due to several limitations of the theory.; The measurements of upper critical field, both perpendicular and parallel to the direction of layering, are examined in light of the current models. Again, only qualitative agreement is found, due to the assumptions in the models. Three-dimensional to two-dimensional crossover has been observed, and an empirical definition of dimensional crossover is proposed for metallic multilayers. A new phase transition, predicted for multilayers composed of two superconductors with different diffusion constants, has been observed.; The critical current measurements for the magnetic field perpendicular to the layers show a substantial increase in the flux pinning as (LAMDA) increases, which is due to the small, but finite, strain in the superlattice. The nature of the pinning goes from single particle-like for large (LAMDA) to collective pinning at small (LAMDA), with the pinning interaction due to dislocations. For the magnetic field parallel to the layers, an increase in the flux pinning is observed, along with a substantial change in the nature of the flux pinning force. Both of these are due to pinning resulting from the multilayering, as well as from dislocations. |
