Transition Temperatures and Critical Fields of Nb/Cu Superlattices

We present experimental results on the superconducting properties of Nb / Cu superlattices. The transition temperature of the superlattice implies a decrease in the transition temperature of single Nb films as a function of layer thickness. This is interpreted as due to the mean free path-induced decrease in the density of states at the Fermi surface, in agreement with experimentally measured magnetic susceptibilities. The temperature dependence of anisotropic critical fields is in qualitative agreement with predictions based on effective mass theories. However, the behavior of the angular dependence is considerably more complicated. In addition to the anisotropy due to the layering, there is an anisotropy due to surface superconductivity. These results are discussed in light of theories of, anisotropic critical fields.