Dimensionality Crossovers in the Parallel Critical Fields of Nb/Nb 0.6 Zr 0.4 Multilayers

We have systematically investigated the parallel critical fields B c2|| in superconducting multilayers of Nb/Nb 0.6 Zr 0.4 by varying the Nb layer thickness at different constant thicknesses of the NbZr layer. With decreasing temperature we find for most samples a 3D-2D transition, followed by a steep upturn in B c2|| . According to the Takahashi-Tachiki theory, the 2D behaviour is due to nucleation of the superconductor order parameter in the Nb layers. Our data indicate that B c2|| in the 2D regime is indeed correlated with the thickness of the Nb layer, i.e. the effective thickness of the layer causing 2D behaviour is given by the thickness of the Nb layer plus parts of the enclosing NbZr layers. Other new results are that the upturn asymptotically leads to ordinary bulk behaviour of Nb 0.6 Zr 0.4 , and the observation of a 3D-3D transition for large thickness of the Nb layer.