Dimensional crossover in superlattice superconductors

We have observed dimensional crossover from anisotropic three-dimensional behavior to two-dimensional behavior in both the temperature dependence and the angular dependence of the upper critical fields of Nb/Cu superlattice superconductors. The crossover occurs when the coherence length ${\xi }_{\perp }$ perpendicular to the layer planes is of the order of the copper layer thickness $D_{\mathrm{Cu}}$. We have observed the crossover (1) by varying the temperature, thereby varying ${\xi }_{\perp }$ relative to $D_{\mathrm{Cu}}$ and (2) by varying $D_{\mathrm{Cu}}$ relative to ${\xi }_{\perp }$. The complicating effects due to surface superconductivity have been eliminated by placing thick copper films as the outside layers of the samples. The measurements are in qualitative agreement with theoretical models of layered superconductors.