Upper critical fields of periodic and quasiperiodic Nb-Ta superlattices

Upper critical fields have been studied for two series of Nb-Ta superlattices grown by molecular-beam epitaxy with both periodic and quasiperiodic (Fibonacci sequence) layering. X-ray results are presented to characterize the nature and quality of the layering. Positive curvature in the perpendicular upper critical field ($H_{c2\perp }$), pronounced negative curvature near $T_c$ in the parallel upper critical field (${\mathit{H}}_{\mathit{c}2\mathrm{?}}$), and dimensional crossover are observed in both types of samples. For quasiperiodic samples two upturns are observed in ${\mathit{H}}_{\mathit{c}2\mathrm{?}}$ with decreasing temperature. These are shown to be associated with dimensional crossover occurring twice as the superconducting coherence length in the growth direction, ${\xi }_{\perp }$, samples the two length scales, 2$d_{\mathrm{Nb}}$ and $d_{\mathrm{Nb}}$, that are present in these structures.