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 (Hc2), pronounced negative curvature near Tc in the parallel upper critical field (Hc2?), and dimensional crossover are observed in both types of samples. For quasiperiodic samples two upturns are observed in Hc2? with decreasing temperature. These are shown to be associated with dimensional crossover occurring twice as the superconducting coherence length in the growth direction, , samples the two length scales, 2dNb and dNb, that are present in these structures.
ASJC Scopus subject areas
- Condensed Matter Physics