Superconducting properties of fractal Nb/Cu multilayers

Abstract

Multilayers of Nb and Cu with a fractal or periodic stacking sequence were prepared by electron-beam evaporation onto sapphire substrates. Their superconductivity was investigated by measurements of Tc and the temperature and angular dependence of the upper critical field Bc2. For low temperatures T≪Tc, all samples show the characteristic behavior of two-dimensional superconductivity independent of the stacking sequence, whereas for temperatures near Tc the type of layering determines the effective dimensionality, resulting in a ‘‘multicrossover’’ behavior in fractal samples.