Dimensional crossover in fractal multilayered superconductors

Superconducting properties of multilayers consisting of alternating Nb and Cu layers with a fractal stacking sequence have been investigated. Dimensional crossover is observed in the temperature dependence of the parallel critical magnetic field Bc2∥ from “fractal behaviour” near Tc where Bc2∥∼(1−T/Tc)ƒ with 0.5 \< ƒ \< 1 to twodimensional behaviour Bc2∥∼(1−T/Tc)12 below a crossover temperature Tcr. Tcr strongly depends on the fractal dimension Dƒ of the multilayered superconductor.