Dimensional and Flux Lattice Transitions in Artificially Layered Superconductors

Artificially layered materials are ideal model systems to tune the dimensionality of the superconducting order as well as the structure of the flux line lattice. Our extensive studies of Pb/Ge multilayers indicate that the anisotropy of the upper critical field H,, strongly depends on the Ge separator thickness d,, . The dimensional crossover from D = 3 towards D = 2, which is revealed by an upturn in the temperature dependence of the parallel critical field H;l2, is also confirmed by the observed excess fluctuation conductivity above T,. In samples for which D = 2 at all temperatures (dGe > 50A), the structure of the flux line lattice for a perpendicular field H , can be probed by measuring the critical current density J , . The variation of J,(H,) exhibits an anomalous minimum, whose amplitude and position depend on dGe , the number of bilayers, temperature, and pinning strength. An interpretation in terms of either fluxlattice decoupling or melting is presented.