| dc.description |
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<sub>c2</sub> strongly depends on the Ge separator thickness d<sub>Ge</sub>. 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<sub>c2</sub><sup>||</sup>, is also confirmed by the observed excess fluctuation conductivity above T<sub>c</sub>. In samples for which D = 2 at all temperatures (d<sub>Ge</sub> > 50 Å), the structure of the flux line lattice for a perpendicular field H<sub>⊥</sub> can be probed by measuring the critical current density J<sub>c</sub>. The variation of J<sub>c</sub>(H<sub>⊥</sub>) exhibits an anomalous minimum, whose amplitude and position depend on d<sub>Ge</sub>, the number of bilayers, temperature, and pinning strength. An interpretation in terms of either fluxlattice decoupling or melting is presented. |
