| Finely layered Niobium/Germanium composites with unique physical and superconducting properties are discussed. These materials are studied as model systems for quasi-two-dimensional superconductors and have provided quantitative verification of dimensional crossover behavior in complete agreement with the Josephson-coupling theory.; The composites consist of alternating sputter-deposited thin films of Nb and Ge. Individual layer thickness ranges from 5 (ANGSTROM) to 100 (ANGSTROM); typical samples contain 50 Nb/Ge layer pairs of fixed thickness ratio.; We have investigated the structural and transport properties of the composites as a function of layer thickness. Likewise the critical temperature and normal-state properties, including the heat capacity and density of states (inferred from critical field measurements), have been studied.; Measurements have been made of the parallel upper critical fields below T(,c) and the fluctuation conductivity above T(,c) of samples with selected Ge thicknesses. These results have revealed a systematic variation in the dimensional nature of these systems both with Ge layer thickness and, in the quasi-2D systems, temperature.; Systematic behavior in anomalous "positive curvature" of the perpendicular upper critical fields has been examined and discussed in the context of a possible manifestation of 2D vortex-lattice melting. |
