Superconducting properties of Nb/Ge metal semiconductor multilayers

We have made layered superconductors composed of alternating thin films of sputter-deposited Nb and Ge. Individual layer thicknesses range from 5 to 100 \AA{}, each sample containing from 50 to 80 Nb/Ge layer pairs with a fixed thickness ratio, $\frac{{D}_{\mathrm{Nb}}}{{D}_{\mathrm{Ge}}}$. As revealed by x-ray analysis, the samples have a distinct superlattice structure consisting of amorphous Ge layers and, except for those with ${D}_{\mathrm{Nb}}<30$ \AA{}, (110) textured polycrystalline Nb layers. Transport studies reveal that each Nb layer itself consists of a clean central portion with damaged regions at the Nb/Ge interfaces. A proximity-effect analysis of the samples indicates that the presence of this damaged Nb interfacial layer largely governs the observed transition temperatures of these multilayers. Measurement of the upper critical fields and fluctuation conductivity have confirmed both the dimensional and the temperature-dependent dimensional crossover effects expected in Josephson-coupled quasi-two-dimensional superconductors. The observed degree of control over interlayer coupling possible with these multilayers suggests they are a model system for the study of quasi-two-dimensional superconductivity.

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