Superconductivity in granular systems built from well-defined rhombohedral Bi-clusters: Evidence for Bi-surface superconductivity

Granular systems built from well-defined Bi clusters with rhombohedral bulk structure show, in contrast to bulk Bi, the occurrence of superconductivity. The experimental facts of a strong size dependence of ${\mathit{T}}_{\mathit{c}}$ and an influence of the surrounding matrix or adsorbed gas on normal-state resistivity ρ(0) and ${\mathit{T}}_{\mathit{c}}$ can be satisfactorily explained by assuming the occurrence of surface superconductivity due to a strongly increased density of states N(${\mathrm{\epsilon }}_{\mathit{F}}$) at the cluster surface.