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 T_c and an influence of the surrounding matrix or adsorbed gas on normal-state resistivity ρ(0) and T_c can be satisfactorily explained by assuming the occurrence of surface superconductivity due to a strongly increased density of states N(ɛ_F) at the cluster surface.