Onset of superconductivity in 2D granular films composed of Bi clusters

Two dimensional films comprising of Bi clusters ranging in size from 25 to 100 Å show an influence of the underlying matrix on normal state resistivity and the superconducting transition temperature Tc. This is in contrast to crystalline Bi. Quantum size effects are observed by changing the deposition temperature, which determines cluster size. These observations can be attributed to superconductivity at the surface. Implications for the requirement of a critical number of superconducting clusters before the establishment of global phase coherence are briefly discussed.