Insulator–superconductor transition in bi-layers of Co clusters and Bi

We have performed electrical resistance measurements on films of pure Bi and hybrid films of Co/Bi consisting of Co nanoclusters having a mean size of ∼4.5 nm covered by Bi. Films were produced by deposition of Bi on top of a layer of well-defined Co clusters onto sapphire substrates kept at low temperatures. As-deposited Co-clusters/Bi films reveal a superconducting-like transition close to T c of amorphous Bi, however, with variable range hopping behavior below T c, i.e., with a diverging resistance for T → 0. Upon annealing at low temperature, a superconducting state with zero resistance for T → 0 is found. Finally, annealing at higher temperature leads to non-superconducting crystalline Bi showing weak-localization behavior typical for a disordered semimetal (or metal). For some films, we find a superconducting transition occurring in two steps, the reason for this is not quite clear.