Electron tunneling studies of ultrathin films near the superconductor-to-insulator transition

Electron tunneling measurements on ultrathin quench-condensed films near the superconductor-to-insulator (SI) transition reveal that the superconducting state degrades with increasing normal state sheet resistance, R_□, in a manner that depends strongly on film morphology. In homogeneously disordered films, the superconducting energy gap Δ_o decreases continuously and appears to go to zero at the SI transition. In granular films the transport properties degrade while Δ_o remains constant. Measurements in the normal state reveal disorder enhanced e^- -e^- interaction corrections to the density of states. These effects are strong and depend on morphology in a manner that is consistent with their playing an important role in driving the SI transition.