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 Δo decreases continuously and appears to go to zero at the SI transition. In granular films the transport properties degrade while Δo 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.