Superconductor-insulator transition induced by over-deposited Ge in the insulating ultrathin a-Nb film

We have observed the superconductor-insulator (S-I) transition induced by over-deposited Ge in the insulating ultrathin amorphous Nb (a-Nb) film. The experiments are performed by depositing Ge onto the insulating a-Nb film with a thickness of 1.04 nm. For dGe>0.3nm, where dGe was a thickness for Ge film, the reduction of electrical sheet resistances was seen at low temperature region, suggesting superconductor. The normal sheet resistance at 8 K decreased monotonically with increasing dGe. The simplest explanation of this S-I transition is that the electron localization effect is weakened due to the addition of Ge film.