Tunneling spectroscopy measurements directly reveal the existence of a superconducting gap in the insulating state of homogenously disordered amorphous indium oxide films. Two films on both sides of the disorder induced superconductor to insulator transition show the same energy gap scale.
This energy gap persists up to relatively high magnetic fields and is observed across the magnetoresistance peak typical of disordered superconductors. The results provide useful information for understanding the nature of the insulating state in the disorder induced superconductor to insulator transition.
We provide the first direct evidence for the existence of a superconductive gap in an insulator state. The gap amplitude is hardly affected by the dramatic change in the global transport properties. The results strongly suggest that homogeneously disordered superconducting films near the SIT contain superconducting islands that are similar on both sides of the transition. The DOS is not influenced by the SIT nor by the magnetoresistance peak indicating that the insulator contains regions of finite order parameter amplitude that are similar to those in the superconductor.