Tunneling Density of States of Indium Oxide Films Through the Superconductor to Insulator Transition

We present planar tunneling junction spectroscopy measurements on disordered amorphous indium oxide films on both sides of the superconductor-insulator transition. Our measurements directly reveal a superconducting gap in the insulating phase. The measured energy gap has the same energy scale on both sides of the transition. Unlike the case of granular films, the tunneling curves cannot be fitted to the BCS density of state expression, even when introducing a broadening parameter to account for nonthermal broadening sources. The results are consistent with the presence of superconducting islands of which superconducting properties depend on film disorder and on the carrier density of the superconducting material.