Experimental Study of the Disorder Effect in Strongly Disordered Superconducting Amorphous Composite Indium/indium Oxide Films.

We have studied the effect of disorder in strongly disordered amorphous composite Indium/Indium Oxide films using tunneling spectroscopy. We have performed measurements on high resistance Al/A lO_{rm x} /a-InO_{rm x} tunnel junctions as a function of bias voltage V, temperature T and supercurrent I_{rm s} in the superconducting state of the a-InO _{rm x} films, and as a function of V in the normal state. From the measurement in the normal state we found that the density of states of heavily disordered a-InO _{rm x} films substantially differs from the predictions, which raises a question about the universality of the theory. We developed a new tunneling technique that provides much information on disordered superconductors, which are otherwise obliterated by its own normal properties. This method was also applied to wide InO_{ rm x} films in order to determine the perpendicular magnetic penetration depth lambda _{rm p} as a function of temperature and disorder. The measured T and R _square-dependence of lambda _{rm p} agree well with the dirty-limit theory except for a factor of 2 discrepancy in magnitude. From studies on the superconducting density of states using the technique, we conclude that the inelastic scattering rate 1/tau_{rm in}, which is most likely due to the disorder -enhanced electron-electron interaction, has a T ^3 dependence for T << T_{rm co} where hbar/tau_{ rm in}Delta<< l. This does not agree with the exponential T-dependence predicted by conventional theory.