The superconductor-insulator transition: is there a new insulating state?

We present nonlinear conductivity measurements on the insulating side of the superconductor-insulator transition in amorphous indium oxide. The results agree with previous data$^{1,2}$, and show conductance jumps at well-defined voltage bias thresholds. The current in the sample changes by as much as a factor of 10$^ {6}$ at the threshold, from our noise floor of 3x10$^{-14}$A to over 10$^{-8}$A. The jumps disappear above a magnetic-field- dependent temperature T$^{*}$, which is 0.11K or lower. The threshold voltage changes from 20$\mu$V to over 0.2V (4 orders of magnitude) by application of a magnetic field. We ask whether a true zero conductance state exists in our samples. DC measurements reveal pseudo-exponential I-V characteristics, which can be extrapolated to find the high Ohmic resistance of these samples at low temperatures. The extrapolated R(T) curves typically show a sub-activated trend at low T. Our results suggest that our samples have zero conductance only at the absolute zero of temperature. \\[4pt] (1) Sambandamurthy et al. PRL 92, 107005\\[0pt] (2) Baturina et al. Nature Letters 452, p613