  The magnetical field tuned superconductor-insulator transition in amorphous
thin films, e.g., Ta and InO, exhibits a range of yet unexplained curious
phenomena, such as a putative low-resistance metallic phase intervening the
superconducting and the insulating phase, and a huge peak in the
magnetoresistance at large magnetic field. Qualitatively, the phenomena can be
explained equally well within several significantly different pictures,
particularly the condensation of quantum vortex liquid, and the percolation of
superconducting islands embedded in normal region. Recently, we proposed and
analyzed a new measurement that should be able to decisively point to the
correct picture: a drag resistance measurement in an amorphous thin-film
bilayer setup. Neglecting interlayer tunneling, we found that the drag
resistance within the vortex paradigm has opposite sign and is orders of
magnitude larger than that in competing paradigms. For example, two identical
films as in Sambandamurthy et al. 2004 with 25 nm layer separation at 0.07 K
would produce a drag resistance $\sim10^{-4}\Omega$ according the vortex
theory, but only $\sim10^{-12}\Omega$ for the percolation theory. We provide
details of our theoretical analysis of the drag resistance within both
paradigms, and report some new results as well.

Superconductivity;Disordered Systems and Neural Networks