Pairbreaking Induced by Magnetic Impurity Doping Shrinks the Giant Magnetoresistance Peak of the Cooper Pair Insulator*

Films near the critical point of a boson fluctuation dominated superconductor to insulator transition exhibit a giant magnetoresistance peak that grows exponentially with decreasing temperature. Experiments indicate that Cooper pair transport dominates the low magnetic field side of the MR peak so that the resistance rise indicates increasing Cooper pair localization. To probe whether the peak arises from magnetic field induced orbital dephasing of the Cooper pair states or Cooper pair breaking, our group investigated its response to a pure pairbreaking perturbation: magnetic impurity doping. We quench deposited a-Bi films onto nanoporous anodized aluminum oxide substrates and tuned their thickness to produce an insulating state of Cooper pairs. Gd impurity doping 1) reduces the size of the MR peak while pushing it to lower magnetic fields and 2) causes changes in the zero magnetic field resistance that are much smaller than the MR peak. How this behavior suggests that orbital dephasing effects are primarily responsible for the MR peak will be discussed.