  Ultrathin amorphous Bi films, patterned with a nano-honeycomb array of holes,
can exhibit an insulating phase with transport dominated by the incoherent
motion of Cooper pairs of electrons between localized states. Here we show that
the magnetoresistance of this Cooper pair insulator phase is positive and grows
exponentially with decreasing temperature, for temperatures well below the pair
formation temperature. It peaks at a field estimated to be sufficient to break
the pairs and then decreases monotonically into a regime in which the film
resistance assumes the temperature dependence appropriate for weakly localized
single electron transport. We discuss how these results support proposals that
the large MR peaks in other unpatterned, ultrathin film systems disclose a
Cooper Pair Insulator phase and provide new insight into the Cooper pair
localization.
