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Fate Of The Bose Insulator In The Limit Of Strong Localization And Low Cooper-Pair Density In Ultrathin Films, Shawna M. Hollen, G. E. Fernandes, J. M. Xu, J M. Valles Jr.

Physics & Astronomy

A Bose insulator composed of a low density of strongly localized Cooper pairs develops at the two-dimensional superconductor to insulator transition (SIT) in a number of thin film systems. Investigations of ultrathin amorphous PbBi films far from the SIT described here provide evidence that the Bose insulator gives way to a second insulating phase with decreasing film thickness. At a critical film thickness dc the magnetoresistance changes sign from positive, as expected for boson transport, to negative, as expected for fermion transport, signs of local Cooper-pair phase coherence effects on transport vanish, and the transport activation energy exhibits a kink. …