Magnetic Field Enhanced Insulating Behavior in Thin Films with Local Cooper Pairing

The effects of a perpendicular magnetic field on insulating amorphous indium oxide thin films exhibiting local superconductivity have been investigated. At very low temperatures the application of relatively low magnetic fields produces a giant positive magnetoresistance that increases with decreasing temperature. This suggests that the ground state in zero field may be a Cooper pair insulator. At nonzero temperatures a magnetic field enhances this insulating behavior. The low temperature I-V characteristics exhibit strong nonlinearities with threshold voltages for enhanced conduction whose magnitudes are independent of both temperature and magnetic field. This behavior indicates a connection with Cooper pairing and might be associated with the depinning of a charge structure. Although X-ray analyses characterize the films being amorphous, AFM images exhibit significant roughness that might be responsible for the formation of regions with a high local density of Cooper pairs. This work was supported in part by the National Science Foundation under grant NSF/DMR-0455121.