Electrical transport in indium oxide thin films near the magnetic field-induced superconductor-insulator transition

The effects of a perpendicular and a parallel magnetic field on amorphous indium oxide thin films near the superconductor-insulator transition have been investigated. The transport in insulating films exhibiting local superconductivity is found to be governed by three-dimensional Mott variable range hopping. At very low temperatures the application of relatively low perpendicular magnetic fields produces a giant positive magnetoresistance that increases with decreasing temperature. This suggests that the ground state in zero magnetic field is a Cooper pair insulator or a vortex superfluid. At nonzero temperatures a magnetic field induces this insulating behavior. The current-voltage characteristics at low temperatures which are nonlinear, exhibit temperature- and magnetic field-independent threshold voltages for enhanced conduction. This behavior suggests a connection with the presence of Cooper pairs and might be associated with the depinning of a charge structure. In superconducting films, the application of a relatively high magnetic field results in a non-superconducting state. The electrical transport in this non-superconducting regime of the thinner of two superconducting films follows two-dimensional Efros-Shklovskii variable range hopping, whereas that of the thicker one obeys a T 1/3 law characteristic of three-dimensional dirty metals. In contrast with what has been reported for amorphous indium oxide thin films with similar transition temperatures and critical fields, finite-size scaling analyses failed to produce a conclusive critical exponent product vz. This failure suggests that the phase transition in these films is to a normal metal with quantum corrections to their conductivity, rather than to an insulator. Structure and surface characterizations show that the films are amorphous, but have rough surfaces. This roughness may be responsible for the formation of regions with a high local density of Cooper pairs, which make the films effectively granular.