2D Mott Hopping of Vortices in an Amorphous Indium Oxide Film

Abstract

The electron transport behavior of a thin film of amorphous indium oxide was studied as it was driven across the superconductor-insulator transition by a perpendicular magnetic field. For the range of field values between zero and the critical field of the transition, a positive slope in temperature dependence of the resistance was observed in the data. These data are best described by the form of two dimensional Mott variable range hopping as applied to vortices. The quality of this fit is demonstrated over several orders of magnitude in resistance and over a broad range of fields using several methods of analysis. The observation of variable range hopping of vortices is the main result of this work. Data from a second sample were also found to be consistent with vortex variable range hopping, as were data extracted from a paper within the literature. These examples suggest that this behavior has probably been overlooked in the past. The field-dependence of the characteristic hopping temperature T_0 at very low fields was predicted using a granular model for the thin film. This is consistent with the picture of effective granularity induced in a highly disordered superconductor, which also explains various properties of the film, including the magnetoresistance peak observed at high fields. What was not observed, however, was a crossover from Mott to some other hopping behavior at high fields, where corrections to the hopping exponent due to vortex-vortex interactions were expected. The reason for this is an open question.