Superconductivity effect on electrical conduction in insulating granular films

We report the temperature and magnetic-field dependence of electrical resistance of indium/indium-oxide (In/InOx) composite granular films in the insulating regime. The films show Mott variable-range hopping behavior at high temperatures, crossing over to a region of a much stronger temperature dependence of resistance near the superconducting transition temperature Tc of the grains. Below the crossover temperature the films show a large and negative magnetoresistance with a field dependence of ln[R(T,H)/R(T,0)]H2 in the low-field limit. We show that the temperature dependence of the zero-field resistance is well explained by a modified version of the conduction model for granular systems proposed by Adkins et al., which indicates that the crossover behavior of zero-field resistance is an effect of the grain superconductivity rather than the crossover from Mott to Efros-Shklovskii conduction. We also show that the magnetoresistance data can be explained quantitatively with the same model by taking into account the suppression of the gap in an external field.