Negative magnetoresistance in the nearest-neighbor hopping conduction

We propose a size effect which leads to the negative magnetoresistance in granular metal-insulator materials in which the hopping between two nearest-neighbor clusters is the main transport mechanism. We show that the hopping probability increases with magnetic field. This is originated from the level crossing in a few-electron cluster. Thus, the overlap of electronic states of two neighboring clusters increases, and negative magnetoresistance results.