Negative Magnetoresistance in the Nearest-neighbour Hopping Conduction in Granular Gold Film

The low temperature (0.5–55 K) conduction of semicontinuous gold film vacuum deposited at T ≈ 50 K is studied. The film is near the percolation threshold (thickness 3.25 nm). Its resistance is extremely sensitive to the applied voltage U . At low enough U the film behaves as an insulator (two-dimensional granular metal). In this state the dependences R(T ) ∝ exp(1/T ) (for T ≤ 20 K) and R(U) ∝ exp(1/U)) (for T ≤ 1 K and U > 0.1 V) are observed. Magnetoresistance (MR) is negative and can be described by ∆R(H)/R(0) ∝ −H/T . This negative MR which manifests itself for nearest-neighbour hopping is rather uncommon and, up to now, has not been clarified. The possible mechanisms of such case of negative MR are discussed.