Negative Magnetoresistance of Granular Metals in a Strong Magnetic Field

The magnetoresistance of a granular superconductor in a strong magnetic field that destroys the gap in each grain is considered. It is assumed that the tunneling between grains is sufficiently large such that all conventional effects of localization can be neglected. A nontrivial sensitivity to the magnetic field comes from superconducting fluctuations leading to the formation of virtual Cooper pairs and reducing the density of states. At low temperature, the pairs do not contribute to the macroscopic transport but their existence can drastically reduce the conductivity. Increasing the magnetic field destroys the fluctuations, which improves the metallic properties and leads to the negative magnetoresistance.