Theory of the magnetoresistance of disordered superconducting films

Recent experimental studies of magnetoresistance in disordered superconducting thin films on the insulating side of the superconductor- insulator transition reveal a huge peak (about 5 orders of magnitude compared with the resistance at the transition). While it may be expected that magnetic field destroys superconductivity, leading to an enhanced resistance, attenuation of the resistance at higher magnetic fields is surprising. We propose a model which accounts for the experimental results in the entire range of magnetic fields, based on the formation of superconducting islands due to fluctuations in the superconducting order parameter amplitude in the disordered sample. At strong magnetic fields, due to Coulomb blockade in these islands, transport is mainly through the normal areas, and thus a decrease is the size and density of the superconducting islands leads to an enhanced conductance and a negative magnetoresistance. As the magnetic field is reduced and the size and density of these islands increase, the conductance is eventually dominated by transport through the superconducting islands and the magnetoresistance changes sign. Numerical calculations show a good qualitative agreement with experimental data.