Effect of a magnetic field on the conductivity of superconductor-insulator-normal metal tunnel junctions

The influence of a magnetic field on the conductivity of superconductivity-insulator-normal metal tunnel junctions with aluminum superconducting electrodes has been investigated. The area of the junctions was 2$\mu$m$^2$ and the thickness of the superconducting aluminum electrodes was 80 nm. The nonlinearity of the current-voltage characteristics has been observed at temperatures of 100–400 mK in a transverse field of up to 28 mT, which is approximately three times greater than the critical field for pure bulk aluminum. The conductivity at zero bias voltage increased with the field by 2–4 orders of magnitude. The effect is hysteretic. It can be caused by the emergence of a vortex structure in the magnetic field above the first critical field in the aluminum film, which becomes a type-II superconductor owing to its small thickness, impurities, and structural defects.