Imaging of vortices in superconductors by electron beam scanning

Abrikosov vortices trapped in a superconducting tunnel junction and oriented perpendicular to the barrier plane were imaged by electron beam scanning at 1.6 K. We have used NbAlOxNb junctions. As an important feature, the top Nb electrode was covered with a SiO2 film of 300 nm thickness, absorbing most of the 5 keV beam energy. The signal generating the image is explained by a model, assuming that the beam-induced electronic excitations in the SiO2 overlay film are trapped in the local magnetic field protruding from a vortex, resulting in an increased recombination rate. In addition to providing a novel approach to the imaging of the vortices in superconductors, our results are important for understanding quasiparticle losses in tunnel junction detectors.