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 ${\mathrm{NbAlO}}_x\mathrm{Nb}$ junctions. As an important feature, the top Nb electrode was covered with a ${\mathrm{SiO}}_{\mathrm{2}}$ 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 ${\mathrm{SiO}}_{\mathrm{2}}$ 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.