Spatially resolved observation of static magnetic flux states in YBa₂Cu₃O_7-δ grain boundary Josephson junctions

With low-temperature scanning electron microscopy, the magnetic flux states in high critical temperature Josephson junctions have been imaged. The experiments were performed with YBa₂Cu₃O_7-δ thin-film grain boundary Josephson junctions fabricated on [001] tilt SrTiO₃ bicrystals. For applied magnetic fields parallel to the grain boundary plane, which correspond to local maxima of the magnetic field dependence of the critical current, the images clearly show the corresponding magnetic flux states in the grain boundary junction. The spatial modulation of the Josephson current density by the external magnetic field is imaged directly with a spatial resolution of about 1 micrometer.