LORENTZ CONTRACTION OF FLUX QUANTA OBSERVED IN EXPERIMENTS WITH ANNULAR JOSEPHSON TUNNEL-JUNCTIONS

Vortices (magnetic flux quanta) in Josephson tunnel junctions can move at velocities near the propagation velocity of light in the junction, and they undergo the Lorentz contraction. In annular junctions, pairs of vortices and antivortices are created; these move in opposite dire ctions and collide with each other. Using low-temperature scanning ele ctron microscopy, we can visualize the collision region. We observe th e contraction of the collision region with increasing vortex velocity. With the assumption that the length of the collision region is propor tional to the length of the vortices, we can directly image the Lorent z contraction of magnetic flux quanta.