Magnetic flux quanta in annular Josephson junctions in a barrier-parallel dc magnetic field

Flux quanta (fluxons) trapped in an annular Josephson junction reside in a spatially periodic pinning potential when a uniform external magnetic field is applied parallel to the plane of the junction's tunnel barrier. We investigate the fluxon behavior by measuring the current versus voltage curves, the magnetic dependence of the junction critical current, and by means of spatially resolved measurements imaging the pinned fluxons. We derive the fluxon potential energy including fluxon-fluxon interaction. Fair agreement of this calculation with our experimental results is obtained.