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Flux quanta ͑fluxons͒ 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.

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Two-dimensional arrays of overdamped Josephson junctions under microwave irradiation have been examined experimentally. We show that the shape of the Shapiro steps in the current–voltage curve of the array is closely related to the critical current spread of the junctions and that its dynamic resistance is closely related to the number of frequency locked junctions which are visualized by low-temperature scanning electron microscopy.