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Abstract:
In long Josephson junctions with multiple discontinuities of the Josephson phase, fractional vortex molecules are spontaneously formed. At each discontinuity point a fractional Josephson vortex carrying a magnetic flux |Φ|<Φ0, where Φ0≈2.07×10−15 Wb is the magnetic flux quantum, is pinned. Each vortex has an oscillatory eigenmode with a frequency that depends on Φ/Φ0 and lies inside the plasma gap. We experimentally investigate the dependence of the eigenfrequencies of a two-vortex molecule on the distance between the vortices, on their topological charge ℘=2πΦ/Φ0, and on the bias current γ applied to the Josephson junction. We find that, with decreasing distance between vortices, a splitting of the eigenfrequencies occurs that corresponds to the emergence of collective oscillatory modes of both vortices. We use a resonant microwave spectroscopy technique and find good agreement between experimental results and theoretical predictions.
