Spectroscopy of a fractional Josephson vortex molecule.

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 \PΦ\ < |PΦ|0, where PΦ0 &ap; 2.07 × 10-15 Wb is the magnetic flux quantum, is pinned. Each vortex has an oscillatory eigenmode with a frequency that depends on PΦ/PΦ0 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 p = 2πPΦ/PΦ0, and on the bias current &ggr; 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. [ABSTRACT FROM AUTHOR]