Breather decay into a vortex/antivortex pair in a Josephson ladder

We present experimental evidence for a new behavior which involves discrete breathers and vortices in a Josephson Ladder. Breathers can be visualized as the creation and subsequent annihilation of vortex/anti-vortex pairs. An externally applied magnetic field breaks the vortex/anti-vortex symmetry and causes the breather to split apart. The motion of the vortex or anti-vortex creates multi-site breathers, which are always to one side or the other of the original breather depending on the sign of the applied field. This asymmetry in applied field is experimentally observed. (PACS numbers: 05.45.Yv, 63.20.Pw, 74.50.+r, 74.81.Fa) In complex nonlinear systems there can often be spatially or temporally coherent structures which emerge with marked particle-like properties [1] [2] [3] [4] [5]. Examples include solitons in nonlinear optics [6], kink dislocations in solids [7], skyrmions in magnetic materials [8] and vortices in superconductors [9] [10] or superfluids [11] [12]. Understanding these structures can be fundamental for many problems in physics and related fields. While many of these have been well-studied independently, how different types of structures within the same system interact and relate to each other is still very much an area of active research. Arrays of superconducting Josephson junctions are excellent model systems to study such coherent structures [13] [14] [15]. They can be fabricated with adjustable parameters, easily scaled to large numbers and measured in a straightforward way. In addition, they are also inherently nonlinear due to the sinusoidal relationship between the phase of the superconducting wavefunction and the junction’s supercurrent [16]. Two of the most fundamental coherent excitations in Josephson junction arrays are Josephson vortices and discrete breathers. Vortices are excitations which have spatially localized flux and an associated circulating current. They carry a topological charge. Discrete breathers [17], or more specifically rotational breathers or rotobreathers [18] [19] [20] [21], are time-periodic excitations which have spatially localized energy and no net topological charge. Of the different geometries of Josephson arrays, the Josephson Ladder has been demonstrated to support both vortices and rotobreathers in prior experiments. In previous work [22] it has been noted that a rotobreather in a Josephson Ladder can be equivalently thought of as a time sequence of intermittent creation and subsequent annihilation of vortex/antivortex pairs. This stems from the fundamental relation between the vorticity, n, and the circulation of the superconducting phase gradient around a plaquette in the ladder [23]: