Three-vortex configurations in trapped Bose-Einstein condensates

We report on the creation of three-vortex clusters in a ${}^{87}\mathrm{Rb}$ Bose-Einstein condensate by oscillatory excitation of the condensate. This procedure can create vortices of both circulations, so that we are able to create several types of vortex clusters using the same mechanism. The three-vortex configurations are dominated by two types, namely, an equilateral-triangle arrangement and a linear arrangement. We interpret these most stable configurations respectively as three vortices with the same circulation and as a vortex-antivortex-vortex cluster. The linear configurations are very likely experimental signatures of predicted stationary vortex clusters.

Figure 1

Atomic optical density images showing configurations of stable vortices forming (a) an equilateral triangle or (b) a linear array. Images were taken after 15 ms of free expansion.

Figure 2

(a) Definition of the largest internal angle $\alpha$. (b) Schematics of the tripole configuration of vortices; arrows indicate the vortex circulation direction.

Figure 3

Observed relative frequency of three-vortex configurations as a function of the angle $\alpha$. The inset shows the expected distribution of $\alpha$ when the vortices are distributed at random positions.

Figure 4

Evolution of the largest angle $\alpha$, in Gross-Pitaevskii simulations starting from various three-vortex configurations in a circularly trapped 2D BEC. Initial configurations are shown on the right.