Observation of stable 3-vortex/anti-vortex clusters in a superfluid Bose-Einstein condensate

We report on the observation of stable configurations of clusters with three vortices in a Rb87 Bose-Einstein condensate. We observe preferentially two types of 3-vortex/anti-vortex configurations. Vortices are either distributed as an equilateral triangle or as a liner array. The results are consistent with the theory where the triangular configuration is formed by vortices with the same circulation sign, while, in contrast, the linear array is constituted of a central vortex with a sign of circulation opposite to the side ones.