Distortion of Interference Fringes and the Resulting Vortex Production of Merging Bose-Einstein Condensates

We investigate the distortion of interference fringes in colliding and merging Bose-Einstein Condensates due to inter-atomic interaction effects. We also consider the effect of the distortion on the spontaneous formation of vortices. The interference pattern depends crucially on two relevant parameters, the relative center-of-mass velocity and peak density of the initial state, which comprises a pair of initially well-separated, but coherent, condensate clouds. We identify three qualitatively distinct regimes of behavior during the subsequent evolution of the condensates: collision; expansion; merging. Using a comprehensive set of numerical simulations based on the Gross-Pitaevskii equation, we specify the cross-overs between these regimes and identify the system parameters required for dynamical instabilities and vortex creation. Interference fringes, which would be planar in the absence of interactions, are distorted by the combined effects of interaction and the nonuniform density profile of a harmonically trapped condensate. In appropriate parameter regimes, the distortion is sufficiently large to produce a net circulation and thereby a dynamical instability occurs by which vortices form with their cores on the interference minima.