Mechanism for producing smooth flow by stirring a racetrack BEC

We studied the production of smooth flow in a Bose-Einstein-condensed (BEC) gas in a racetrack atom-circuit potential. The BEC is confined into a thin horizontal sheet by laser light. The racetrack potential is a channel having two straight channels connected by circular endcaps. The system is assumed to follow the Gross-Pitaevskii model. Flow is produced by stirring the BEC with a rectangular barrier. We have studied in-depth how flow is produced by this stirring. As the barrier strength increases, a backflow develops in the barrier region producing a vortex/anti-vortex pair. The outside vortex circulates like the stir. Above a critical barrier height, the vortices swap places. This generates two disturbances: (1) the vortex/antivortex pair moves off in the anti-stir direction and (2) a compression wave moving the other way. This repeats until the flow produced overtakes the barrier speed. Each new unit of flow creates another pair of disturbances. These disturbances convert the localized circulation of the vortices into macroscopic flow around the racetrack. The flow oscillates around this value until the barrier falls below the critical value determining the final flow.