Evolution of large-scale flow from turbulence in a two-dimensional superfluid
In two-dimensional turbulent flow the seemingly random swirling motion of a fluid can evolve towards persistent large-scale vortices. To explain such behavior, Lars Onsager proposed a statistical hydrodynamic model based on quantized vortices in which the persistent large-scale vortices correspond to negative temperature states. I will describe our experimental confirmation of Onsager's model in a superfluid gas of atoms. We drag grid barriers through an oblate atomic gas Bose-Einstein condensate to generate non-equilibrium distributions of vortices. In the subsequent evolution of the superfluid, we observe signatures of an inverse energy cascade driven by the evaporative heating of vortices, leading to steady-state configurations of clustered vortices characterized by negative absolute temperatures.