Emergence of Order from Turbulence in an Isolated Planar Superfluid

We study the relaxation dynamics of an isolated zero temperature quasi-two-dimensional superfluid Bose-Einstein condensate that is imprinted with a spatially random distribution of quantum vortices. Following a period of vortex annihilation the remaining vortices self-organize into two macroscopic coherent Onsager vortex clusters that are stable indefinitely-despite the absence of driving or external dissipation in the dynamics. We demonstrate that this occurs due to a novel physical mechanism-the evaporative heating of the vortices-that results in a negative-temperature phase transition in the vortex degrees of freedom. At the end of our simulations the system is trapped in a nonthermal state. Our computational results provide a pathway to observing Onsager vortex states in a superfluid Bose gas.