Abstract
We study the relaxation of a twodimensional (2D) ultracold Bose gas from a nonequilibrium initial state containing vortex excitations in experimentally realizable square and rectangular traps. We show that the subsystem of vortex gas excitations results in the spontaneous emergence of a coherent superfluid flow with a nonzero coarsegrained vorticity field. The stream function of this emergent quasiclassical 2D flow is governed by a PoissonBoltzmann equation. This equation reveals that maximum entropy states of a neutral vortex gas that describe the spectral condensation of energy can be classified into types of flow depending on whether or not the flow spontaneously acquires angular momentum. Numerical simulations of a neutral point vortex model and a Bose gas governed by the 2D GrossPitaevskii equation in a square reveal that a largescale monopole flow field with net angular momentum emerges that is consistent with predictions of the PoissonBoltzmann equation. The results allow us to characterize the spectral energy condensate in a 2D quantum fluid that bears striking similarity to similar flows observed in experiments of 2D classical turbulence. By deforming the square into a rectangular region, the resulting maximum entropy state switches to a dipolar flow field with zero net angular momentum.By deforming the square into a rectangular region, the resulting maximum entropy state switches to a dipolar flow field with zero net angular momentum.
Original language  English 

Article number  043642 
Journal  Physical Review A 
Volume  94 
DOIs  
Publication status  Published  25 Oct 2016 
Profiles

Hayder Salman
 School of Mathematics  Associate Professor in Applied Mathematics
 Centre for Photonics and Quantum Science  Member
 Quantum Fluids  Member
Person: Research Group Member, Academic, Teaching & Research