The true mechanism of spontaneous order from turbulence in two-dimensional superfluid manifolds

TL;DR

This paper investigates the mechanisms behind spontaneous vortex order in 2D superfluid turbulence, revealing that vortex escape from boundaries, rather than vortex clustering via annihilation, drives the observed order.

Contribution

It identifies vortex boundary escape as the key mechanism for spontaneous vortex order in 2D superfluids, challenging previous beliefs about vortex clustering.

Findings

Vortex pair annihilation emits sound waves that damp vortex motion.

Vortex clusters do not form in boundaryless 2D spherical BECs.

Vortex escape from boundaries explains spontaneous vortex order.

Abstract

In a two-dimensional (2D) turbulent fluid containing point-like vortices, Lars Onsager predicted that adding energy to the fluid can lead to the formation of persistent clusters of like-signed vortices, i.e., Onsager vortex (OV) clusters. In the evolution of 2D superfluid turbulence in a uniform disk-shaped Bose-Einstein condensate (BEC), it was discovered that a pair of OV clusters with opposite signs can form without any energy input. This striking spontaneous order was explained as due to a vortex evaporative-heating mechanism, i.e., annihilations of vortex-antivortex pairs which remove the lowest-energy vortices and thereby boost the mean energy per vortex. However, in our search for exotic OV states in a boundaryless 2D spherical BEC, we found that OV clusters never form despite the annihilations of vortex pairs. Our analysis reveals that contrary to the general belief, vortex-pair annihilation emits intense sound waves, which damp the motion of all vortices and hence suppress the formation of OV clusters. We also present unequivocal evidences showing that the true mechanism underlying the observed spontaneous OV state is the escaping of vortices from the BEC boundary. Uncovering this mechanism paves the way for a comprehensive understanding of emergent vortex orders in 2D manifolds of superfluids driven far from equilibrium.