Universal expansion of vortex clusters in a dissipative two-dimensional superfluid

TL;DR

This paper uncovers a universal expansion behavior of vortex clusters in dissipative superfluids, supported by theory, simulations, and experiments, revealing new out-of-equilibrium dynamics distinct from classical fluids.

Contribution

It introduces a new universality class for vortex cluster dynamics in dissipative superfluids, validated through analytic solutions, numerical simulations, and experimental observations.

Findings

Vortex clusters exhibit a universal expanding Rankine vortex distribution.

Numerical simulations confirm the universal expansion dynamics.

Experimental results in Bose-Einstein condensates agree with vortex fluid theory.

Abstract

A large ensemble of quantum vortices in a superfluid may itself be treated as a novel kind of fluid that exhibits anomalous hydrodynamics. Here we consider the dynamics of vortex clusters with thermal friction, and present an analytic solution that uncovers a new universality class in the out-of-equilibrium dynamics of dissipative superfluids. We find that the long-time dynamics of the vorticity distribution is an expanding Rankine vortex (i.e.~top-hat distribution) independent of initial conditions. This highlights a fundamentally different decay process to classical fluids, where the Rankine vortex is forbidden by viscous diffusion. Numerical simulations of large ensembles of point vortices confirm the universal expansion dynamics, and further reveal the emergence of a frustrated lattice structure marked by strong correlations. We present experimental results in a quasi-two-dimensional Bose-Einstein condensate that are in excellent agreement with the vortex fluid theory predictions, demonstrating that the signatures of vortex fluid theory can be observed with as few as $N\sim 11$ vortices. Our theoretical, numerical, and experimental results establish the validity of the vortex fluid theory for superfluid systems.