Critical Dynamics of a Two-dimensional Superfluid near a Non-Thermal Fixed Point

TL;DR

This paper investigates the non-equilibrium critical dynamics of a 2D ultracold Bose gas, revealing vortex behavior and power-law decay regimes near a non-thermal fixed point, with implications for turbulence and phase transitions.

Contribution

It introduces the concept of a non-thermal fixed point in 2D Bose gases and analyzes vortex dynamics and decay regimes during far-from-equilibrium evolution.

Findings

Two distinct power-law decay regimes of vortex density identified

Vortex-antivortex correlations characterize the approach to the non-thermal fixed point

Relation to classical turbulence decay discussed

Abstract

Critical dynamics of an ultracold Bose gas far from equilibrium is studied in two spatial dimensions. Superfluid turbulence is created by quenching the equilibrium state close to zero temperature. Instead of immediately re-thermalizing, the system approaches a meta-stable transient state, characterized as a non-thermal fixed point. A focus is set on the vortex density and vortex-antivortex correlations which characterize the evolution towards the non-thermal fixed point and the departure to final (quasi-)condensation. Two distinct power-law regimes in the vortex-density decay are found and discussed in terms of a vortex binding-unbinding transition and a kinetic description of vortex scattering. A possible relation to decaying turbulence in classical fluids is pointed out. By comparing the results to equilibrium studies of a two-dimensional Bose gas, an intuitive understanding of the location of the non-thermal fixed point in a reduced phase space is developed.