Universal dynamics on the way to thermalisation

TL;DR

This paper explores how many-body systems far from equilibrium can exhibit universal dynamics near a non-thermal fixed point, demonstrated through Bose-Einstein condensation and vortex dynamics.

Contribution

It reveals the conditions under which universal scaling and critical slowing down occur during Bose-Einstein condensation far from equilibrium.

Findings

Power-law spectrum n(k) ~ k^{-5} observed during strong quenches

Critical slowing down occurs near the non-thermal fixed point

Vortex lines are associated with the fixed point dynamics

Abstract

It is demonstrated how a many-body system far from thermal equilibrium can exhibit universal dynamics in passing a non-thermal fixed point. As an example, the process of Bose-Einstein (BE) condensation of a dilute cold gas is considered. If the particle flux into the low-energy modes, induced, e.g., by a cooling quench, is sufficiently strong, the Bose gas develops a characteristic power-law single-particle spectrum $n(k)\sim k^{-5}$, and critical slowing down in time occurs. The fixed point is shown to be marked by the creation and dilution of tangled vortex lines. Alternatively, for a weak cooling quench and particle flux, the condensation process runs quasi adiabatically, passing by the fixed point in far distance, and signatures of critical scaling remain absent.