Non-equilibrium Berezinskii-Kosterlitz-Thouless Transition in a Driven Open Quantum System

TL;DR

This paper demonstrates that the Berezinskii-Kosterlitz-Thouless phase transition, characterized by vortex binding and unbinding, persists in a driven, non-equilibrium quantum system of polaritons, revealing enhanced robustness of order beyond equilibrium limits.

Contribution

It shows that BKT transitions occur in non-equilibrium driven systems and reveals a surprising increase in the correlation decay exponent beyond equilibrium bounds.

Findings

BKT transition occurs in driven polariton superfluids.

The ordered phase exhibits a power-law decay with an exponent exceeding equilibrium limits.

The phase transition demonstrates greater robustness against quantum fluctuations in non-equilibrium conditions.

Abstract

The Berezinskii-Kosterlitz-Thouless mechanism, in which a phase transition is mediated by the proliferation of topological defects, governs the critical behaviour of a wide range of equilibrium two-dimensional systems with a continuous symmetry, ranging from superconducting thin films to two-dimensional Bose fluids, such as liquid helium and ultracold atoms. We show here that this phenomenon is not restricted to thermal equilibrium, rather it survives more generally in a dissipative highly non-equilibrium system driven into a steady-state. By considering a light-matter superfluid of polaritons, in the so-called optical parametric oscillator regime, we demonstrate that it indeed undergoes a vortex binding-unbinding phase transition. Yet, the exponent of the power-law decay of the first order correlation function in the (algebraically) ordered phase can exceed the equilibrium upper limit -- a surprising occurrence, which has also been observed in a recent experiment. Thus we demonstrate that the ordered phase is somehow more robust against the quantum fluctuations of driven systems than thermal ones in equilibrium.