Observation of the diffusive Nambu-Goldstone mode of a non-equilibrium phase transition

TL;DR

This paper reports the first experimental observation of a diffusive Nambu-Goldstone mode in a non-equilibrium Bose-Einstein condensate of microcavity polaritons, confirming theoretical predictions and advancing understanding of non-equilibrium phase transitions.

Contribution

It provides the first experimental evidence of a diffusive NG mode in a non-equilibrium system, demonstrating spectral narrowing and the effect of symmetry breaking.

Findings

Observation of spectral narrowing indicating a diffusive NG mode

Symmetry breaking opens a gap and removes the NG mode

Confirms diffusive dynamics in non-equilibrium phase transitions

Abstract

Second-order phase transitions are governed by spontaneous symmetry breaking, which yield collective excitations with a gapless spectrum called Nambu-Goldstone (NG) modes. While NG modes in conservative systems are propagating excitations, non-equilibrium phase transitions have been predicted to feature a diffusive NG mode. We present the first experimental evidence of a diffusive NG mode in a non-equilibrium Bose-Einstein condensate of microcavity polaritons. The NG mode is observed as a spectral narrowing in the spectroscopic response of the condensate. Additionally, explicitly breaking the symmetry causes the opening of a gap in the spectrum and the disappearance of the NG mode. Our observations confirm the diffusive dynamics of the NG mode of non-equilibrium phase transitions and establish a promising framework to investigate fundamental questions in statistical mechanics.