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<p>Second-order phase transitions are governed by a spontaneous symmetry-breaking mechanism, which yields collective excitations with a gapless spectrum called Nambu–Goldstone modes. Although these modes propagate as sound waves in conservative systems, non-equilibrium phase transitions have been predicted to feature a diffusive Nambu–Goldstone mode. Here, we present the experimental characterization of such a mode in a non-equilibrium Bose–Einstein condensate of microcavity polaritons. The mode appears in the spectroscopic response of the condensate to an extra probe laser as spectral narrowing, along with the emergence of a tilted frequency plateau. Breaking the symmetry with another phase-fixing beam causes a gap to open in the imaginary part of the spectrum and the disappearance of the Nambu–Goldstone mode. These observations confirm theoretical predictions for the Nambu–Goldstone mode of non-equilibrium phase transitions and reveal the symmetry-breaking mechanism underlying polariton condensation.</p>

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

  • 期刊论文

  • 发表日期：

    2025年5月2日

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  • Ferdinand Claude,
  • Maxime J. Jacquet,
  • Quentin Glorieux,
  • Michiel Wouters,
  • Elisabeth Giacobino,
  • Iacopo Carusotto,
  • Alberto Bramati

摘要

Second-order phase transitions are governed by a spontaneous symmetry-breaking mechanism, which yields collective excitations with a gapless spectrum called Nambu–Goldstone modes. Although these modes propagate as sound waves in conservative systems, non-equilibrium phase transitions have been predicted to feature a diffusive Nambu–Goldstone mode. Here, we present the experimental characterization of such a mode in a non-equilibrium Bose–Einstein condensate of microcavity polaritons. The mode appears in the spectroscopic response of the condensate to an extra probe laser as spectral narrowing, along with the emergence of a tilted frequency plateau. Breaking the symmetry with another phase-fixing beam causes a gap to open in the imaginary part of the spectrum and the disappearance of the Nambu–Goldstone mode. These observations confirm theoretical predictions for the Nambu–Goldstone mode of non-equilibrium phase transitions and reveal the symmetry-breaking mechanism underlying polariton condensation.