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<p>In ordered magnets, the elementary excitations are spin waves (magnons), which obey Bose–Einstein statistics. Similarly to Cooper pairs in superconductors, magnons can be paired into bound states under attractive interactions. The Zeeman coupling to a magnetic field is able to tune the particle density through a quantum critical point, beyond which a ‘hidden order’ is predicted to exist. Here we report direct observation of the Bose–Einstein condensation of the two-magnon bound state in Na<sub>2</sub>BaNi(PO<sub>4</sub>)<sub>2</sub>. Comprehensive thermodynamic measurements confirmed the two-dimensional Bose–Einstein condensation quantum critical point at the saturation field. Inelastic neutron scattering experiments were performed to establish the microscopic model. An exact solution revealed stable two-magnon bound states that were further confirmed by electron spin resonance and nuclear magnetic resonance experiments, demonstrating that the quantum critical point is due to the pair condensation, and the phase below the saturation field is likely the long-sought-after spin nematic phase.</p>

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Bose–Einstein condensation of a two-magnon bound state in a spin-1 triangular lattice

  • 期刊论文

  • 发表日期：

    2025年1月20日

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  • Jieming Sheng,
  • Jia-Wei Mei,
  • Le Wang,
  • Xiaoyu Xu,
  • Wenrui Jiang,
  • Lei Xu,
  • Han Ge,
  • Nan Zhao,
  • Tiantian Li,
  • Andrea Candini,
  • Bin Xi,
  • Jize Zhao,
  • Ying Fu,
  • Jiong Yang,
  • Yuanzhu Zhang,
  • Giorgio Biasiol,
  • Shanmin Wang,
  • Jinlong Zhu,
  • Ping Miao,
  • Xin Tong,
  • Dapeng Yu,
  • Richard Mole,
  • Yi Cui,
  • Long Ma,
  • Zhitao Zhang,
  • Zhongwen Ouyang,
  • Wei Tong,
  • Andrey Podlesnyak,
  • Ling Wang,
  • Feng Ye,
  • Dehong Yu,
  • Weiqiang Yu,
  • Liusuo Wu,
  • Zhentao Wang

摘要

In ordered magnets, the elementary excitations are spin waves (magnons), which obey Bose–Einstein statistics. Similarly to Cooper pairs in superconductors, magnons can be paired into bound states under attractive interactions. The Zeeman coupling to a magnetic field is able to tune the particle density through a quantum critical point, beyond which a ‘hidden order’ is predicted to exist. Here we report direct observation of the Bose–Einstein condensation of the two-magnon bound state in Na2BaNi(PO4)2. Comprehensive thermodynamic measurements confirmed the two-dimensional Bose–Einstein condensation quantum critical point at the saturation field. Inelastic neutron scattering experiments were performed to establish the microscopic model. An exact solution revealed stable two-magnon bound states that were further confirmed by electron spin resonance and nuclear magnetic resonance experiments, demonstrating that the quantum critical point is due to the pair condensation, and the phase below the saturation field is likely the long-sought-after spin nematic phase.