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<p>The study of collective behaviour driven by spontaneous symmetry breaking and topology is crucial for understanding phase transitions in quantum matter. The two-dimensional (2D) XY model, describing spins with continuous in-plane rotational symmetry, hosts the topological Berezinskii–Kosterlitz–Thouless (BKT) transition, where vortex–antivortex binding induces quasi-long-range order. This model was later extended to include anisotropy fields, leading to the six-state clock model, which predicts the instability of the BKT phase toward true long-range order at low temperatures. Here we investigate this physics in the van der Waals antiferromagnet NiPS<sub>3</sub> using nonlinear optical micropolarimetry. As the material is thinned to a monolayer, its magnetic response switches abruptly from the 3D XXZ behaviour of multilayers to a distinct 2D regime consistent with a BKT state. Upon further cooling, the monolayer BKT phase becomes unstable and transforms into a pinned state with long-range order. These results, corroborated by Monte Carlo simulations, open pathways to explore spin vortices and topological dynamics in 2D antiferromagnets.</p>

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Six-state clock physics in an atomically thin antiferromagnet

  • 期刊论文

  • 发表日期：

    2026年2月23日

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  • Frank Y. Gao,
  • Dong Seob Kim,
  • Chao Lei,
  • Ajesh Kumar,
  • Xinyue Peng,
  • Xiaohui Liu,
  • Francesco Barantani,
  • Shangjie Zhang,
  • Kyoung Pyo Lee,
  • Kalaivanan Raju,
  • David Lujan,
  • Saba Arash,
  • Sankar Raman,
  • Shang-Fan Lee,
  • Mengxing Ye,
  • Xiaoqin Li,
  • Allan H. MacDonald,
  • Edoardo Baldini

摘要

The study of collective behaviour driven by spontaneous symmetry breaking and topology is crucial for understanding phase transitions in quantum matter. The two-dimensional (2D) XY model, describing spins with continuous in-plane rotational symmetry, hosts the topological Berezinskii–Kosterlitz–Thouless (BKT) transition, where vortex–antivortex binding induces quasi-long-range order. This model was later extended to include anisotropy fields, leading to the six-state clock model, which predicts the instability of the BKT phase toward true long-range order at low temperatures. Here we investigate this physics in the van der Waals antiferromagnet NiPS3 using nonlinear optical micropolarimetry. As the material is thinned to a monolayer, its magnetic response switches abruptly from the 3D XXZ behaviour of multilayers to a distinct 2D regime consistent with a BKT state. Upon further cooling, the monolayer BKT phase becomes unstable and transforms into a pinned state with long-range order. These results, corroborated by Monte Carlo simulations, open pathways to explore spin vortices and topological dynamics in 2D antiferromagnets.