XY型蜂窝范德华磁体中从三维锯齿形反铁磁性到二维残余向列性的维度跨越
Dimensionality crossover to 2D vestigial nematicity from 3D zigzag antiferromagnetism in an XY-type honeycomb van der Waals magnet
AI总结：
本研究结合NV自旋弛豫、光谱与Monte Carlo模拟，揭示NiPS3从三维锯齿反铁磁相在约10 nm临界厚度下转变为二维Z3残余Potts向列相。
摘要：
在降低的维度中，涨落和无序效应会显著增强。尽管它们通常被视为长程有序的敌人，但涨落和无序也可以激发新物相的出现，例如残余有序。以二维磁性为平台，现有努力主要集中在通过抑制涨落来维持二维长程磁有序，而另一面，即利用涨落实现新的二维磁相，仍是一片未知领域。在这里，我们结合使用NV自旋弛豫测量、光谱学和Monte Carlo模拟，报道了在XY型蜂窝磁体NiPS3中，从三维块体的锯齿形AFM（反铁磁）有序到二维少层中一种新的Z3残余Potts向列性的相变。利用NV自旋弛豫测量和光学Raman准弹性散射表明，随着NiPS3层数减少，自旋涨落在GHz–THz范围内显著增强。结果，块体NiPS3中锯齿形AFM的Raman特征——由平移对称性破缺（PBTS）导致的约30 cm^-1处的区折叠声子，以及由三重旋转对称性破缺（PBRS）导致的约180 cm^-1处两个声子的简并解除——在少层NiPS3中演变为PBTS消失而PBRS保留，临界厚度约为10 nm。光学线性二向色性显微成像在单个少层NiPS3薄片中呈现出全部三种向列畴态。对双层NiPS3模型进行的大规模Monte Carlo模拟证实不存在长程锯齿形AFM有序，但形成了Z3残余Potts向列相，这与实验发现相符。我们的结果证明了强涨落在破坏更常规物相之后创造新物相的积极作用，并为开发新型二维物相提供了一条前所未有的途径。
英文摘要：
Fluctuations and disorder effects are substantially enhanced in reduced dimensionalities. While they are mostly considered as the foe for long-range orders, fluctuations and disorders can also stimulate the emergence of novel phases of matter, for example, vestigial orders. Taking 2D magnetism as a platform, existing efforts have been focused on maintaining 2D long-range magnetic orders by suppressing fluctuations, whereas the other side, exploiting fluctuations for realizing new 2D magnetic phases, remains as an uncharted territory. Here, using a combination of NV spin relaxometry, optical spectroscopy, and Monte Carlo simulations, we report, in an XY-type honeycomb magnet NiPS3, the phase transition from the zigzag AFM order in 3D bulk to a new Z3 vestigial Potts-nematicity in 2D few layers. Spin fluctuations are shown to significantly enhance over the GHz-THz range as the layer number of NiPS3 reduces, using the NV spin relaxometry and the optical Raman quasi-elastic scattering. As a result, the Raman signatures of the zigzag AFM for bulk NiPS3, a zone-folded phonon at ~30cm-1 from the broken translational symmetry (PBTS) and a degeneracy lift of two phonons at ~180cm-1 for the broken 3-fold rotational symmetry (PBRS), evolve into the disappearance of PBTS and the survival of PBRS in few-layer NiPS3, with a critical thickness of ~10nm. The optical linear dichroism microscopy images all three nematic domain states in a single few-layer NiPS3 flake. The large-scale Monte Carlo simulations for bilayer NiPS3 model confirms the absence of long-range zigzag AFM order but the formation of the Z3 vestigial Potts-nematic phase, corroborating with the experimental finding. Our results demonstrate the positivity of strong fluctuations in creating new phases of matter after destroying more conventional ones, and offer an unprecedented pathway for developing novel 2D phases.
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