RIKEN Center for Emergent Matter Science (CEMS), Wako351-0198, Japan.
School of Physical Sciences and CAS Key Laboratory of Vacuum Physics, University of Chinese Academy of Sciences, Beijing100049, China.
ACS Nano. 2023 Feb 14;17(3):1916-1924. doi: 10.1021/acsnano.2c04995. Epub 2023 Jan 26.
Stacking order is expected to have a significant impact on the properties of van der Waals layered magnets, as it determines the crystallographic and magnetic symmetry. Recent synchrotron-based optical studies on antiferromagnetic MnPS have revealed a thickness-dependent symmetry crossover, suggesting possible different stackings in few-layer crystals from the bulk, which, however, has not been explicitly identified. Here, by using a combination of atomic-scale electron microscopy and theoretical calculations, we show that despite the bulk monoclinic stacking persists macroscopically down to bilayer, additional local rippling effect lifts the monoclinic symmetry of the few layers while preserving the trigonal symmetry of individual monolayers, leading to possible monolayer-like behavior in ultrathin MnPS samples. This finding reveals the profound impact of rippling on the microscopic symmetry of two-dimensional materials with weak interlayer interactions and raises the possibility of approaching the paradigmatic two-dimensional Néel antiferromagnetic honeycomb lattice in MnPS without reaching monolayer thickness.
堆积顺序预计会对范德华层状磁体的性质产生重大影响，因为它决定了晶体和磁对称性。最近基于同步加速器的反铁磁 MnPS 的光学研究揭示了厚度相关的对称性转变，表明在少层晶体中可能存在与体相不同的堆积方式，但尚未明确确定。在这里，我们通过结合原子尺度电子显微镜和理论计算，表明尽管在宏观上直到双层，块状的单斜堆积仍然存在，但额外的局部波纹效应会提升少层的单斜对称性，同时保持各单层的三角对称性，从而导致在超薄 MnPS 样品中可能表现出类似单层的行为。这一发现揭示了波纹对弱层间相互作用的二维材料微观对称性的深远影响，并提出了在不达到单层厚度的情况下接近 MnPS 中典型二维奈尔反铁磁蜂窝晶格的可能性。
