Nanoscale exotic polar topological structures, such as vortices and skyrmions, hold promise for next-generation electronic devices, yet their spontaneous formation in 2D van der Waals (vdW) materials remains quite challenging. Herein, we demonstrate from first-principles that ultrahigh-density polar vortices emerge in the square moiré bilayer formed by twisting two layers of centrosymmetric PbS with the D_4h point group. The emerged ferroelectricity arises from the inherent complex strain associated with the twisted structures, and the resulting electron polarization is much greater than that obtained in sliding ferroelectricity. Notably, the engineered strain patterns are characterized by peculiar inhomogeneous in-plane fields with a checkerboard distribution of uniaxial tension. This nanoscale nonuniform strain produces an ultrahigh-density vortex polarization lattice. The results from our study not only reveals a new mechanism for electric polarization and polar topologies in moiré bilayers but also provides opportunities for designing 2D ultrahigh-density electric devices.

中文翻译：

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硫属化铅方形莫尔条纹双层中的超高密度极性涡旋晶格


纳米级奇异的极拓扑结构，如涡流和斯格明子，为下一代电子设备带来了希望，但它们在 2D 范德华 （vdW） 材料中的自发形成仍然相当具有挑战性。在此，我们从第一性原理证明，超高密度极性涡流出现在由两层中心对称 PbS 与 D_4h 点群扭曲形成的方形莫尔条纹双层中。出现的铁电性是由与扭曲结构相关的固有复杂应变产生的，所得的电子极化远大于滑动铁电性中获得的电子极化。值得注意的是，工程应变模式的特点是特殊的不均匀面内场，具有单轴张力的棋盘状分布。这种纳米级非均匀应变产生超高密度涡旋极化晶格。我们的研究结果不仅揭示了莫尔双层中电极化和极性拓扑的新机制，还为设计二维超高密度电气器件提供了机会。