Key Laboratory of Computational Physical Sciences (Ministry of Education), Institute of Computational Physical Sciences, and Department of Physics, Fudan University, Shanghai 200433, China.
Shanghai Qi Zhi Institute, Shanghai 200030, China.
Phys Rev Lett. 2023 Apr 7;130(14):146801. doi: 10.1103/PhysRevLett.130.146801.
Two-dimensional (2D) ferroelectrics, which are rare in nature, enable high-density nonvolatile memory with low energy consumption. Here, we propose a theory of bilayer stacking ferroelectricity (BSF), in which two stacked layers of the same 2D material, with different rotation and translation, exhibit ferroelectricity. By performing systematic group theory analysis, we find all the possible BSF in all 80 layer groups (LGs) and discover the rules about the creation and annihilation of symmetries in the bilayer. Our general theory can not only explain all the previous findings (including sliding ferroelectricity), but also provide a new perspective. Interestingly, the direction of the electric polarization of the bilayer could be totally different from that of the single layer. In particular, the bilayer could become ferroelectric after properly stacking two centrosymmetric nonpolar monolayers. By means of first-principles simulations, we predict that the ferroelectricity and thus multiferroicity can be introduced to the prototypical 2D ferromagnetic centrosymmetric material CrI_{3} by stacking. Furthermore, we find that the out-of-plane electric polarization in bilayer CrI_{3} is interlocked with the in-plane electric polarization, suggesting that the out-of-plane polarization can be manipulated in a deterministic way through the application of an in-plane electric field. The present BSF theory lays a solid foundation for designing a large number of bilayer ferroelectrics and thus colorful platforms for fundamental studies and applications.
二维（2D）铁电体在自然界中较为罕见，但其具有高密度、低能耗的非易失性存储特性。在这里，我们提出了一种双层堆叠铁电体（BSF）理论，其中相同 2D 材料的两层以不同的旋转和平移堆叠，表现出铁电性。通过进行系统的群论分析，我们发现了所有可能的 BSF 在所有 80 个层群（LG）中，并发现了双层中对称创造和消弭的规则。我们的一般理论不仅可以解释所有以前的发现（包括滑动铁电性），而且还提供了一个新的视角。有趣的是，双层的电偶极子方向可以与单层完全不同。特别地，通过适当堆叠两个中心对称的非极性单层，双层可以变成铁电体。通过第一性原理模拟，我们预测堆叠可以将铁电性和多铁性引入到典型的 2D 铁磁中心对称材料 CrI_{3}中。此外，我们发现双层 CrI_{3}中的面外电偶极子与面内电偶极子相互锁定，这表明通过施加面内电场可以以确定的方式操纵面外极化。本 BSF 理论为设计大量双层铁电体奠定了坚实的基础，从而为基础研究和应用提供了丰富多彩的平台。
