Zhao Xing-Ju, Yang Yang, Zhang Dong-Bo, Wei Su-Huai
College of Nuclear Science and Technology, Beijing Normal University, Beijing 100875, People's Republic of China.
Beijing Computational Science Research Center, Beijing 100193, People's Republic of China.
Phys Rev Lett. 2020 Feb 28;124(8):086401. doi: 10.1103/PhysRevLett.124.086401.
The existence of Bloch flat bands of electrons provides a facile pathway to obtain exotic quantum phases owing to strong correlation. Despite the established magic angle mechanism for twisted bilayer graphene, understanding of the emergence of flat bands in twisted bilayers of two-dimensional polar crystals remains elusive. Here, we show that due to the polarity between constituent elements in the monolayer, the formation of complete flat bands in twisted bilayers is triggered as long as the twist angle is less than a certain critical value. Using the twisted bilayer of hexagonal boron nitride (hBN) as an example, our simulations using the density-functional tight-binding method reveal that the flat band originates from the stacking-induced decoupling of the highest occupied (lowest unoccupied) states, which predominantly reside in the regions of the moiré superlattice where the anion (cation) atoms in both layers are overlaid. Our findings have important implications for the future search for and study of flat bands in polar materials.
由于强关联作用，电子的布洛赫平带的存在为获得奇异量子相提供了一条便捷途径。尽管对于扭曲双层石墨烯已经确立了魔角机制，但对于二维极性晶体扭曲双层中平带的出现仍缺乏理解。在此，我们表明，由于单层中组成元素之间的极性，只要扭曲角小于某个临界值，就会触发扭曲双层中完整平带的形成。以六方氮化硼（hBN）的扭曲双层为例，我们使用密度泛函紧束缚方法进行的模拟表明，平带起源于最高占据（最低未占据）态的堆叠诱导解耦，这些态主要位于莫尔超晶格中两层阴离子（阳离子）原子重叠的区域。我们的发现对于未来极性材料中平带的寻找和研究具有重要意义。
