前沿: 2D范德瓦尔斯异质结：能谷劈裂扭转增强效应

将不同的二维材料进行堆叠形成异质结形式，可以综合不同组分的新奇物理特性，同时还能弥补各自的缺点，为二维材料的应用打开了新方向。将过渡金属硫化物（TMDs）与本征磁性二维材料结合成异质结，利用界面间产生的磁近邻效应，可以破坏TMDs中相邻能谷的能级简并。在二维范德瓦尔斯异质结中，可以通过调控上下两层材料相互扭转，从而对其电子、磁性质进行精准调控，是异质结性能调控的新变量。该研究通过对不同扭转角度的WSe₂和CrI₃进行堆叠，发现扭转可以加强上下两原子层间耦合。

Fig. 1 Structural model and stability of non-twisted WSe2/CrI3 heterostructure.

来自山西师范大学的张均锋教授团队与美国伦斯勒理工学院的张绳百教授及其合作者们，利用第一性原理考察了不同扭转角下，二维范德瓦尔斯WSe₂/CrI₃异质结的结构稳定性、电子和磁性质。发现随着扭转角度的变大，异质结变得越来越稳定。更为重要的是，发现WSe₂中的能谷劈裂，与非扭转的堆叠方式相比，有十几倍的显著增强。这揭示了扭转的范德瓦尔斯异质结中能谷劈裂增强与上下两层原子之间的耦合强度有着很大的关系。量子力学模型分析等效磁场发现，扭转角为23.4^o的异质结中产生的能谷劈裂值，相当于将WSe₂放置于20.9T的超高强度磁场中产生的劈裂值大小。这一工作，为TMDs中能谷劈裂调控提供了新的方向。该文近期发表于npj Computational Materials 8，32 (2022)。

Fig. 2 Structure models of twisted WSe2/CrI3 heterostructures.

Editorial Summary

Van der Waals (vdW) heterostructures composed of different two-dimensional (2D) materials offer an easily accessible way to combine properties of individual materials for applications. The magnetic proximity effect at interfaces of 2D vdW heterostructures formed by transition metal dichalcogenides (TMDs) and intrinsic magnetic materials, can break the time-reversal symmetry and degenerate energy valleys in momentum space. Further on, 2D vdW twisted heterostructures, in which one layer is rotated with respect to the other by an angle, are also interesting. Twist may be new variable to tune the properties of electronic and magnetic. The enhanced coupling of atoms between the upper and lower layers of the twisted vdW WSe₂/CrI₃ heterostructures is reported in this work. 

A team led by Prof. Junfeng Zhang from Shanxi Normal University, Prof. Shengbai Zhang from Rensselaer Polytechnic Institute, USA, and their coworkers, investigated a set of 2D vdW WSe₂/CrI₃ heterostructures with different twist angle with an emphasis on the magnetic properties, using first-principles calculation. The results of the formation energy indicate that the twisted heterostructures are more stable than non-twisted ones. More importantly, in twisted heterostructures, there is an order of magnitude enhancement of valley splitting. The analysis of the partial charge density suggests that the coupling of Cr atoms and W atoms is responsible for the enhanced valley splitting. With the help of a k·p model, the equivalent magnetic field is deduced (for the 23.4 twist angle, the equivalent magnetic field CrI₃ generated is about 20.9 T, corresponding to 5.18 meV valley splitting in WSe₂). It shows that not only the magnetic field strength is sufficiently strong but also twisting can be an effective way to amplify the magnetic proximity effect at interfaces. This article was recently published in npj Computational Materials 8:32 (2022).

Fig. 4 Band structures of WSe2/CrI3 heterostructures.

原文Abstract及其翻译

Enhanced valley splitting of WSe₂ in twisted van der Waals WSe₂/CrI₃ heterostructures  (扭转的WSe₂/CrI₃范德瓦尔斯异质结中WSe₂能谷劈裂的增强效应)

Mei Ge, Han Wang, Jizheng Wu, Chen Si, Junfeng Zhang & Shengbai Zhang

Abstract Van der Waals (vdW) heterostructures composed of different two-dimensional (2D) materials offer an easily accessible way to combine properties of individual materials for applications. Owing to the discovery of a set of unanticipated physical phenomena, the twisted 2D vdW heterostructures have gained considerable attention recently. Here, we report enhanced valley splitting in twisted 2D vdW WSe2/CrI3  heterostructures. In particular, the splitting can be 1200% (or 5.18 meV) of the value for a non-twisted heterostructure. According to the k·p model, this value is equivalent to a ~20 T external magnetic field applied perpendicular to the 2D sheet. The thermodynamic stability of 2D vdW WSe2/CrI3 heterostructures, on the other hand, depends linearly on the interlayer twisting angle.

Fig. 5 Partial charge density differences of the VBM states between K and K′ for different θ. 

摘要 将不同的二维材料堆叠成异质结形式，为二维材料的广泛应用打开了新的窗口。近年来，由于层间效应引起的新奇物理特性，扭转的二维范德瓦尔斯异质结引起了人们的重点关注。在本工作中，我们将二维材料WSe₂和CrI₃构建成范德瓦尔斯异质结，详细讨论了层间扭转引起的能谷劈裂增强效应。我们发现，与非扭转的异质结相比，层间旋转能将异质结的能谷劈裂增强大约12倍（5.18 meV）。根据量子力学k·p模型，这个能谷劈裂值相当于将单层WSe₂放在大约20T的外磁场中产生的能谷劈裂。另外，计算结果还表明，WSe₂/CrI₃范德瓦尔斯异质结的热力学稳定性也与层间扭转角有一定的依赖关系。

Fig. 6 Dipole moment and KK′ valley splitting as functions of applied E-field.

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