Exciton fine structure in twisted transition metal dichalcogenide heterostructures 

Sudipta Kundu, Tomer Amit, H. R. Krishnamurthy, Manish Jain & Sivan Refaely-Abramson 

npj Computational Materials 9: 186 (2023).

编辑概述：扭曲过渡金属硫化物异质结中的激子行为

二维材料层间晶格不匹配产生的莫尔图案为新型电子和激子物理提供了一个崭新平台。特别具有类型-II能带对齐的过渡金属二硫化物（TMD）扭曲异质结构，由于扭曲引起的莫尔势导致了激子定域变化，从而呈现出了有趣的光学特性。层间扭曲角度决定了莫尔布里渊区（MBZ）与单层晶胞布里渊区（UBZs）之间的关联，引发了相关的莫尔势。这导致了在UBZs中具有不同动量态之间的光允许电子-空穴跃迁，这可能与吸收过程中激子的精细结构有关。为了对激子结构与扭曲角度之间关系进行全面理解，需要对扭曲结构中的激子进行基于GW-BSE的多体微扰理论分析。在这项工作中，来自印度科学研究所物理系凝聚态理论中心的Sudipta Kundu等人，提出了一种使用GW-BSE研究TMD异质结中双层扭曲角度对激子性质和光学选择规则影响的方法。作者在一个相对旋转角度为16∘的扭曲MoS₂/MoSe₂异质结构上演示了其方法，该结构允许在UBZs中不同高对称点之间发生跃迁，由原子重构和层间不匹配确定。通过分析，作者揭示了一种独特的动量混合激子性质，其状态由层间和层内电子-空穴激发组成。这些结果指出了动量直接激子和光学活跃激子的一般性质，其混合了层UBZs中的不同点，并且在较小的角度下同样有效。研究表明，激子性质和光谱特征与扭曲的TMD异质结构的底层结构之间存在直接的关系，为通过选择层间扭曲角度来调节激子性质提供了潜在可能性。

Editorial Summary: Exciton Behaviors in twisted TMD heterostructures

Moiré patterns generated due to a lattice mismatch between layers of two-dimensional materials serve as an emerging platform for novel correlated electronic and excitonic physics. In particular, twisted heterostructures of transition metal dichalcogenides (TMDs) with a type-II band alignment hold intriguing optical properties due to the varying exciton localization associated with the twist-induced moiré potential. The interlayer twist angle dictates the relation between the moiré Brillouin Zone (MBZ) and the unit-cell Brillouin Zones (UBZs) of the separate layers, inducing an associated moiré potential. This leads to optically-allowed electron-hole transitions between states of different momenta in the UBZs, which can be associated with the exciton fine structure in absorption. A full GW-BSE based many-body theory analysis of excitons in the twisted structure is required for a general ab initio understanding of the relation between the twist angle and the exciton structure. In this work, Sudipta Kundu et al. from the Centre for Condensed Matter Theory, Department of Physics, Indian Institute of Science, presented an approach to study the effect of twist angle on the exciton nature and optical selection rules in TMD hetero-bilayers by using GW-BSE. They demonstrated their approach on a twisted MoS₂/MoSe₂heterostructure with a relative rotation angle of 16°, which allows transitions between distinct high-symmetry points in the UBZs, determined by the atomic reconstruction and interlayer mismatch. Analyses reveal a unique momentum-mixed excitonic nature, with states that are comprised of both inter- and intra-layer electron-hole excitations. These results point to a general property of momentum-direct and optically-active excitons that mix different points in the layer UBZs, and is valid at smaller angles as well. Their findings suggest a direct relation between exciton nature and spectral features to the underlying structure in twisted TMD heterostructures, offering potential tunability of excitonic properties upon the choice of interlayer twist angle.