Stacking atomically thin films enables artificial construction of van der Waals heterostructures with exotic functionalities such as superconductivity, the quantum Hall effect, and engineered light-matter interactions. In particular, heterobilayers composed of monolayer transition metal dichalcogenides have attracted significant interest due to their controllable interlayer coupling and trapped valley excitons in moiré superlattices. However, the identification of twist-angle-modulated optical transitions in heterobilayers is sometimes controversial since both momentum-direct (K-K) and -indirect excitons reside on the low energy side of the bright exciton in the monolayer constituents. Here, we attribute the optical transition at approximately 1.35 eV in the WS2/WSe2 heterobilayer to an indirect Γ-K transition based on a systematic analysis and comparison of experimental PL spectra with theoretical calculations. The exciton wavefunction obtained by the state-of-the-art GW-Bethe-Salpeter equation (GW-BSE) approach indicates that both the electron and hole of the exciton are contributed by the WS2 layer. Polarization-resolved k-space imaging further confirms that the transition dipole moment of this optical transition is dominantly in-plane and is independent of the twist angle. The calculated absorption spectrum predicts that the usually called interlayer exciton peak coming from the K-K transition is located at 1.06 eV, but with a much weaker amplitude. Our work provides new insights into understanding the steady-state and dynamic properties of twist-angle-dependent excitons in van der Waals heterostructures.

中文翻译：

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WS2/WSe2 异双分子层中与扭转角相关的激子的识别

堆叠原子薄膜可以人工构建具有奇异功能的范德华异质结构，例如超导性、量子霍尔效应和工程光物质相互作用。特别是，由单层过渡金属二硫属化物组成的异质双层由于其可控的层间耦合和莫尔超晶格中的俘获谷激子而引起了极大的兴趣。然而，异质双层中扭曲角调制的光学跃迁的识别有时是有争议的，因为动量直接（KK）和间接激子都位于单层成分中明亮激子的低能侧。在这里，我们将光学跃迁归因于大约 1。基于对实验 PL 光谱与理论计算的系统分析和比较，WS2/WSe2 异质双层中的 35 eV 到间接 Γ-K 跃迁。通过最先进的 GW-Bethe-Salpeter 方程 (GW-BSE) 方法获得的激子波函数表明激子的电子和空穴均由 WS2 层贡献。偏振分辨的 k 空间成像进一步证实了这种光学跃迁的跃迁偶极矩主要在平面内，并且与扭转角无关。计算的吸收光谱预测来自 KK 跃迁的通常称为层间激子峰位于 1.06 eV，但幅度要弱得多。