堆叠原子薄膜可以人工构建具有奇特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合物组成的异双层由于其可控的层间耦合和在莫尔超晶格中捕获的谷激子而引起了人们的极大兴趣。然而,由于动量直接(K-K)和-间接激子都位于单层成分中亮激子的低能量侧,因此在异对双层中扭曲角调制的光学跃迁的识别有时是有争议的。这里,我们根据对实验光致发光光谱与理论计算的系统分析和比较,将WS2/WSe2异质双层中的〜1.35eV处的光学跃迁归因于间接的Γ-K跃迁。通过最先进的GW-Bethe-Salpeter方程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k空间成像进一步证实,该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K-K跃迁的所谓层间激子峰位于1.06eV,但是振幅要弱得多。我们的工作为范德华异质结构中与扭曲角相关的激子的稳态和动态特性提供了新的见解。
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 ∼1.35 eV in the WS2/WSe2 heterobilayer to an indirect Γ-K transition based on a systematic analysis and comparison of experimental photoluminescence spectra with theoretical calculations. The exciton wavefunction obtained by the state-of-the-art GW-Bethe-Salpeter equation approach indicates that both the electron and hole of the excitons 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 so-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 insight into the steady-state and dynamic properties of twist-angle-dependent excitons in van der Waals heterostructures.