Bera Krishna Prasad, Solanki Darshit, Mandal Shinjan, Biswas Rabindra, Taniguchi Takashi, Watanabe Kenji, Raghunathan Varun, Jain Manish, Sood A K, Das Anindya
Department of Physics, Indian Institute of Science, Bangalore 560012, India.
Centre for Condensed Matter Theory, Indian Institute of Science, Bangalore 560012, India.
ACS Nano. 2024 Sep 3;18(35):24379-24390. doi: 10.1021/acsnano.4c06767. Epub 2024 Aug 19.
The emerging moiré superstructure of twisted transition metal dichalcogenides (TMDs) leads to various correlated electronic and optical properties compared to those of twisted bilayer graphene. In such a versatile architecture, phonons can also be renormalized and evolve due to atomic reconstruction, which, in turn, depends on the twist angle. However, observing this reconstruction and its relationship to phonon behavior with conventional, cost-effective imaging methods remains challenging. Here, we used noninvasive Raman spectroscopy on twisted WSe/WSe (t-WSe) homobilayers to examine the evolution of phonon modes due to interlayer coupling and atomic reconstruction. Unlike in the natural bilayer (NB), ∼0° as well as ∼60° t-WSe samples, the nearly degenerate A/E mode in the twisted samples (1-7°) split into a doublet in addition to the nondegenerate B mode, and the maximum splitting is observed around 2-3°. Our detailed theoretical calculations qualitatively capture the splitting and its dependence as a function of the twist angle and highlight the role of the moiré potential in phonon hybridization. Additionally, we found that around the 2° twist angle, the anharmonic phonon-phonon interaction is higher than the natural bilayer and decreases for larger twist angles. Interestingly, we observed anomalous Raman frequency softening and line-width increase with the decreasing temperature below 50 K, pointing to the combined effect of enhanced electron-phonon coupling and cubic anharmonic interactions in moiré superlattice.
与扭曲双层石墨烯相比，扭曲过渡金属二硫属化物（TMD）中出现的莫尔超结构导致了各种相关的电子和光学性质。在这种多功能结构中，声子也会由于原子重构而重新归一化并发生演化，而原子重构又取决于扭曲角。然而，用传统的、经济高效的成像方法观察这种重构及其与声子行为的关系仍然具有挑战性。在这里，我们对扭曲的WSe/WSe（t-WSe）同质双层膜进行了非侵入性拉曼光谱研究，以考察由于层间耦合和原子重构引起的声子模式的演化。与天然双层膜（NB）、0°以及60°的t-WSe样品不同，扭曲样品（1-7°）中几乎简并的A/E模式除了非简并的B模式外还分裂成了一个双峰，并且在2-3°左右观察到最大分裂。我们详细的理论计算定性地捕捉了分裂及其作为扭曲角函数的依赖性，并突出了莫尔势在声子杂化中的作用。此外，我们发现，在2°扭曲角附近，非谐声子-声子相互作用高于天然双层膜，并且随着扭曲角增大而减小。有趣的是，我们观察到随着温度降至50 K以下，拉曼频率出现异常软化且线宽增加，这表明在莫尔超晶格中增强的电子-声子耦合和三次非谐相互作用的综合效应。
