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1. Shedding light on moiré excitons: A first-principles perspective

   https://doi.org/10.1126/sciadv.abc5638  

   Guo, Hongli; Zhang, Xu; Lu, Gang (October 2020, Science Advances)

   null (Ed.)

   Moiré superlattices in van der Waals (vdW) heterostructures could trap long-lived interlayer excitons. These moiré excitons could form ordered quantum dot arrays, paving the way for unprecedented optoelectronic and quantum information applications. Here, we perform first-principles simulations to shed light on moiré excitons in twisted MoS 2 /WS 2 heterostructures. We provide direct evidence of localized interlayer moiré excitons in vdW heterostructures. The interlayer and intralayer moiré potentials are mapped out based on spatial modulations of energy gaps. Nearly flat valence bands are observed in the heterostructures. The dependence of spatial localization and binding energy of the moiré excitons on the twist angle of the heterostructures is examined. We explore how vertical electric field can be tuned to control the position, polarity, emission energy, and hybridization strength of the moiré excitons. We predict that alternating electric fields could modulate the dipole moments of hybridized moiré excitons and suppress their diffusion in moiré lattices. 

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2. Light Confinement in Twisted Single-Layer 2D+ Moiré Photonic Crystals and Bilayer Moiré Photonic Crystals

   https://doi.org/10.3390/photonics11010013  

   Kamau, Steve; Hurley, Noah; Kaul, Anupama B; Cui, Jingbiao; Lin, Yuankun (January 2024, Photonics)

   Twisted photonic crystals are photonic analogs of twisted monolayer materials such as graphene and their optical property studies are still in their infancy. This paper reports optical properties of twisted single-layer 2D+ moiré photonic crystals where there is a weak modulation in z direction, and bilayer moiré-overlapping-moiré photonic crystals. In weak-coupling bilayer moiré-overlapping-moiré photonic crystals, the light source is less localized with an increasing twist angle, similar to the results reported by the Harvard research group in References 37 and 38 on twisted bilayer photonic crystals, although there is a gradient pattern in the former case. In a strong-coupling case, however, the light source is tightly localized in AA-stacked region in bilayer PhCs with a large twist angle. For single-layer 2D+ moiré photonic crystals, the light source in Ex polarization can be localized and forms resonance modes when the single-layer 2D+ moiré photonic crystal is integrated on a glass substrate. This study leads to a potential application of 2D+ moiré photonic crystal in future on-chip optoelectronic integration. 

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3. Tuning moiré excitons and correlated electronic states through layer degree of freedom

   https://doi.org/10.1038/s41467-022-32493-9  

   Chen, Dongxue; Lian, Zhen; Huang, Xiong; Su, Ying; Rashetnia, Mina; Yan, Li; Blei, Mark; Taniguchi, Takashi; Watanabe, Kenji; Tongay, Sefaattin; et al (August 2022, Nature Communications)

   Abstract Moiré coupling in transition metal dichalcogenides (TMDCs) superlattices introduces flat minibands that enable strong electronic correlation and fascinating correlated states, and it also modifies the strong Coulomb-interaction-driven excitons and gives rise to moiré excitons. Here, we introduce the layer degree of freedom to the WSe₂/WS₂moiré superlattice by changing WSe₂from monolayer to bilayer and trilayer. We observe systematic changes of optical spectra of the moiré excitons, which directly confirm the highly interfacial nature of moiré coupling at the WSe₂/WS₂interface. In addition, the energy resonances of moiré excitons are strongly modified, with their separation significantly increased in multilayer WSe₂/monolayer WS₂moiré superlattice. The additional WSe₂layers also modulate the strong electronic correlation strength, evidenced by the reduced Mott transition temperature with added WSe₂layer(s). The layer dependence of both moiré excitons and correlated electronic states can be well described by our theoretical model. Our study presents a new method to tune the strong electronic correlation and moiré exciton bands in the TMDCs moiré superlattices, ushering in an exciting platform to engineer quantum phenomena stemming from strong correlation and Coulomb interaction. 

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4. Twist-angle dependence of moiré excitons in WS2/MoSe2 heterobilayers

   https://doi.org/10.1038/s41467-020-19466-6  

   Zhang, Long; Zhang, Zhe; Wu, Fengcheng; Wang, Danqing; Gogna, Rahul; Hou, Shaocong; Watanabe, Kenji; Taniguchi, Takashi; Kulkarni, Krishnamurthy; Kuo, Thomas; et al (November 2020, Nature Communications)

   Abstract Moiré lattices formed in twisted van der Waals bilayers provide a unique, tunable platform to realize coupled electron or exciton lattices unavailable before. While twist angle between the bilayer has been shown to be a critical parameter in engineering the moiré potential and enabling novel phenomena in electronic moiré systems, a systematic experimental study as a function of twist angle is still missing. Here we show that not only are moiré excitons robust in bilayers of even large twist angles, but also properties of the moiré excitons are dependant on, and controllable by, the moiré reciprocal lattice period via twist-angle tuning. From the twist-angle dependence, we furthermore obtain the effective mass of the interlayer excitons and the electron inter-layer tunneling strength, which are difficult to measure experimentally otherwise. These findings pave the way for understanding and engineering rich moiré-lattice induced phenomena in angle-twisted semiconductor van der Waals heterostructures. 

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5. Ultrahigh-resolution scanning microwave impedance microscopy of moiré lattices and superstructures

   https://doi.org/10.1126/sciadv.abd1919  

   Lee, Kyunghoon; Utama, M. Iqbal; Kahn, Salman; Samudrala, Appalakondaiah; Leconte, Nicolas; Yang, Birui; Wang, Shuopei; Watanabe, Kenji; Taniguchi, Takashi; Altoé, M. Virginia; et al (December 2020, Science Advances)

   null (Ed.)

   Two-dimensional heterostructures composed of layers with slightly different lattice vectors exhibit new periodic structure known as moiré lattices, which, in turn, can support novel correlated and topological phenomena. Moreover, moiré superstructures can emerge from multiple misaligned moiré lattices or inhomogeneous strain distributions, offering additional degrees of freedom in tailoring electronic structure. High-resolution imaging of the moiré lattices and superstructures is critical for understanding the emerging physics. Here, we report the imaging of moiré lattices and superstructures in graphene-based samples under ambient conditions using an ultrahigh-resolution implementation of scanning microwave impedance microscopy. Although the probe tip has a gross radius of ~100 nm, spatial resolution better than 5 nm is achieved, which allows direct visualization of the structural details in moiré lattices and the composite super-moiré. We also demonstrate artificial synthesis of novel superstructures, including the Kagome moiré arising from the interplay between different layers. 

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