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1. Quadrupolar excitons and hybridized interlayer Mott insulator in a trilayer moiré superlattice

   https://doi.org/10.1038/s41467-023-40288-9  

   Lian, Zhen ; Chen, Dongxue ; Ma, Lei ; Meng, Yuze ; Su, Ying ; Yan, Li ; Huang, Xiong ; Wu, Qiran ; Chen, Xinyue ; Blei, Mark ; et al ( August 2023 , Nature Communications)

   Transition metal dichalcogenide (TMDC) moiré superlattices, owing to the moiré flatbands and strong correlation, can host periodic electron crystals and fascinating correlated physics. The TMDC heterojunctions in the type-II alignment also enable long-lived interlayer excitons that are promising for correlated bosonic states, while the interaction is dictated by the asymmetry of the heterojunction. Here we demonstrate a new excitonic state, quadrupolar exciton, in a symmetric WSe₂-WS₂-WSe₂trilayer moiré superlattice. The quadrupolar excitons exhibit a quadratic dependence on the electric field, distinctively different from the linear Stark shift of the dipolar excitons in heterobilayers. This quadrupolar exciton stems from the hybridization of WSe₂valence moiré flatbands. The same mechanism also gives rise to an interlayer Mott insulator state, in which the two WSe₂layers share one hole laterally confined in one moiré unit cell. In contrast, the hole occupation probability in each layer can be continuously tuned via an out-of-plane electric field, reaching 100% in the top or bottom WSe₂under a large electric field, accompanying the transition from quadrupolar excitons to dipolar excitons. Our work demonstrates a trilayer moiré system as a new exciting playground for realizing novel correlated states and engineering quantum phase transitions.

    

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2. Exciton Superposition across Moiré States in a Semiconducting Moiré Superlattice

   https://doi.org/10.1038/s41467-023-40783-z  

   Lian, Zhen ; Chen, Dongxue ; Meng, Yuze ; Chen, Xiaotong ; Su, Ying ; Banerjee, Rounak ; Taniguchi, Takashi ; Watanabe, Kenji ; Tongay, Sefaattin ; Zhang, Chuanwei ; et al ( August 2023 , Nature Communications)

   Moiré superlattices of semiconducting transition metal dichalcogenides enable unprecedented spatial control of electron wavefunctions, leading to emerging quantum states. The breaking of translational symmetry further introduces a new degree of freedom: high symmetry moiré sites of energy minima behaving as spatially separated quantum dots. We demonstrate the superposition between two moiré sites by constructing a trilayer WSe₂/monolayer WS₂moiré heterojunction. The two moiré sites in the first layer WSe₂interfacing WS₂allow the formation of two different interlayer excitons, with the hole residing in either moiré site of the first layer WSe₂and the electron in the third layer WSe₂. An electric field can drive the hybridization of either of the interlayer excitons with the intralayer excitons in the third WSe₂layer, realizing the continuous tuning of interlayer exciton hopping between two moiré sites and a superposition of the two interlayer excitons, distinctively different from the natural trilayer WSe₂.

    

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3. Strong interaction between interlayer excitons and correlated electrons in WSe2/WS2 moiré superlattice

   https://doi.org/10.1038/s41467-021-23732-6  

   Miao, Shengnan ; Wang, Tianmeng ; Huang, Xiong ; Chen, Dongxue ; Lian, Zhen ; Wang, Chong ; Blei, Mark ; Taniguchi, Takashi ; Watanabe, Kenji ; Tongay, Sefaattin ; et al ( December 2021 , Nature Communications)

   null (Ed.)

   Abstract Heterobilayers of transition metal dichalcogenides (TMDCs) can form a moiré superlattice with flat minibands, which enables strong electron interaction and leads to various fascinating correlated states. These heterobilayers also host interlayer excitons in a type-II band alignment, in which optically excited electrons and holes reside on different layers but remain bound by the Coulomb interaction. Here we explore the unique setting of interlayer excitons interacting with strongly correlated electrons, and we show that the photoluminescence (PL) of interlayer excitons sensitively signals the onset of various correlated insulating states as the band filling is varied. When the system is in one of such states, the PL of interlayer excitons is relatively amplified at increased optical excitation power due to reduced mobility, and the valley polarization of interlayer excitons is enhanced. The moiré superlattice of the TMDC heterobilayer presents an exciting platform to engineer interlayer excitons through the periodic correlated electron states. 

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4. Proximity-enhanced valley Zeeman splitting at the WS 2 /graphene interface

   https://doi.org/10.1088/2053-1583/acd5df  

   Faria Junior, Paulo E. ; Naimer, Thomas ; McCreary, Kathleen M. ; Jonker, Berend T. ; Finley, Jonathan J. ; Crooker, Scott A. ; Fabian, Jaroslav ; Stier, Andreas V. ( May 2023 , 2D Materials)

   The valley Zeeman physics of excitons in monolayer transition metal dichalcogenides provides valuable insight into the spin and orbital degrees of freedom inherent to these materials. Being atomically-thin materials, these degrees of freedom can be influenced by the presence of adjacent layers, due to proximity interactions that arise from wave function overlap across the 2D interface. Here, we report 60 T magnetoreflection spectroscopy of the A- and B- excitons in monolayer WS₂, systematically encapsulated in monolayer graphene. While the observed variations of the valley Zeeman effect for the A- exciton are qualitatively in accord with expectations from the bandgap reduction and modification of the exciton binding energy due to the graphene-induced dielectric screening, the valley Zeeman effect for the B- exciton behaves markedly different. We investigate prototypical WS₂/graphene stacks employing first-principles calculations and find that the lower conduction band of WS₂at the$K/K^{\mathrm{\prime }}$valleys (the${\mathrm{C}\mathrm{B}}^{-}$band) is strongly influenced by the graphene layer on the orbital level. Specifically, our detailed microscopic analysis reveals that the conduction band at theQpoint of WS₂mediates the coupling between${\mathrm{C}\mathrm{B}}^{-}$and graphene due to resonant energy conditions and strong coupling to the Dirac cone. This leads to variations in the valley Zeeman physics of the B- exciton, consistent with the experimental observations. Our results therefore expand the consequences of proximity effects in multilayer semiconductor stacks, showing that wave function hybridization can be a multi-step energetically resonant process, with different bands mediating the interlayer interactions. Such effects can be further exploited to resonantly engineer the spin-valley degrees of freedom in van der Waals and moiré heterostructures.

    

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5. Topological minibands and interaction driven quantum anomalous Hall state in topological insulator based moiré heterostructures

   https://doi.org/10.1038/s41467-024-46717-7  

   Yang, Kaijie ; Xu, Zian ; Feng, Yanjie ; Schindler, Frank ; Xu, Yuanfeng ; Bi, Zhen ; Bernevig, B. Andrei ; Tang, Peizhe ; Liu, Chao-Xing ( March 2024 , Nature Communications)

   The presence of topological flat minibands in moiré materials provides an opportunity to explore the interplay between topology and correlation. In this work, we study moiré minibands in topological insulator films with two hybridized surface states under a moiré superlattice potential created by two-dimensional insulating materials. We show the lowest conduction (highest valence) Kramers’ pair of minibands can be$${{\mathbb{Z}}}_{2}$$$Z_2$non-trivial when the minima (maxima) of moiré potential approximately form a hexagonal lattice with six-fold rotation symmetry. Coulomb interaction can drive the non-trivial Kramers’ minibands into the quantum anomalous Hall state when they are half-filled, which is further stabilized by applying external gate voltages to break inversion. We propose the monolayer Sb₂on top of Sb₂Te₃films as a candidate based on first principles calculations. Our work demonstrates the topological insulator based moiré heterostructure as a potential platform for studying interacting topological phases.

    

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