TOPICAL REVIEW — Moiré physics in two-dimensional materials

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    Manipulating optical and electronic properties through interfacial ferroelectricity
    Yulu Liu(刘钰璐), Gan Liu(刘敢), and Xiaoxiang Xi(奚啸翔)
    Chin. Phys. B, 2025, 34 (1): 017701.   DOI: 10.1088/1674-1056/ad9456
    Abstract209)   HTML0)    PDF (2472KB)(57)      
    Interfacial ferroelectricity is a recently established mechanism for generating spontaneous reversible electric polarization, arising from the charge transfer between stacked van der Waals layered atomic crystals. It has been realized in both naturally formed multilayer crystals and moiré superlattices. Owing to the large number of material choices and combinations, this approach is highly versatile, greatly expanding the scope of ultrathin ferroelectrics. A key advantage of interfacial ferroelectricity is its potential to couple with preexisting properties of the constituent layers, enabling their electrical manipulation through ferroelectric switching and paving the way for advanced device functionalities. This review article summarizes recent experimental progress in interfacial ferroelectricity, with an emphasis on its coupling with a variety of electronic properties. After introducing the underlying mechanism of interfacial ferroelectricity and the range of material systems discovered to date, we highlight selected examples showcasing ferroelectric control of excitonic optical properties, Berry curvature effects, and superconductivity. We also discuss the challenges and opportunities that await further studies in this field.
    Valley-selective manipulation of moiré excitons through optical Stark effect
    Chenran Xu(徐晨燃), Jichen Zhou(周纪晨), Zhexu Shan(单哲旭), Wenjian Su(苏文健), Kenji Watanabe, Takashi Taniguchi, Dawei Wang(王大伟), and Yanhao Tang(汤衍浩)
    Chin. Phys. B, 2025, 34 (1): 017102.   DOI: 10.1088/1674-1056/ad7c32
    Abstract218)   HTML0)    PDF (4387KB)(74)      
    Semiconductor moiré superlattices provide great platforms for exploring exotic collective excitations. Optical Stark effect, a shift of the electronic transition in the presence of a light field, provides an ultrafast and coherent method of manipulating matter states, which, however, has not been demonstrated in moiré materials. Here, we report the valley-selective optical Stark effect of moiré excitons in the WSe$_{2}$/WS$_{2}$ superlattice by using transient reflection spectroscopy. Prominent valley-selective energy shifts up to 7.8 meV have been observed for moiré excitons, corresponding to pseudo-magnetic fields as large as 34 T. Our results provide a route to coherently manipulate exotic states in moiré superlattices.
    Chiral phonons of honeycomb-type bilayer Wigner crystals
    Dingrui Yang(杨丁睿), Lingyi Li(李令仪), Na Zhang(张娜), and Hongyi Yu(俞弘毅)
    Chin. Phys. B, 2025, 34 (1): 017301.   DOI: 10.1088/1674-1056/ad8eca
    Abstract191)   HTML1)    PDF (1392KB)(53)      
    We theoretically investigated the chiral phonons of honeycomb-type bilayer Wigner crystals recently discovered in van der Waals structures of layered transition metal dichalcogenides. These chiral phonons can emerge under the inversion symmetry breaking introduced by an effective mass imbalance between the two layers or a moiré potential in one layer, as well as under the time-reversal symmetry breaking realized by applying a magnetic field. Considering the wide tunability of layered materials, the frequencies and chirality of phonons can both be tuned by varying the system parameters. These findings suggest that bilayer honeycomb-type Wigner crystals can serve as an exciting new platform for studying chiral phonons.