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Every-other-layer dipolar excitons in a spin-valley locked superlattice

Nature Nanotechnology volume 18, pages 501–506 (2023)Cite this article

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Abstract

Monolayer semiconducting transition metal dichalcogenides possess broken inversion symmetry and strong spin-orbit coupling, leading to a unique spin-valley locking effect. In 2H stacked pristine multilayers, spin-valley locking yields an electronic superlattice structure, where alternating layers correspond to barriers and quantum wells depending on the spin-valley indices. Here we show that the spin-valley locked superlattice hosts a kind of dipolar exciton with the electron and hole constituents separated in an every-other-layer configuration: that is, either in two even or two odd layers. Such excitons become optically bright via hybridization with intralayer excitons. This effect is also manifested by the presence of multiple anti-crossing patterns in the reflectance spectra, as the dipolar exciton is tuned through the intralayer resonance by an electric field. The reflectance spectra further reveal an excited state orbital of the every-other-layer exciton, pointing to a sizable binding energy in the same order of magnitude as the intralayer exciton. As layer thickness increases, the dipolar exciton can form a one-dimensional Bose–Hubbard chain displaying layer number-dependent fine spectroscopy structures.

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Fig. 1: Spin-valley locked superlattice and every-other-layer exciton.
Fig. 2: Electric field tuning of exciton hybridization.
Fig. 3: Stacking-dependent every-other-layer DX.
Fig. 4: Layer number dependence and multiple anti-crossings.

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Data availability

Source data are provided with this paper. All other data that support the plots within this paper and other findings of this study are available from the corresponding authors upon reasonable request.

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Acknowledgements

This work was mainly supported by DoE BES (DE-SC0018171). Sample fabrication and PFM characterization were partially supported by the ARO MURI programme (W911NF-18-1-0431). The atomic force microscope-related measurements were performed using instrumentation supported by the US National Science Foundation through the UW Molecular Engineering Materials Center, a Materials Research Science and Engineering Center (DMR-1719797). W.Y. and C.X. acknowledge support from the University Grant Committee/Research Grants Council of Hong Kong SAR (AoE/P-701/20 and HKU SRFS2122-7S05). W.Y. also acknowledges support from the Tencent Foundation. Bulk WSe2 crystal growth and characterization by J.Y. were supported by the US Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division. K.W. and T.T. acknowledge support from the JSPS KAKENHI (19H05790, 20H00354 and 21H05233). X.X. acknowledges support from the State of Washington-funded Clean Energy Institute and from the Boeing Distinguished Professorship in Physics.

Author information

Authors and Affiliations

  1. Department of Physics, University of Washington, Seattle, WA, USA

    Yinong Zhang, Dmitry Ovchinnikov, Jiayi Zhu, Xi Wang & Xiaodong Xu

  2. Department of Physics, University of Hong Kong, Hong Kong, China

    Chengxin Xiao & Wang Yao

  3. HKU-UCAS Joint Institute of Theoretical and Computational Physics at Hong Kong, Hong Kong, China

    Chengxin Xiao & Wang Yao

  4. Department of Chemistry, University of Washington, Seattle, WA, USA

    Xi Wang

  5. International Center for Materials Nanoarchitectonics, National Institute for Materials Science, Tsukuba, Japan

    Takashi Taniguchi

  6. Research Center for Functional Materials, National Institute for Materials Science, Tsukuba, Japan

    Kenji Watanabe

  7. Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN, USA

    Jiaqiang Yan

  8. Department of Materials Science and Engineering, University of Washington, Seattle, WA, USA

    Xiaodong Xu

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  1. Yinong Zhang
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Contributions

X.X. and W.Y. conceived the project. Y.Z. fabricated samples and performed measurements assisted by D.O., J.Z. and X.W. Y.Z., X.X., C.X. and W.Y. analysed and interpreted the results. C.X. and W.Y. performed calculations. T.T. and K.W. synthesized the hBN crystals. J.Y. synthesized and characterized the bulk WSe2 crystals. Y.Z., X.X., C.X. and W.Y. wrote the paper with input from all authors. All authors discussed the results.

Corresponding authors

Correspondence to Wang Yao or Xiaodong Xu.

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Extended data

Extended Data Fig. 1 Optical reflectance spectra in monolayer and bilayer WSe2.

a-b, Electric field (Ez) dependence of differential optical reflectance measurement in monolayer (a) and bilayer (b) WSe2. Doping is fixed at zero. As expected, the every-other-layer dipolar exciton as well as the anti-crossing feature are not observed.

Source data

Extended Data Fig. 2 Electric field EZ dependence of photoluminescence spectra in trilayer WSe2.

In addition to the dR/R data presented in the maintext, we examine photoluminescence (PL) spectra and its EZ field dependence on the same trilayer WSe2. Near K-K direct transition region (around 1.7 eV), we find similar anti-crossing feature as dR/R spectra, supporting the formation of every-other-layer dipolar excitons. The field-independent PL feature around 1.76 eV is instrument artefact.

Source data

Extended Data Fig. 3 Modeling and simulating the every-other-layer dipolar exciton.

a, EZ dependence of measured differential reflectance spectra in trilayer WSe2. The green dashed lines show the curve fitting results based on the Hamiltonian Eq. (2) + (3). b, Simulation results after considering the spectral intensity and width. c, d, Same plots for 5-layer WSe2.

Source data

Source data

Source Data Fig. 1

Statistical data for Fig. 1.

Source Data Fig. 2

Statistical data for Fig. 2.

Source Data Fig. 3

Statistical data for Fig. 3.

Source Data Fig. 4

Statistical data for Fig. 4.

Source Data Extended Data Fig.1

Statistical data for Extended Data Fig. 1.

Source Data Extended Data Fig. 2

Statistical data for Extended Data Fig. 2.

Source Data Extended Data Fig. 3

Statistical data for Extended Data Fig. 3.

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Zhang, Y., Xiao, C., Ovchinnikov, D. et al. Every-other-layer dipolar excitons in a spin-valley locked superlattice. Nat. Nanotechnol. 18, 501–506 (2023). https://doi.org/10.1038/s41565-023-01350-1

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