Imaging two-dimensional generalized Wigner crystals

Hongyuan Li, Shaowei Li (), Emma C. Regan, Danqing Wang, Wenyu Zhao, Salman Kahn, Kentaro Yumigeta, Mark Blei, Takashi Taniguchi, Kenji Watanabe, Sefaattin Tongay, Alex Zettl, Michael F. Crommie () and Feng Wang ()
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Hongyuan Li: University of California at Berkeley
Shaowei Li: University of California at Berkeley
Emma C. Regan: University of California at Berkeley
Danqing Wang: University of California at Berkeley
Wenyu Zhao: University of California at Berkeley
Salman Kahn: University of California at Berkeley
Kentaro Yumigeta: Arizona State University
Mark Blei: Arizona State University
Takashi Taniguchi: National Institute for Materials Science
Kenji Watanabe: National Institute for Materials Science
Sefaattin Tongay: Arizona State University
Alex Zettl: University of California at Berkeley
Michael F. Crommie: University of California at Berkeley
Feng Wang: University of California at Berkeley

Nature, 2021, vol. 597, issue 7878, 650-654

Abstract: Abstract The Wigner crystal1 has fascinated condensed matter physicists for nearly 90 years2–14. Signatures of two-dimensional (2D) Wigner crystals were first observed in 2D electron gases under high magnetic field2–4, and recently reported in transition metal dichalcogenide moiré superlattices6–9. Direct observation of the 2D Wigner crystal lattice in real space, however, has remained an outstanding challenge. Conventional scanning tunnelling microscopy (STM) has sufficient spatial resolution but induces perturbations that can potentially alter this fragile state. Here we demonstrate real-space imaging of 2D Wigner crystals in WSe2/WS2 moiré heterostructures using a specially designed non-invasive STM spectroscopy technique. This employs a graphene sensing layer held close to the WSe2/WS2 moiré superlattice. Local STM tunnel current into the graphene layer is modulated by the underlying Wigner crystal electron lattice in the WSe2/WS2 heterostructure. Different Wigner crystal lattice configurations at fractional electron fillings of n = 1/3, 1/2 and 2/3, where n is the electron number per site, are directly visualized. The n = 1/3 and n = 2/3 Wigner crystals exhibit triangular and honeycomb lattices, respectively, to minimize nearest-neighbour occupations. The n = 1/2 state spontaneously breaks the original C3 symmetry and forms a stripe phase. Our study lays a solid foundation for understanding Wigner crystal states in WSe2/WS2 moiré heterostructures and provides an approach that is generally applicable for imaging novel correlated electron lattices in other systems.

Date: 2021
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DOI: 10.1038/s41586-021-03874-9

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