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The Wigner crystal state, first predicted by Eugene Wigner in 1934, hasfascinated condensed matter physicists for nearly 90 years2-14. Studies oftwo-dimensional (2D) electron gases first revealed signatures of the Wignercrystal in electrical transport measurements at high magnetic fields2-4. Morerecently optical spectroscopy has provided evidence of generalized Wignercrystal states in transition metal dichalcogenide (TMDC) moir\'e superlattices.Direct observation of the 2D Wigner crystal lattice in real space, however, hasremained an outstanding challenge. Scanning tunneling microscopy (STM) inprinciple has sufficient spatial resolution to image a Wigner crystal, butconventional STM measurements can potentially alter fragile Wigner crystalstates in the process of measurement. Here we demonstrate real-space imaging of2D Wigner crystals in WSe2/WS2 moir\'e heterostructures using a novelnon-invasive STM spectroscopy technique. We employ a graphene sensing layer inclose proximity to the WSe2/WS2 moir\'e superlattice for Wigner crystalimaging, where local STM tunneling current into the graphene sensing layer ismodulated by the underlying electron lattice of the Wigner crystal in theWSe2/WS2 heterostructure. Our measurement directly visualizes different latticeconfigurations associated with Wigner crystal states at fractional electronfillings of n = 1/3, 1/2, and 2/3, where n is the electron number per site. Then=1/3 and n=2/3 Wigner crystals are observed to exhibit a triangle and ahoneycomb lattice, respectively, in order to minimize nearest-neighboroccupations. The n = 1/2 state, on the other hand, spontaneously breaks theoriginal C3 symmetry and forms a stripe structure in real space. Our study laysa solid foundation toward the fundamental understanding of rich Wigner crystalstates in WSe2/WS2 moir\'e heterostructures.

Imaging Generalized Wigner Crystal States in a WSe2/WS2 Moir\'e Superlattice