Kavli Affiliate: Michael F. Crommie
| First 5 Authors: Hongyuan Li, Shaowei Li, Emma C. Regan, Danqing Wang, Wenyu Zhao
| Summary:
The Wigner crystal state, first predicted by Eugene Wigner in 1934, has
fascinated condensed matter physicists for nearly 90 years2-14. Studies of
two-dimensional (2D) electron gases first revealed signatures of the Wigner
crystal in electrical transport measurements at high magnetic fields2-4. More
recently optical spectroscopy has provided evidence of generalized Wigner
crystal states in transition metal dichalcogenide (TMDC) moir’e superlattices.
Direct observation of the 2D Wigner crystal lattice in real space, however, has
remained an outstanding challenge. Scanning tunneling microscopy (STM) in
principle has sufficient spatial resolution to image a Wigner crystal, but
conventional STM measurements can potentially alter fragile Wigner crystal
states in the process of measurement. Here we demonstrate real-space imaging of
2D Wigner crystals in WSe2/WS2 moir’e heterostructures using a novel
non-invasive STM spectroscopy technique. We employ a graphene sensing layer in
close proximity to the WSe2/WS2 moir’e superlattice for Wigner crystal
imaging, where local STM tunneling current into the graphene sensing layer is
modulated by the underlying electron lattice of the Wigner crystal in the
WSe2/WS2 heterostructure. Our measurement directly visualizes different lattice
configurations associated with Wigner crystal states at fractional electron
fillings of n = 1/3, 1/2, and 2/3, where n is the electron number per site. The
n=1/3 and n=2/3 Wigner crystals are observed to exhibit a triangle and a
honeycomb lattice, respectively, in order to minimize nearest-neighbor
occupations. The n = 1/2 state, on the other hand, spontaneously breaks the
original C3 symmetry and forms a stripe structure in real space. Our study lays
a solid foundation toward the fundamental understanding of rich Wigner crystal
states in WSe2/WS2 moir’e heterostructures.
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