Imaging 2D Wigner Crystals*

The Wigner crystal has fascinated condensed matter physicists for nearly 90 years. Direct observation of the 2D Wigner crystal lattice in real space, however, has remained an outstanding challenge. Conventional scanning tunneling 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 that employs a graphene sensing layer. 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. Particularly, 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 moiré heterostructures and provides an approach that is generally applicable for imaging novel correlated electron lattices in other systems.