Experimental observations of Wigner crystals

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In 1934, Eugene Wigner, who was studying at Princeton University, predicted the existence of electronic crystals. Electrons have both kinetic energy and potential energy of interaction. When the density of electronic states satisfies certain conditions, due to the repulsion between electrons, electrons will tend to arrange themselves in a regular lattice structure, forming electron crystals, which is also known as Wigner crystals. For nearly 90 years, Wigner crystals have fascinated condensed matter physicists. Physicists have designed many ingenious semiconductor heterojunctions to obtain lower electron densities and added magnetic fields to achieve larger effective mass of electron. In 1979, experiments revealed the existence of a phase transition from an electron liquid phase to an electron crystal on the surface of liquid helium, and subsequent experiments observed the characteristics of two-dimensional(2D) Wigner crystals in 2D electron gas under high magnetic fields. However, direct observation of 2D Wigner lattices in real space remains a formidable challenge. Through the graphene sensing layer of <inline-formula><tex-math id="M3">\begin{document}$ {WSe}_{2} $\end{document}</tex-math></inline-formula>/<inline-formula><tex-math id="M4">\begin{document}$ {WS}_{2} $\end{document}</tex-math></inline-formula>moiré superlattice, Hongyuan Li, Wang Feng et al. observed the real-space morphologies of Wigner crystals in their experiments. And in a recent study, researchers used high-resolution scanning tunneling microscopy to directly image magnetic field-induced Wigner crystals in bernal stacking bilayer graphene and investigated their structural properties as a function of electron density, magnetic field, and temperature. In this paper, we will introduce some interesting things about Wigner crystals through four representative researches briefly.