Valence electrons in crystalline solids can behave like gases, liquids or solids. The solid state of electrons called a Wigner crystal is difficult to detect and so far only indirect evidence of crystallization has been reported. Now, Feng Wang and coworkers directly image 2D Wigner crystals in a moiré heterostructure using a specially designed non-invasive scanning tunneling microscope. They image different Wigner crystal configurations by tuning the charge carrier density. The results allow a better understanding of Wigner crystals and introduce a new technique to image surfaces and 2D materials with minimal perturbation by the experimental probe.
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