https://www.nature.com/articles/d41586-021-02657-6 Skip to main content Thank you for visiting nature.com. You are using a browser version with limited support for CSS. To obtain the best experience, we recommend you use a more up to date browser (or turn off compatibility mode in Internet Explorer). In the meantime, to ensure continued support, we are displaying the site without styles and JavaScript. Advertisement Advertisement Nature * View all journals * Search * My Account Login * Explore content * About the journal * Publish with us Subscribe * Sign up for alerts * RSS feed 1. nature 2. news 3. article * NEWS * 29 September 2021 This is what a solid made of electrons looks like Physicists have imaged elusive 'Wigner crystals' for the first time. * Davide Castelvecchi 1. Davide Castelvecchi View author publications You can also search for this author in PubMed Google Scholar * Share on Twitter * Share on Facebook * Share via E-Mail You have full access to this article via your institution. Download PDF Raw image for the generalized Wigner crystal states. This scanning tunneling microscope image of a graphene sheet reveals that a 'Wigner crystal' -- a honeycomb arrangement of electrons -- has formed inside a layered structure underneath.Credit: H. Li et al./ Nature If the conditions are just right, some of the electrons inside a material will arrange themselves into a tidy honeycomb pattern -- like a solid within a solid. Physicists have now directly imaged these 'Wigner crystals', named after the Hungarian-born theorist Eugene Wigner, who first imagined them almost 90 years ago. Researchers had convincingly created Wigner crystals and measured their properties before, but this is the first time that anyone has actually taken a snapshot of the patterns, says study co-author Feng Wang, a physicist at the University of California, Berkeley. "If you say you have an electron crystal, show me the crystal," he says. The results were published on 29 September in Nature^1. [d41586-021] How 'magic angle' graphene is stirring up physics To create the Wigner crystals, Wang's team built a device containing atom-thin layers of two similar semiconductors: tungsten disulfide and tungsten diselenide. The team then used an electric field to tune the density of the electrons that moved freely along the interface between the two layers. In ordinary materials, electrons zoom around too quickly to be significantly affected by the repulsion between their negative charges. But Wigner predicted that if electrons travelled slowly enough, that repulsion would begin to dominate their behaviour. The electrons would then find arrangements that minimize their total energy, such as a honeycomb pattern. So Wang and his colleagues slowed the electrons in their device by cooling it to just a few degrees above absolute zero. A mismatch between the two layers in the device also helped the electrons to form Wigner crystals. The atoms in each of the two semiconductor layers are slightly different distances apart, so pairing them together creates a honeycomb 'moire pattern', similar to that seen when overlaying two grids. That repeating pattern created regions of slightly lower energy, which helped the electrons settle down. Graphene trick The team used a scanning tunnelling microscope (STM) to see this Wigner crystal. In an STM, a metal tip hovers above the surface of a sample, and a voltage causes electrons to jump down from the tip, creating an electric current. As the tip moves across the surface, the changing intensity of the current reveals the location of electrons in the sample. [d41586-021] Welcome anyons! Physicists find best evidence yet for long-sought 2D structures Initial attempts to image the Wigner crystal by applying the STM directly on the double-layer device were unsuccessful, Wang says, because the current destroyed the fragile Wigner arrangements. So the team added a layer of graphene, a single-atom sheet of carbon, on top. The presence of the Wigner crystal slightly changed the electron structure of the graphene directly above, which was then picked up by the STM. The images clearly show the neat arrangement of the underlying Wigner electrons. As expected, consecutive electrons in the Wigner crystal are nearly 100 times farther apart than are the atoms in the semiconductor device's actual crystals. "I think that's a great advancement, being able to perform STM on this system," says Carmen Rubio Verdu, a physicist at Columbia University in New York City. She adds that the same graphene-based method will enable STM studies of a number of other interesting physical phenomena beyond Wigner crystals. Kin Fai Mak, a physicist at Cornell University in Ithaca, New York, agrees. "The technique is non-invasive to the state you want to probe. To me, it is a very clever idea." doi: https://doi.org/10.1038/d41586-021-02657-6 Read the related News & Views. References 1. 1. Li, H. et al. Nature 597, 650-654 (2021). Article Google Scholar Download references Related Articles * [d41586-021] How 'magic angle' graphene is stirring up physics * [d41586-021] Welcome anyons! Physicists find best evidence yet for long-sought 2D structures * [d41586-021] The super materials that could trump graphene Subjects * Condensed-matter physics * Physics Latest on: Condensed-matter physics Cascade of correlated electron states in a kagome superconductor CsV3Sb5 Article 29 SEP 21 Roton pair density wave in a strong-coupling kagome superconductor Article 29 SEP 21 Imaging two-dimensional generalized Wigner crystals Imaging two-dimensional generalized Wigner crystals Article 29 SEP 21 Physics Cascade of correlated electron states in a kagome superconductor CsV3Sb5 Article 29 SEP 21 Roton pair density wave in a strong-coupling kagome superconductor Article 29 SEP 21 Imaging two-dimensional generalized Wigner crystals Imaging two-dimensional generalized Wigner crystals Article 29 SEP 21 Nature Careers Jobs * Bacteriologist California State University Long Beach (CSULB) Long Beach, CA, United States * Postdoctoral Fellow The University of Texas at Austin (UT Austin) Austin, TX, United States * Postdoctoral Fellow - Transcriptional Regulation The University of Texas Health Science Center at San Antonio (UT Health Science Center at San Antonio) San Antonio, TX, United States * Assistant Professor Position - Plant Biochemistry and Synthetic Biology University of California Davis (UC Davis) Davis, CA, United States You have full access to this article via your institution. Download PDF Related Articles * [d41586-021] How 'magic angle' graphene is stirring up physics * [d41586-021] Welcome anyons! Physicists find best evidence yet for long-sought 2D structures * [d41586-021] The super materials that could trump graphene Subjects * Condensed-matter physics * Physics Advertisement Sign up to Nature Briefing An essential round-up of science news, opinion and analysis, delivered to your inbox every weekday. Email address [ ] [ ] Yes! Sign me up to receive the daily Nature Briefing email. I agree my information will be processed in accordance with the Nature and Springer Nature Limited Privacy Policy. Sign up * Close Nature Briefing Sign up for the Nature Briefing newsletter -- what matters in science, free to your inbox daily. Email address [ ] Sign up [ ] I agree my information will be processed in accordance with the Nature and Springer Nature Limited Privacy Policy. Close Get the most important science stories of the day, free in your inbox. 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