Signatures of Wigner crystal of electrons in a monolayer semiconductor

• Published in: Nature (London) Vol. 595; no. 7865; pp. 53 - 57
• Main Authors: Smoleński, Tomasz, Dolgirev, Pavel E., Kuhlenkamp, Clemens, Popert, Alexander, Shimazaki, Yuya, Back, Patrick, Lu, Xiaobo, Kroner, Martin, Watanabe, Kenji, Taniguchi, Takashi, Esterlis, Ilya, Demler, Eugene, Imamoğlu, Ataç
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 01.07.2021; Nature Publishing Group
• Subjects: 140/125; 639/301/119/995; 639/301/119/999; Crystals; Electrons; Energy; Excitons; Humanities and Social Sciences; Kinetic energy; Magnetic fields; Monolayers; multidisciplinary; Optical reflection; Phase transitions; Science; Science (multidisciplinary); Semiconductors; Spectroscopy; Spectrum analysis; Symmetry; Transition metal compounds
• DOI: 10.1038/s41586-021-03590-4
• ISSN: 0028-0836

When the Coulomb repulsion between electrons dominates over their kinetic energy, electrons in two-dimensional systems are predicted to spontaneously break continuous-translation symmetry and form a quantum crystal 1 . Efforts to observe 2 – 12 this elusive state of matter, termed a Wigner crystal, in two-dimensional extended systems have primarily focused on conductivity measurements on electrons confined to a single Landau level at high magnetic fields. Here we use optical spectroscopy to demonstrate that electrons in a monolayer semiconductor with density lower than 3 × 10 11  per centimetre squared form a Wigner crystal. The combination of a high electron effective mass and reduced dielectric screening enables us to observe electronic charge order even in the absence of a moiré potential or an external magnetic field. The interactions between a resonantly injected exciton and electrons arranged in a periodic lattice modify the exciton bandstructure so that an umklapp resonance arises in the optical reflection spectrum, heralding the presence of charge order 13 . Our findings demonstrate that charge-tunable transition metal dichalcogenide monolayers 14 enable the investigation of previously uncharted territory for many-body physics where interaction energy dominates over kinetic energy. The signature of a Wigner crystal—the analogue of a solid phase for electrons—is observed via the optical reflection spectrum in a monolayer transition metal dichalcogenide.