Signatures of Wigner crystal of electrons in a monolayer semiconductor

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. 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 x 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 moire 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.