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 crystal¹. Efforts to observe^{2,3,4,5,6,7,8,9,10,11,12} 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 × 10¹¹ 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 order¹³. Our findings demonstrate that charge-tunable transition metal dichalcogenide monolayers¹⁴ enable the investigation of previously uncharted territory for many-body physics where interaction energy dominates over kinetic energy.

当电子之间的库仑斥力超过其动能时，预计二维系统中的电子会自发地破坏连续平移对称性并形成量子晶体¹。在二维扩展系统中观察^{2,3,4,5,6,7,8,9,10,11,12}这种难以捉摸的物质状态（称为 Wigner 晶体）的努力主要集中在限制电子的电导率测量上在高磁场下达到单个朗道能级。在这里，我们使用光谱学来证明密度低于 3 × 10 ^{11的单层半导体中的电子} 每平方厘米形成一个维格纳晶体。高电子有效质量和减少的介电屏蔽相结合，使我们即使在没有莫尔电位或外部磁场的情况下也能观察到电荷顺序。共振注入的激子和排列在周期性晶格中的电子之间的相互作用改变了激子的能带结构，从而在光学反射光谱中出现 umklapp 共振，预示着电荷顺序¹³的存在。我们的研究结果表明，电荷可调的过渡金属二硫属化物单分子层¹⁴能够研究以前未知的多体物理领域，其中相互作用能优于动能。