In strongly correlated materials, quasiparticle excitations can carry fractional quantum numbers. An intriguing possibility is the formation of fractionalized, charge-neutral fermions—for example, spinons¹ and fermionic excitons^2,3—that result in neutral Fermi surfaces and Landau quantization^4,5 in an insulator. Although previous experiments in quantum spin liquids¹, topological Kondo insulators^6,7,8 and quantum Hall systems^3,9 have hinted at charge-neutral Fermi surfaces, evidence for their existence remains inconclusive. Here we report experimental observation of Landau quantization in a two-dimensional insulator, monolayer tungsten ditelluride (WTe₂), a large-gap topological insulator^10,11,12,13. Using a detection scheme that avoids edge contributions, we find large quantum oscillations in the material’s magnetoresistance, with an onset field as small as about 0.5 tesla. Despite the huge resistance, the oscillation profile, which exhibits many periods, mimics the Shubnikov–de Haas oscillations in metals. At ultralow temperatures, the observed oscillations evolve into discrete peaks near 1.6 tesla, above which the Landau quantized regime is fully developed. Such a low onset field of quantization is comparable to the behaviour of high-mobility conventional two-dimensional electron gases. Our experiments call for further investigation of the unusual ground state of the WTe₂ monolayer, including the influence of device components and the possible existence of mobile fermions and charge-neutral Fermi surfaces inside its insulating gap.

在强相关材料中，准粒子激发可以携带分数量子数。一个有趣的可能性是形成分级的、电荷中性的费米子——例如，旋子¹和费米子激子^2,3——这会导致绝缘体中的中性费米表面和朗道量子化^4,5。尽管之前在量子自旋液体¹、拓扑 Kondo 绝缘体^6,7,8和量子霍尔系统^3,9中的实验已经暗示了电荷中性费米表面，但它们存在的证据仍然没有定论。在这里，我们报告了在二维绝缘体、单层二碲化钨（WTe ₂)，大间隙拓扑绝缘体^10,11,12,13。使用避免边缘贡献的检测方案，我们发现材料的磁阻中有很大的量子振荡，起始场小至约 0.5 特斯拉。尽管阻力很大，但表现出许多周期的振荡曲线模仿了金属中的 Shubnikov-de Haas 振荡。在超低温下，观察到的振荡演变成接近 1.6 特斯拉的离散峰，在该峰之上，朗道量子化状态得到充分发展。这种低起始量子化场可与高迁移率的传统二维电子气的行为相媲美。我们的实验要求进一步研究 WTe _{2的异常基态}单层，包括器件组件的影响以及在其绝缘间隙内可能存在的移动费米子和电荷中性费米表面。