Monolayer semiconducting transition-metal dichalcogenides (TMDs) represent a unique class of two-dimensional (2D) electron systems. Their atomically thin structure facilitates gate tunability just like graphene does, but unlike graphene, TMDs have the advantage of a sizable band gap and strong spin–orbit coupling. Measurements under large magnetic fields have revealed an unusual Landau level (LL) structure^1,2,3, distinct from other 2D electron systems. However, owing to the limited sample quality and poor electrical contact, probing the lowest LLs has been challenging, and observation of electron correlations within the fractionally filled LL regime has not been possible. Here, through bulk electronic compressibility measurements, we investigate the LL structure of monolayer WSe₂ in the extreme quantum limit, and observe fractional quantum Hall states in the lowest three LLs. The odd-denominator fractional quantum Hall sequences demonstrate a systematic evolution with the LL orbital index, consistent with generic theoretical expectations. In addition, we observe an even-denominator state in the second LL that is expected to host non-Abelian statistics. Our results suggest that the 2D semiconductors can provide an experimental platform that closely resembles idealized theoretical models in the quantum Hall regime.

单层半导体过渡金属二卤化物（TMD）代表一类独特的二维（2D）电子系统。与石墨烯一样，它们的原子薄结构可促进栅极可调谐性，但是与石墨烯不同，TMD具有带隙较大和自旋轨道耦合强的优点。在大磁场下的测量显示出与其他2D电子系统不同的不寻常的Landau能级（LL）结构^1,2,3。然而，由于有限的样品质量和不良的电接触，探测最低的LLs一直具有挑战性，并且不可能在分数填充的LL范围内观察电子相关性。在这里，通过体电子可压缩性测量，我们研究了单层WSe ₂的LL结构在极限量子极限中，观察最低的三个LL中的分数量子霍尔态。奇分母分数阶霍尔分子序列证明，随着LL轨道指数的发生，系统进化，与一般理论预期一致。此外，我们在第二个LL中观察到一个偶数分母状态，该状态有望承载非阿贝尔统计数据。我们的结果表明，二维半导体可以提供一个与量子霍尔理论中的理想理论模型非常相似的实验平台。