Excitonic insulators (EIs) arise from the formation of bound electron–hole pairs (excitons)^1,2 in semiconductors and provide a solid-state platform for quantum many-boson physics^3,4,5,6,7,8. Strong exciton–exciton repulsion is expected to stabilize condensed superfluid and crystalline phases by suppressing both density and phase fluctuations^8,9,10,11. Although spectroscopic signatures of EIs have been reported^6,12,13,14, conclusive evidence for strongly correlated EI states has remained elusive. Here we demonstrate a strongly correlated two-dimensional (2D) EI ground state formed in transition metal dichalcogenide (TMD) semiconductor double layers. A quasi-equilibrium spatially indirect exciton fluid is created when the bias voltage applied between the two electrically isolated TMD layers is tuned to a range that populates bound electron–hole pairs, but not free electrons or holes^15,16,17. Capacitance measurements show that the fluid is exciton-compressible but charge-incompressible—direct thermodynamic evidence of the EI. The fluid is also strongly correlated with a dimensionless exciton coupling constant exceeding 10. We construct an exciton phase diagram that reveals both the Mott transition and interaction-stabilized quasi-condensation. Our experiment paves the path for realizing exotic quantum phases of excitons⁸, as well as multi-terminal exciton circuitry for applications^18,19,20.

激子绝缘体 (EI) 源于半导体中束缚电子-空穴对（激子）^{1,2的形成，并为量子多玻色子物理学3,4,5,6,7,8}提供了一个固态平台。预计强激子-激子排斥会通过抑制密度和相波动^8,9,10,11来稳定凝聚的超流体和晶相。尽管已经报道了 EI 的光谱特征^6,12,13,14, 强相关 EI 状态的确凿证据仍然难以捉摸。在这里，我们展示了在过渡金属二硫化物 (TMD) 半导体双层中形成的强相关二维 (2D) EI 基态。当两个电隔离 TMD 层之间施加的偏置电压被调谐到填充束缚电子-空穴对但不填充自由电子或空穴的范围时，会产生准平衡空间间接激子流体^15,16,17. 电容测量表明流体是激子可压缩的但电荷不可压缩——EI 的直接热力学证据。流体还与超过 10 的无量纲激子耦合常数密切相关。我们构建了一个激子相图，揭示了莫特转变和相互作用稳定的准凝聚。我们的实验为实现激子⁸的奇异量子相以及应用^18,19,20的多端激子电路铺平了道路。