Strongly correlated excitonic insulator in atomic double layers

Abstract

Excitonic insulators (EI) arise from the formation of bound electron-hole pairs (excitons) in semiconductors and provide a solid-state platform for quantum many-boson physics. Strong exciton-exciton repulsion is expected to stabilize condensed superfluid and crystalline phases by suppressing both density and phase fluctuations. Although spectroscopic signatures of EIs have been reported, conclusive evidence for strongly correlated EI states has remained elusive. Here, we demonstrate a strongly correlated spatially indirect two-dimensional (2D) EI ground state formed in transition metal dichalcogenide (TMD) semiconductor double layers. An equilibrium interlayer exciton fluid is formed 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. 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 further construct an exciton phase diagram that reveals both the Mott transition and interaction-stabilized quasi-condensation. Our experiment paves the path for realizing the exotic quantum phases of excitons, as well as multi-terminal exciton circuitry for applications.

21 pages, 12 figures

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• Evidence of high-temperature exciton condensation in 2D atomic double layers
• High-temperature superfluidity with indirect excitons in van der Waals heterostructures
• Evidence for an excitonic insulator phase in 1T-TiSe${}_2$
• Excitonic Bose-Einstein condensation in Ta2NiSe5 above room temperature
• Strongly Interacting Two-Dimensional Bose Gases
• Electrically Controlled Two-Dimensional Electron-Hole Fluids
• Effects of strong correlations for 2D Bose-Einstein condensed dipolar excitons
• Second order Josephson effect in excitonic insulators

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