Strongly correlated excitonic insulator in atomic double layers

TL;DR

This paper reports the experimental discovery of a strongly correlated excitonic insulator state in transition metal dichalcogenide double layers, providing thermodynamic evidence and a phase diagram for this quantum phase.

Contribution

It demonstrates the formation of a strongly correlated 2D excitonic insulator in TMD double layers with direct thermodynamic evidence and detailed phase diagram.

Findings

Observation of an exciton fluid that is charge-incompressible but exciton-compressible.

Identification of a dimensionless exciton coupling constant exceeding 10.

Revelation of a phase diagram showing Mott transition and quasi-condensation.

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.