Electron-Exciton Coupling in 1T-TiSe2 Bilayer

Excitons in solid state are bosons generated by electron-hole pairs as the Coulomb screening is sufficiently reduced. The exciton condensation can result in exotic physics such as super-fluidity and insulating state. In charge density wave (CDW) state, 1T-TiSe2 is one of the candidates that may host the exciton condensation. However, to envision its excitonic effect is still challenging, particularly at the two-dimensional limit, which is applicable to future devices. Here, we realize the epitaxial 1T-TiSe2 bilayer, the two-dimensional limit for its 2 x 2 x 2 CDW order, to explore the exciton-associated effect. By means of high-resolution scanning tunneling spectroscopy and quasiparticle interference, we discover an unexpected state residing below the conduction band and right within the CDW gap region. As corroborated by our theoretical analysis, this mysterious phenomenon is in good agreement with the electron-exciton coupling. Our study provides a material platform to explore exciton-based electronics and opto-electronics.

Automated summary

Excitons in solid state are bosons generated by electron-hole pairs as the Coulomb screening is reduced. The condensation of excitons can lead to exotic physics like superfluidity and insulating state. 1T-TiSe2 in charge density wave (CDW) state is a candidate for exciton condensation, but it is still challenging to envision its excitonic effect in the two-dimensional limit applicable to future devices. In this study, we create a 2D epitaxial bilayer of 1T-TiSe2 to explore the exciton-associated effect and discover an unexpected state below the conduction band and within the CDW gap region, which is found to be in good agreement with the electron-exciton coupling through theoretical analysis. Our research provides a material platform for investigating exciton-based electronics and opto-electronics.