Electron-Exciton Coupling in 1T-TiSe$_{2}$ Bilayer
Li Zhu1†, Wei-Min Zhao1†, Zhen-Yu Jia1, Huiping Li2,3, Xuedong Xie1, Qi-Yuan Li1, Qi-Wei Wang1, Li-Guo Dou1, Ju-Gang Hu1, Yi Zhang1, Wenguang Zhu2,3, Shun-Li Yu1*, Jian-Xin Li1*, and Shao-Chun Li1,4*
1National Laboratory of Solid State Microstructures, Collaborative Innovation Center of Advanced Microstructures, School of Physics, Nanjing University, Nanjing 210093, China 2International Center for Quantum Design of Functional Materials (ICQD), Hefei National Laboratory for Physical Sciences at the Microscale, and Synergetic Innovation Center of Quantum Information and Quantum Physics, University of Science and Technology of China, Hefei 230026, China 3Key Laboratory of Strongly Coupled Quantum Matter Physics of Chinese Academy of Sciences, School of Physical Sciences, University of Science and Technology of China, Hefei 230026, China 4Jiangsu Provincial Key Laboratory for Nanotechnology, Nanjing University, Nanjing 210093, China
Abstract：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-TiSe$_{2}$ 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-TiSe$_{2}$ bilayer, the two-dimensional limit for its $2 \times 2\times 2$ 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.
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2023, Vol. 40 
