Zhen Lian1,Dongxue Chen1,Xiong Huang2,Ying Su3,Mina Rashetnia4,Lei Ma1,Li Yan1,Mark Blei5,Li Xiang6,Sefaattin Tongay5,Dmitry Smirnov6,Zenghui Wang7,Chuanwei Zhang3,Yong-Tao Cui4,Su-Fei Shi1
Rensselaer Polytechnic Institute1,Columbia University2,The University of Texas at Dallas3,University of California4,Arizona State University5,National High Magnetic Field Lab6,University of Electronic Science and Technology of China7
Zhen Lian1,Dongxue Chen1,Xiong Huang2,Ying Su3,Mina Rashetnia4,Lei Ma1,Li Yan1,Mark Blei5,Li Xiang6,Sefaattin Tongay5,Dmitry Smirnov6,Zenghui Wang7,Chuanwei Zhang3,Yong-Tao Cui4,Su-Fei Shi1
Rensselaer Polytechnic Institute1,Columbia University2,The University of Texas at Dallas3,University of California4,Arizona State University5,National High Magnetic Field Lab6,University of Electronic Science and Technology of China7
The moiré superlattices of 2D semiconductors exhibit many novel physics phenomena, including moiré excitons and correlated insulating states. While extensive studies have been conducted to investigate the properties of the moiré superlattices between monolayer TMDCs, the effect of the layer degree of freedom on moiré excitons and correlated insulating states remains uninvestigated. In the first part of this work, we demonstrate layer-modulated moiré excitons and correlated insulating states in WSe<sub>2</sub>/WS<sub>2</sub> moiré superlattices. We investigate reflectance contrast spectra of WS<sub>2</sub>/WSe<sub>2</sub> moiré heterostructures with varying WSe<sub>2</sub> layer number. The layer degree of freedom strongly modifies the resonant energies of the moiré excitons. In addition, the 1L/2L and 1L/3L WS<sub>2</sub>/WSe<sub>2</sub> show an extra excitonic peak close to the original WSe<sub>2</sub> A-exciton resonance, a result of the interfacial nature of the moiré coupling. The carrier density to reach each correlated insulating state is the same for 1L/1L, 1L/2L and 1L/3L WS<sub>2</sub>/WSe<sub>2</sub>, indicating the moiré flat band is isolated from the trivial valence bands from the additional WSe<sub>2</sub> layers.<br/>Based on the above result, we further show that the valence bands of the 1<sup>st</sup> and the 2<sup>nd</sup> WSe<sub>2 </sub>layer in 1L/2L WS<sub>2</sub>/WSe<sub>2 </sub>can be tuned to overlap by applying an electric field. Holes can partially populate the 2<sup>nd</sup> WSe<sub>2</sub> layer at a total filling of one hole per moiré supercell, which is evidenced by the emergence of exciton-polarons from the 2<sup>nd</sup> WSe<sub>2</sub> layer. The abrupt changes of peak positions and intensities of moiré excitons suggest that the system is in an insulating state at n=-1. From particle-hole transformation, electron-like carriers reside in the moiré flat band of the 1<sup>st</sup> WSe<sub>2</sub> layer and are bound with holes in the 2<sup>nd</sup> WSe<sub>2</sub> layer due to strong Coulomb interaction, forming an excitonic insulator phase. Our result demonstrates a new moiré system for the study of correlated many-body physics in two dimensions.