Electronic phase diagram and nonlinear Hall effect in ionic liquid gated monolayer transition metal dichalcogenides semiconductor

The semiconducting monolayer transition metal dichalcogenides (MoS₂, WSe₂, WS₂, etc.) (TMDCs) have attracted great interest due to their large direct band-gap, Berry phase-related physical properties, and strong spin-orbit coupling that leads to spin-valley locking. Although they are well studied by optical means, the study of transport properties is still lacking because of the difficulties in achieving Ohmic contact. In my talk, I will focus on the transport properties of the semiconducting monolayer TMDCs, which are detected by ionic liquid gating technique. Taking advantage of the broad doping capability, we can access the whole electronic phase diagram of monolayer WSe₂ and observe a cascade of phases “band insulator-superconductor-emergent insulator-quasi metal” as the doping level increases. We find that the emergent insulator is derived from a split narrow subband, the half-filling of which corresponds to the insulating dome peak. This correlation picture is supported by a charge density wave that possesses an isolated flat band. Then I will introduce a new optical stimulus that enables the ionic liquid gating transistor to operate at cryogenic temperature and discuss the underlying mechanism. Finally, I will present the realization of the nonlinear Hall effect under time-reversal-symmetric conditions in the strained monolayer WSe₂. Our observation indicates that the C_3v symmetry of monolayer WSe₂ is broken by the strain and the finite Berry curvature dipole emerges. Therefore, upon applying an electric field E, the Berry curvature dipole D can lead to an out-of-plane orbital magnetization M∝D·E, which further induces this novel second-harmonic Hall effect. Remarkably, the magnitude of the nonlinear Hall signals can be effectively tuned by the strain, which is very promising for practical applications.