Probing unconventional electron Landau levels with strong interaction effects in atomically thin transition metal dichalcogenides

Atomically thin transition metal dichalcogenides (TMDCs) are emerging as a new platform for exploring many-body effects. Coulomb interactions are markedly enhanced in these materials because of the reduced dimensionality and large effective masses. Although many-body excitonic effects in TMDCs have been extensively studied by optical means, not until recently did probing their strongly correlated electronic effects become possible in transport. In this talk, I demonstrate our recent experimental study on quantum transport of few-layer WSe₂ and MoS₂ with unconventional electron Landau levels (LLs) and strong interaction effects. We fabricate high-quality n-type MoS₂ and p-type WSe₂ devices and study their valley-resolved SdH oscillations relevant to the spin-valley locked massive Dirac electron LLs. Encapsulating these TMDCs in ultra-clean hexagonal boron nitride sheets effectively eliminates impurity scattering and provides clean interfaces for making high-quality low-temperature ohmic contacts. Few-layer WSe₂ and MoS₂ field-effect devices with mobilities up to 20,000 cm²/V s have been achieved at cryogenic temperatures. We observe interesting quantum Hall transport phenomena involving the Q valley, Γ valley and K valley, such as the Q valley Zeeman effect in all odd-layer MoS₂ devices and the spin Zeeman effect in all even-layer MoS₂ devices and highly density-dependent quantum Hall states of Γ valley holes below 12T, whose predominant sequences alternate between odd- and even-integers. By tilting the magnetic field to induce Landau level crossings, we show that the strong Coulomb interaction enhances the Zeeman-to-cyclotron energy ratio from 2.67 to 3.55 as the density is reduced from 5.7 to 4.0×10¹² cm^-2, giving rise to the even-odd alternation. With decreasing the carrier density in the conductance band (K valley) of few-layer MoS₂, we observe LL crossing induced valley ferrimagnet-to-ferromagnet transitions, as a result of the interaction enhancement of the g-factor from 5.6 to 21.8. Near integer ratios of Zeeman-to-cyclotron energies, we discover LL anti-crossings. Our results provide compelling evidence for many-body interaction effects in few-layer WSe₂ and MoS₂.