Department of Physics and Center for Quantum Materials, the Hong Kong University of Science and Technology, Hong Kong
Wednesday, March 14, 2018
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.