Atomically thin transition metal dichalcogenides (TMDs, e.g. MoS2, WSe2) and heterostructures host heavy charge carriers with strong Coulomb interactions, which favor the formation of correlated states such as excitons and Wigner crystals. In this talk, I will present our research on the Wigner (nano)-crystals in monolayer WSe2 and the generalized Wigner crystals in WSe2/WS2 moiré superlattices by optical spectroscopy. We observe the exciton Umklapp scattering signatures of both electron and hole Wigner nano-crystals in monolayer WSe2. The transition temperature reaches a record-high Tc ~27 K. The excitons also exhibit valley-dependent scattering with the Wigner nano-crystals. On the other hand, we observe the excitonic signatures of generalized Wigner crystals at fractional fillings of MoSe2/WS2 moiré superlattices. The electron crystallization can significantly affect the absorption, emission and valley polarization of some excitonic states. Overall, our results demonstrate novel excitonic and valley phenomena in the regime of electron crystallization in 2D materials.
Prof. Chun Hung (Joshua) Lui joined the Department of Physics and Astronomy at the University of California Riverside in July 2015. Before that, he was a postdoc at MIT. He obtained his Ph.D. in physics at Columbia University in 2011. His group investigates novel quantum phenomena in two-dimensional materials and heterostructures by optical spectroscopy combined with nanofabrication.