Research Seminar - Yoseob Yoon

Two-dimensional van der Waals heterostructures host a moiré lattice potential that can induce strongly correlated phases of electrons such as Mott and generalized Wigner crystalline phases. Their electronic and optical properties strongly depend on the interactions between atomic lattices and their structural reconstruction at the moiré periodicity. It is an outstanding challenge to understand how lattice vibrations are modified by the structural reconstruction and how carriers and optical excitations are transferred between moiré sites or between layers. In this talk, I will show that ultrafast pump-probe spectroscopy provides signatures of such atomic and electronic motion in real time. We measured oscillations of exciton peaks after a perturbation, which reveal interlayer moiré phonon modes in a WSe2/WS2 heterostructure. We compared them with classical force-field calculations to identify the corresponding lattice motion within the moiré cell and showed that they are very sensitive to the twist angle. In addition, I will show that the charge transfer process in a MoSe2/WSe2 heterostructure, which is known to be ultrafast (<100 fs), is significantly slowed down by a thin layer of hexagonal boron nitride (hBN) between MoSe2 and WSe2 layers. To explain the observed density-dependent charge transfer dynamics using a coupled rate equation model, exciton-exciton annihilation and nonlinear charge transfer processes must be included. Our work provides a deeper understanding of lattice and charge transfer dynamics, and the role of interlayer coupling.

Yoseob is a postdoctoral researcher in the Wang group at UC Berkeley, where he uses ultrafast pump-probe spectroscopy to investigate the effect of moiré potential on excitons and phonons in van der Waals heterostructures. He received his Ph.D. in Physical Chemistry from Massachusetts Institute of Technology (MIT), where he studied strong light-matter interactions in various excitonic systems in the Nelson group. During his undergraduate program in Pohang University of Science and Technology (POSTECH), he spent a year in the Alivisatos group at UC Berkeley to study the properties of PbSe nanocrystals and their clusters.