Proximity control of interlayer excitonic correlation and exciton-phonon hybridization in van der Waals heterostructures

Chaw-Keong Yong^1*

¹Physics, National Taiwan University, Taipei, Taiwan

* Presenter:Chaw-Keong Yong, email:chawkyong@phys.ntu.edu.tw

Heterostructures of atomically thin materials provide a unique laboratory to explore novel quantum states of matter. By van der Waals stacking, band structures and electronic correlations have been tailored, shaping moiré excitons, Mott insulating, super¬conducting, and (anti-)ferromagnetic states. Theory has predicted that also electron-phonon coupling critically influences the quantum ground state of low-dimensional systems, involving Cooper pairs or charge density waves. Here we introduce proximity-controlled strong-coupling between Coulomb correlations and lattice dynamics in neighbouring van der Waals materials, creating new electrically neutral hybrid eigenmodes. Specifically, we first explore the roles of orbital overlaps at the atomic interface in defining the excitonic correlation, many-body dynamics and exciton lifetime in series of twisted bilayer tungsten diselenide. Using ultrafast optical pump-THz probe spectroscopy, we reveal how the internal orbital 1s-2p transition of Coulomb-bound electron-hole pairs in monolayer tungsten diselenide resonantly hybridizes with lattice vibrations of a polar capping layer of gypsum, giving rise to exciton-phonon mixed eigenmodes, called excitonic Lyman polarons. Tuning the orbital exciton resonance across the vibrational modes, we observe distinct anti¬crossing and polarons with adjustable exciton and phonon compositions. This hybridization can be further controlled by quantum designing the spatial wavefunction overlap of excitons and phonons. Proximity-induced phonon coupling even with uncharged quasiparticles opens a promising new strategy to engineer novel ground states of two-dimensional systems.

Keywords: twistronics, excitonic Lyman resonance, electron-phonon hybridization, ultrafast optical spectroscopy, electron dynamics