Ultrafast dynamics of valley-polarized correlated excitonic states in WS₂/WSe₂ moire superlattice heterostructures

Chaw-Keong Yong^1*

¹Physics, National Taiwan University, Taipei, Taiwan

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

Moiré superlattices in van der Waals heterostructures are emerging as transformative platforms for probing complex quantum phenomena. Upon optical excitation, these structures generate charge-neutral interlayer excitons with out-of-plane electric dipoles, fostering strong dipolar interactions that may lead to exotic correlated bosonic states, analogous to Mott states in electronic systems. In this study, we employed polarization-dependent, time-resolved optical spectroscopy to explore the exciton and valley dynamics within hBN-encapsulated WS₂/WSe₂ moiré superlattices. Using a femtosecond optical pump resonant with the A-exciton of WSe₂, we observe a pronounced new resonance approximately 27 meV above the ground state, triggered only when exciton density exceeds one per moiré lattice site. This resonance signals the emergence of a strongly correlated exciton state, where two excitons are confined within a single moiré unit cell (IX₂), exhibiting a significantly enhanced recombination rate and accelerate the valley depolarization, compared to the singly occupied state (IX₁). The depletion of IX₂ is followed by frozen recombination of IX₁ with prominent valley polarization, which extended over ~ 10 ns, signifying a quantum phase transition into incompressible excitonic Mott insulating phase. When the excitation density is below one-exciton-per-site threshold, the exciton recombination rates remain largely independent of exciton density, while valley polarization display a distinct progression: as initial exciton density approaches one per moiré site, the valley recombination time constant reduces from microsecond to 100 ns timescale. These findings reveal an unprecedented level of optical control over exciton-exciton correlations and valley dynamics, advancing the tunability of quantum states in engineered moiré superlattices.

Keywords: Bosonic Mott insulator, 2D moire superlattice , Ultrafast spectroscopy, quantum phase transition