[Paper Review] Anomalous Interlayer Exciton Diffusion in Twist-Angle-Dependent Moiré Potentials of WS$_2$-WSe$_2$ Heterobilayers

Long Yuan, Biyuan Zheng+8 more|arXiv (Cornell University)|Oct 7, 2019

2D Materials and Applications6 references48 citations

TL;DR

This paper investigates how twist-angle-dependent moiré potentials affect interlayer exciton localization and diffusion in WS2-WSe2 heterobilayers, revealing anomalous, density- and angle-dependent transport.

ABSTRACT

The nanoscale periodic potentials introduced by moiré patterns in semiconducting van der Waals (vdW) heterostructures provide a new platform for designing exciton superlattices. To realize these applications, a thorough understanding of the localization and delocalization of interlayer excitons in the moiré potentials is necessary. Here, we investigated interlayer exciton dynamics and transport modulated by the moiré potentials in WS$_2$-WSe$_2$ heterobilayers in time, space, and momentum domains using transient absorption microscopy combined with first-principles calculations. Experimental results verified the theoretical prediction of energetically favorable K-Q interlayer excitons and unraveled exciton-population dynamics that was controlled by the twist-angle-dependent energy difference between the K-Q and K-K excitons. Spatially- and temporally-resolved exciton-population imaging directly visualizes exciton localization by twist-angle-dependent moiré potentials of ~100 meV. Exciton transport deviates significantly from normal diffusion due to the interplay between the moiré potentials and strong many-body interactions, leading to exciton-density- and twist-angle-dependent diffusion length. These results have important implications for designing vdW heterostructures for exciton and spin transport as well as for quantum communication applications.

Motivation & Objective

• Understand how moiré potentials from twist angles localize interlayer excitons in WS2-WSe2 heterobilayers.
• Characterize exciton dynamics in time, space, and momentum domains using transient absorption microscopy.
• Determine how twist-angle dependent energy differences between K-Q and K-K interlayer excitons govern population dynamics.
• Quantify exciton diffusion behavior and its deviation from normal diffusion under moiré potentials.

Proposed method

• Combine transient absorption microscopy with first-principles calculations to study interlayer exciton dynamics.
• Identify energetically favorable K-Q interlayer excitons and compare with K-K excitons.
• Image exciton population spatially and temporally to observe localization due to moiré potentials (~100 meV).
• Analyze how twist angle modulates energy landscapes and exciton transport.
• Assess the role of many-body interactions in modifying diffusion length and transport regimes.

Experimental results

Research questions

• RQ1 How do twist-angle-dependent moiré potentials localize interlayer excitons in WS$_2$-WSe$_2$ heterobilayers?
• RQ2 What is the role of K-Q versus K-K interlayer excitons in population dynamics and diffusion?
• RQ3 How does exciton diffusion deviate from normal diffusion under moiré potentials and many-body interactions?
• RQ4 How does exciton diffusion length depend on exciton density and twist angle?

Key findings

• Exciton localization is directly visualized and is governed by twist-angle-dependent moiré potentials of ~100 meV.
• The energy difference between K-Q and K-K interlayer excitons controls exciton-population dynamics.
• Exciton transport deviates from normal diffusion due to interplay between moiré potentials and strong many-body interactions.
• Diffusion length becomes dependent on both exciton density and twist angle.
• K-Q interlayer excitons are energetically favorable, influencing overall transport behavior.