Strongly bound excitons dominate electronic relaxation in resonantly excited twisted bilayer graphene

TL;DR

This study reveals that strongly bound dark excitons dominate electronic relaxation in resonantly excited twisted bilayer graphene, creating a relaxation bottleneck and coexisting with free-electron states, a novel phenomenon in metallic 2D materials.

Contribution

It demonstrates the existence of stable, strongly bound dark excitons in twisted bilayer graphene and their role in electronic relaxation processes, using advanced microscopy and TEM techniques.

Findings

Resonant electronic population is significantly enhanced in tBLG.

A dark exciton state lies 0.37 eV below the absorption resonance.

Coexistence of bound excitons and free-electron states observed in metallic 2D material.

Abstract

When two sheets of graphene stack in a twisted bilayer graphene (tBLG) configuration, the resulting constrained overlap between interplanar 2p orbitals produce angle-tunable electronic absorption resonances. Using a novel combination of multiphoton transient absorption (TA) microscopy and TEM, we resolve the resonant electronic structure, and ensuing electronic relaxation inside single tBLG domains. Strikingly, we find that the transient electronic population in resonantly excited tBLG domains is enhanced many fold, forming a major electronic relaxation bottleneck. 2-photon TA microscopy shows this bottleneck effect originates from a strongly bound, dark exciton state lying 0.37 eV below the 1-photon absorption resonance. This stable coexistence of strongly bound excitons alongside free-electron continuum states has not been previously observed in a metallic, 2D material.