Collective excitations in twisted bilayer graphene close to the magic angle

TL;DR

This study uncovers a unique collective plasmon mode in charge-neutral twisted bilayer graphene near the magic angle, revealing new optical properties and insights into carrier dynamics in this quantum system.

Contribution

We identify and characterize a novel interband plasmon mode in TBG near the magic angle using near-field optical microscopy, highlighting its distinct dispersion and enhanced optical transition strength.

Findings

Discovery of a gapped, linearly dispersing plasmon mode in TBG

Observation of higher plasmon group velocity than expected

Indication of weaker interlayer coupling in AA regions

Abstract

The electronic properties of twisted bilayer graphene (TBG) can be dramatically different from those of a single graphene layer, in particular when the two layers are rotated relative to each other by a small angle. TBG has recently attracted a great deal of interest, sparked by the discovery of correlated insulating and superconducting states, for twist angle $\theta$ close to a so-called 'magic angle' $\approx 1.1{\deg}$. In this work, we unveil, via near-field optical microscopy, a collective plasmon mode in charge-neutral TBG near the magic angle, which is dramatically different from the ordinary single-layer graphene intraband plasmon. In selected regions of our samples, we find a gapped collective mode with linear dispersion, akin to the bulk magnetoplasmons of a two-dimensional (2D) electron gas. We interpret these as interband plasmons and associate those with the optical transitions between quasi-localized states originating from the moir\'e superlattice. Surprisingly, we find a higher plasmon group velocity than expected, which implies an enhanced strength of the corresponding optical transition. This points to a weaker interlayer coupling in the AA regions. These intriguing optical properties offer new insights, complementary to other techniques, on the carrier dynamics in this novel quantum electron system.