Plasmons in a network of topological states in twisted bilayer graphene

In this work, we study theoretically plasmons in minimally twisted bilayer graphene (mTBG) that hosts a triangular network of one-dimensional electron states confined inside the domain walls that form a moiré superlattice. Previous work in this field has concentrated on the single-particle spectrum of this system and largely neglected interaction effects. In contrast, we develop a model for collective modes—the plasmons—that propagate along the network links and scatter at the network nodes. Our study shows how the network of domain-wall states in mTBG gives rise to an unusual plasmonic spectrum with unique properties rooted in the underlying electron topology. We compare our detailed predictions with those of the conventional random phase approximation and clarify the regimes in which each approach is valid. Additionally, we numerically simulate plasmon waves launched by local scatterers to guide future terahertz near-field imaging experiments with this system.