Twisted bilayer graphene as a terahertz plasmonic crystal

Abstract

We study surface plasmons in minimally-twisted gapped bilayer graphene that contains a triangular network of partial dislocations (or AB-BA domain walls) hosting topologically protected one-dimensional electronic states. We show that this system behaves as a plasmonic crystal and we calculate its band structure by solving classical equations of motion for charge dynamics on the network links with impedance boundary conditions at the network nodes. The plasmon dispersion exhibits several notable features such as multiple gapless branches, flat bands, and dissipationless modes at high-symmetry points. We compare our network-based formalism with the conventional random phase approximation and discuss when each approach is valid. Calculations of plasmon waves launched by local scatterers are presented to simulate terahertz nano-imaging experiments.