We discuss plasmons of biased twisted bilayer graphene when the Fermi level lies inside the gap. The collective excitations are a network of chiral edge plasmons (CEP) entirely composed of excitations in the topological electronic edge states that appear at the AB-BA interfaces. The CEP form a hexagonal network with a unique energy scale ${\epsilon }_p=(e^2)/({\epsilon }_0\epsilon t_0)$ with $t_0$ the moiré lattice constant and $\epsilon$ the dielectric constant. From the dielectric matrix we obtain the plasmon spectra that has two main characteristics: (i) a diverging density of states at zero energy, and (ii) the presence of a plasmonic Dirac cone at $\hslash \omega \sim {\epsilon }_p/2$ with sound velocity $v_D=0.0075c$, which is formed by zigzag and armchair current oscillations. A network model reveals that the antisymmetry of the plasmon bands implies that CEP scatter at the hexagon vertices maximally in the deflected chiral outgoing directions, with a current ratio of $4/9$ into each of the deflected directions and $1/9$ into the forward one. We show that scanning near-field microscopy should be able to observe the predicted plasmonic Dirac cone and its broken symmetry phases.

• Received 21 June 2020
• Accepted 16 November 2020

DOI:https://doi.org/10.1103/PhysRevLett.125.256804