Plasmonic Dirac Cone in Twisted Bilayer Graphene

TL;DR

This paper predicts a unique plasmonic Dirac cone in twisted bilayer graphene with topological edge states, revealing novel collective excitations and potential for experimental observation via near-field microscopy.

Contribution

It introduces the concept of a plasmonic Dirac cone in twisted bilayer graphene, arising from topological edge states, with detailed spectral and scattering properties.

Findings

Presence of a plasmonic Dirac cone at b4b5a4/2 energy

Diverging density of states at zero energy

Predicted observable features in near-field microscopy

Abstract

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 (EES) that appear at the AB-BA interfaces. The CEP form an hexagonal network with an unique energy scale $\epsilon_p=\frac{e^2}{\epsilon_0\epsilon t_0}$ with $t_0$ the moir\'e 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 $\hbar\omega\sim\epsilon_p/2$ 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.