Plasmonic Dirac Cone in Twisted Bilayer Graphene

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 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.

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• Plasmonic Transverse Dipole Moment in Chiral Fermion Nanowires
• Twisted bilayer graphene as a terahertz plasmonic crystal
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