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arXiv · 2010.05946

Lattice Collective Modes from a Continuum Model of Magic-Angle Twisted Bilayer Graphene

Ajesh Kumar·Ming Xie·A. H. MacDonald
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Abstract

We show that the insulating states of magic-angle twisted bilayer graphene support a series of collective modes corresponding to local particle-hole excitations on triangular lattice sites. Our theory is based on a continuum model of the magic angle flat bands. When the system is insulating at moir\'e band filling $\nu=-3$, our calculations show that the ground state supports seven low-energy modes that lie well below the charge gap throughout the moir\'e Brillouin zone, one of which couples strongly to THz photons. The low-energy collective modes are faithfully described by a model with a local $SU(8)$ degree of freedom in each moir\'e unit cell that we identify as the direct product of spin, valley, and an orbital pseudospin. Apart from spin and valley-wave modes, the collective mode spectrum includes a low-energy intra-flavor exciton mode associated with transitions between flat valence and conduction band orbitals.

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Ajesh Kumar, Ming Xie, A. H. MacDonald. 2020-10-12. Lattice Collective Modes from a Continuum Model of Magic-Angle Twisted Bilayer Graphene. https://doi.org/10.1103/physrevb.104.035119

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