The Plasmon Spectrum of Twisted Bilayer Graphene

Low energy electron collective modes (acoustic plasmons) are an expected feature of layered systems. The unique band structure of Twisted Bilayer Graphene (TBLG) supports long-lived acoustic plasmons in the meV energy range. We summarize the relevant features of the continuum band structure with a focus on the flat moiré bands. We then discuss how they shape the plasmon spectrum for several twist angles at varying doping levels.
We observe sharp plasmons for chemical potentials within the flat bands. Unlike in typical metals, the acoustic plasmons are sharp at high momenta and damped at low momenta. We show the importance of the gap separating flat bands from other bands for the existence of well-defined collective excitations. We discuss the possible relation between plasmons and superconductivity in TBLG.