Role of electron-electron interaction in the plasmon modes of twisted bilayer graphene

The unconventional properties of magic angle twisted bilayer graphene (TBG), such as correlated insulator states and superconductivity, have drawn significant attention to areas where electron-electron interactions play an important role. However, the role of electron-electron interactions on the plasmon properties of TBG has rarely been studied. In this study, we focus on TBG with a twist angle near the magic angle (1.32 \ifmmode^\circ\else\textdegree\fi{}), and examine the plasmon and optical absorption properties using the continuum model involving Hartree potentials that describe the electron-electron interaction. We find that the Hartree potentials decrease the energy of the intraband plasmon due to the band flattening driven by the interaction between electrons, whereas the energy of the interband plasmon is slightly increased by the Hartree potentials. Moreover, the splitting of the interband absorption peak occurs due to the effects of the Hartree potentials, which can be verified in experiments. Interestingly, the ultrahigh wave location (${\ensuremath{\lambda}}_{\mathrm{air}}/{\ensuremath{\lambda}}_p\ensuremath{\sim}2000$) of the intraband plasmon enables TBG to be a superior platform for future terahertz optoelectronic devices.