Collective excitations and plasmon spectrum in twisted bilayer graphene near the magic angle

TL;DR

This paper investigates collective excitations and plasmon spectra in twisted bilayer graphene near the magic angle using a tight-binding model, revealing tunable undamped plasmon modes linked to flat bands and superconductivity.

Contribution

It introduces a full tight-binding approach to study plasmons in twisted bilayer graphene, accounting for atomic relaxation and providing insights into undamped plasmon modes near the magic angle.

Findings

Discovery of damped and undamped interband plasmon modes near the magic angle.

Identification of a tunable undamped plasmon mode in narrow bands at superconductivity transition temperature.

The flat bands can be detected through collective plasmon measurements.

Abstract

Twisted bilayer graphene with tiny rotation angles have drawn significant attention due to the observation of the unconventional superconducting and correlated insulating behaviors. In this paper, we employ a full tight-binding model to investigate collective excitations in twisted bilayer graphene near magic angle. The polarization function is obtained from the tight-binding propagation method without diagonalization of the Hamiltonian matrix. With the atomic relaxation considered in the simulation, damped and undamped interband plasmon modes are discovered near magic angle under both room temperature and superconductivity transition temperature. In particular, an undamped plasmon mode in narrow bands can be directly probed in magic angle twisted bilayer graphene at superconductivity transition temperature. The undamped plasmon mode is tunable with angles and gradually fades away with both temperature and chemical potential. In practice, the flat bands in twisted bilayer graphene can be detected by exploring the collective plasmons from the measured energy loss function.