Emergent plasmonic excitations in Mexican-hat and bell-shaped bands of hybridized Dirac electrons in graphene/topological insulator heterostructures

Stacking a graphene monolayer onto a three-dimensional topological insulator results in a van der Waals heterostructure that can serve as a versatile platform for rich emergent exotic physics. In particular, the Dirac electrons of the graphene and that of the two-dimensional surface states of the topological insulator hybridize effectively, leading to the emergence of two distinct electronic bands around the Fermi level, with respective Mexican-hat and bell shapes. In this paper, we explore the emergent collective plasmonic excitations in the representative heterostructure of graphene/${\mathrm{Sb}}_{2}{\mathrm{Te}}_{3}$, combining first-principles calculations and low-energy effective Hamiltonian descriptions. We discover two different plasmon modes of the system, and our band-resolved analysis allows us to attribute the two modes to the intraband transitions within the Mexican-hat and bell-shaped bands, respectively. We also analyze the relative strengths and damping rates of the plasmon modes upon variations of the bias potential across the heterojunction, enabling potential experimental detections of those collective excitations. These findings provide alternate avenues for regulating the electronic states and plasmonic excitations in the hybridized Dirac electron systems for potential device applications.