Dynamical conductivity of AA-stacked bilayer graphene

TL;DR

This paper calculates the dynamical conductivity of AA-stacked bilayer graphene, revealing unique absorption features at various doping levels and comparing it with AB-stacking, including effects of spin-orbit coupling.

Contribution

It provides a detailed analysis of the frequency-dependent conductivity in AA-stacked bilayer graphene, including effects of doping, stacking variations, and spin-orbit coupling, which were not previously comprehensively studied.

Findings

Drude absorption at charge neutrality in AA-stacked bilayer graphene.

Interband absorption with onset at twice the interlayer hopping energy.

Effects of doping and stacking on optical conductivity and energy gaps.

Abstract

We calculate the dynamical conductivity of AA-stacked bilayer graphene as a function of frequency and in the presence of a finite chemical potential due to charging. Unlike the monolayer, we find a Drude absorption at charge neutrality in addition to an interband absorption with onset of twice the interlayer hopping energy. At finite doping, the interband absorption exhibits two edges which depend on both chemical potential and interlayer hopping energy. We study the behaviour as a function of varying chemical potential relative to the interlayer hopping energy scale and compute the partial optical sum. The results are contrasted with the previously published case of AB-stacking. While we focus on in-plane conductivity, we also provide the perpendicular conductivity for both AB and AA stacking. We also examine conductivity for other variations with AA-stacking, such as AAA-stacked trilayer. Based on proposed models for topological insulators discussed in the literature, we also consider the effect of spin orbit coupling on the optical properties of an AA-stacked bilayer which illustrates the effect of an energy gap opening at points in the band structure.