# Twisting Dirac fermions: Circular dichroism in bilayer graphene

## TL;DR

This paper investigates the origin of circular dichroism in twisted bilayer graphene, attributing it to the rotation of Dirac fermions' helicities, and provides a theoretical framework for understanding this optical activity.

## Contribution

It introduces a theoretical model linking Dirac fermion helicity rotation to circular dichroism, applicable to chiral layered Dirac materials.

## Key findings

- Derived a compact Hall conductivity expression considering dephasing.
- Confirmed the helicity rotation as the source of optical activity.
- Applicable to general 2D Dirac materials with chiral stacking.

## Abstract

Twisted bilayer graphene is a chiral system which has been recently shown to present circular dichroism. In this work we show that the origin of this optical activity is the rotation of the Dirac fermions' helicities in the top and bottom layer. Starting from the Kubo formula, we obtain a compact expression for the Hall conductivity that takes into account the dephasing of the electromagnetic field between the top and bottom layers and gathers all the symmetries of the system. Our results are based in both a continuum and a tight-binding model, and they can be generalized to any two-dimensional Dirac material with a chiral stacking between layers.