Terahertz conductivity of twisted bilayer graphene

TL;DR

This study measures the terahertz optical conductivity of twisted bilayer graphene across various temperatures, revealing a Drude response and a notable peak at 2.7 THz linked to a van Hove singularity caused by layer twisting.

Contribution

It provides the first terahertz conductivity measurements of twisted bilayer graphene and links spectral features to electronic structure modifications due to twisting.

Findings

Observation of a strong peak at 2.7 THz in conductivity.

Identification of a van Hove singularity caused by twisting.

Temperature dependence of the Drude response.

Abstract

Using terahertz time-domain spectroscopy, the real part of optical conductivity [$\sigma_{1}(\omega)$] of twisted bilayer graphene was obtained at different temperatures (10 -- 300 K) in the frequency range 0.3 -- 3 THz. On top of a Drude-like response, we see a strong peak in $\sigma_{1} (\omega)$ at $\sim$2.7 THz. We analyze the overall Drude-like response using a disorder-dependent (unitary scattering) model, then attribute the peak at 2.7 THz to an enhanced density of states at that energy, that is caused by the presence of a van Hove singularity arising from a commensurate twisting of the two graphene layers.