Terahertz Conductivity of Twisted Bilayer Graphene

Using terahertz time-domain spectroscopy, the real part of optical conductivity [σ₁(ω)] 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 σ₁(ω) at ~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, which is caused by the presence of a van Hove singularity arising from a commensurate twisting of the two graphene layers.