Terahertz conductivity of twisted bilayer graphene*

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 and narrow 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 van Hove singularities arising from a commensurate twisting of the two graphene layers.