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Universal absorption of two-dimensional systems

We discuss the optical conductivity of several non-interacting two-dimensional (2D) semiconducting systems focusing on gapped Dirac and Schrödinger fermions as well as on a system mixing these two types. Close to the band-gap, we can define a universal optical conductivity quantum of $σ_0=\frac{1}{16}\frac{e^2}{\hbar}$ for the pure systems. The effective optical conductivity then depends on the degeneracy factors $g_s$ (spin) and $g_v$ (valley) and on the curvature around the band-gap $ν$, i.e., it generally reads $σ=g_sg_vνσ_0$. For a system composed of both types of carriers, the optical conductivity becomes non-universal.