Intrinsic optical absorption and d.c. conductivity in Dirac metals

In an ideal Dirac metal, optical absorption is absent for frequencies below the Pauli threshold (twice the Fermi energy). In real systems, however, e.g., in doped graphene, both optical absorption [1] and Raman scattering [2] find a very broad transition region around the Pauli threshold. While a number of extrinsic damping mechanisms were proposed to explain this observation in the past, we argue that the effect can be explained by an intrinsic mechanism -- Auger-like recombination of optically excited minority carriers with equilibrium majority carriers. The idea goes back to a similar mechanism proposed for doped gapped semiconductors by Gavoret et al [3]. The width of the transition region in this mechanism is comparable to the Fermi energy. We also discuss certain electron-hole processes that give a \begin{figure}[htbp] \centerline{\includegraphics[width=0.15in,height=0.16in]{300920211.eps}} \label{fig1} \end{figure} scaling to the d.c. conductivity and could possibly be detected in 3-dimensional Dirac systems. \newline \newline [1] Li, Z.,~et al.~Nature Phys.~4, 532--535 (2008) \newline [2] E. Riccardi,~et al. Phys. Rev. Lett.~116, 066805 (2016) \newline [3] J. Gavoret,~et al. Journal de Physique, 1969, 30 (11-12), pp.987-997.