Electrodynamic Properties of the Semimetallic Dirac Material SrMnBi2: Two-carrier-model Analysis

The electrodynamics of free carriers in the semimetallic Dirac material $\mathrm{SrMnB}{\mathrm{i}}_{2}$ was investigated using optical spectroscopy and first-principles calculations. Using a two-carrier-model analysis, the total free-carrier response was successfully decomposed into individual contributions from Dirac fermions and non-Dirac free carriers. Possible roles of chiral pseudospin, spin-orbit interaction (SOI), antiferromagnetism, and electron-phonon $(e\text{\ensuremath{-}}ph)$ coupling in the Dirac fermion transport were also addressed. The Dirac fermions possess a low scattering rate of $\ensuremath{\sim}10\phantom{\rule{0.16em}{0ex}}\mathrm{meV}$ at low temperature and thereby experience coherent transport. However, at high temperatures, we observed that the Dirac fermion transport becomes significantly incoherent, possibly due to strong $e\text{\ensuremath{-}}ph$ interactions. The SOI-induced gap and antiferromagnetism play minor roles in the electrodynamics of the free carriers in $\mathrm{SrMnB}{\mathrm{i}}_{2}$. We also observed a seemingly optical-gap-like feature near 120 meV, which emerges at low temperatures but becomes filled in with increasing temperature. This gap-filling phenomenon is ascribed to phonon-assisted indirect transitions promoted at high temperatures.