Electrodynamic properties of the semimetallic Dirac material SrMnB i2: Two-carrier-model analysis

The electrodynamics of free carriers in the semimetallic Dirac material SrMnBi2 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-ph) coupling in the Dirac fermion transport were also addressed. The Dirac fermions possess a low scattering rate of ∼10meV 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-ph interactions. The SOI-induced gap and antiferromagnetism play minor roles in the electrodynamics of the free carriers in SrMnBi2. 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.