Infrared study of the multiband low-energy excitations of the topological antiferromagnet MnBi 2 Te 4

With infrared spectroscopy, we studied the bulk electronic properties of the topological antiferromagnet $\mathrm{Mn}{\mathrm{Bi}}_{2}{\mathrm{Te}}_{4}$ with ${T}_{N}\ensuremath{\simeq}25\phantom{\rule{4pt}{0ex}}\mathrm{K}$. With the support of band-structure calculations, we assign the intra- and interband excitations and determine the band gap of ${E}_{g}\ensuremath{\approx}0.17$ eV. We also obtain evidence for two types of conduction bands with light and very heavy carriers. The multiband free-carrier response gives rise to an unusually strong increase of the combined plasma frequency, ${\ensuremath{\omega}}_{\mathrm{pl}}$, below 300 K. The band reconstruction below ${T}_{N}$ yields an additional increase of ${\ensuremath{\omega}}_{\mathrm{pl}}$ and a splitting of the transition between the two conduction bands by about 54 meV. Our study thus reveals a complex and strongly temperature-dependent multiband low-energy response that has important implications for the study of the surface states and device applications.