Infrared study of the multiband low-energy excitations of the topological antiferromagnet MnBi2 Te4

With infrared spectroscopy, we studied the bulk electronic properties of the topological antiferromagnet Mn Bi2 Te4 with TN ≃ 25 K . With the support of band-structure calculations, we assign the intra- and interband excitations and determine the band gap of Eg ≈ 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, ωpl , below 300 K. The band reconstruction below TN yields an additional increase of ωpl 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.