Infrared study of the layered, magnetic insulator Mn(Bi${}_{0.07}$Sb${}_{0.93}$)${}_2$Te${}_4$ at low temperatures

Topological insulators with intrinsic magnetic ordering, potentially hosting rare quantum effects, recently attracted extensive attention. MnBi${}_2$Te${}_4$ is the first established example. The Sb-doped variant Mn(Bi${}_{1-x}$Sb${}_x$)${}_2$Te${}_4$ shows a great variety of electronic properties depending on the Sb content~$x$, such as shifts in the Fermi level and the Neel temperature $T_{\mathrm{N}}$, and the change of the free charge carrier type from $n$- to $p$-type at high Sb substitution ratios. Here, we investigate the effect of magnetic ordering on the bulk electronic structure of Mn(Bi${}_{1-x}$Sb${}_x$)${}_2$Te${}_4$ with high Sb content $x=0.93$ by temperature-dependent reflectivity measurements over a broad frequency range. We observe anomalies in the optical response across $T_{\mathrm{N}}$ when the antiferromagnetic order sets, which suggests a coupling between the magnetic ordering and the electronic structure of the material.