Evolution of the optical response of the magnetic topological insulators Mn(Bi$_{1-x}$Sb$_{x}$)$_2$Te$_4$ with Sb content

Abstract: MnBi$2$Te$_4$ is a promising representative of intrinsic antiferromagnetic topological insulators, which could enable rare quantum mechanical effects like the quantum anomalous Hall effect. Especially at low temperatures, numerous studies have been reported, demonstrating the great potential of this compound in the magnetically ordered state below $T{\mathrm{N}}$. Among recent findings, the alloy compound Mn(Bi${1-x}$Sb$_x$)$_2$Te$_4$ has been suggested to be an interesting candidate for the realization of an ideal Weyl semimetal state. By exchanging Bi by Sb, the electronic structure is influenced in terms of a shift of the Fermi energy, and a decrease in the energy gap has been predicted. In this work, we investigate and compare the optical conductivity of Mn(Bi${1-x}$Sb$_x$)$_2$Te$_4$ single crystals with various Sb doping levels $x$ by infrared reflectivity measurements. We observe a big impact of the Sb content on the low-energy excitations characterizing the metallic state of our samples. Different $x$=0.26 crystals show significant differences in their optical response and also a strong position dependence. The findings are compared to the recently reported optical conductivity spectrum of the pure compound MnBi$_2$Te$_4$.