The van der Waals material ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$ and the related Sb-substituted compounds $\mathrm{Mn}{\left({\mathrm{Bi}}_{1-x}{\mathrm{Sb}}_x\right)}_2{\mathrm{Te}}_4$ are prominent members of the family of magnetic topological insulators, in which rare quantum mechanical states can be realized. In this work, we study the evolution of the charge dynamics in ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$ and the Sb-substituted compound $\mathrm{Mn}{\left({\mathrm{Bi}}_{1-x}{\mathrm{Sb}}_x\right)}_2{\mathrm{Te}}_4$ with $x=0.26$ under hydrostatic pressure. For ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$, the pressure dependence of the screened plasma frequency, the dc conductivity, and the reflectance at selected frequencies show weak anomalies at $\sim 2$ and $\sim 4$ GPa, which might be related to an electronic phase transition driven by the enhanced interlayer interaction. We observe a pressure-induced decrease in the optical gap, consistent with the decrease in and closing of the energy gap reported in the literature. Both studied materials show an unusual decrease in the low-energy optical conductivity under pressure, which we attribute to the decreasing spectral weight of the Drude terms describing the free charge carrier excitations. Our results suggest a localization of conduction electrons under pressure, possibly due to hybridization effects.

• Received 9 January 2024
• Revised 17 May 2024
• Accepted 23 May 2024

DOI:https://doi.org/10.1103/PhysRevB.109.245124