Optical study of the charge dynamics evolution in the topological insulators MnBi2Te4 and Mn(Bi0.74Sb0.26)2Te4 under high pressure

The van der Waals material MnBi2Te4 and the related Sb-substituted compounds Mn(Bi1-xSbx)2Te4 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 MnBi2Te4 and the Sb-substituted compound Mn(Bi1-xSbx)2Te4 with x=0.26 under hydrostatic pressure. For MnBi2Te4, the pressure dependence of the screened plasma frequency, the dc conductivity, and the reflectance at selected frequencies show weak anomalies at ∼2 and ∼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.