Optical study of magnetic topological insulator MnBi₄Te₇

Abstract

We present an infrared spectroscopy study of the magnetic topological insulator MnBi$_4$Te$_7$ with antiferromagnetic (AFM) order below the Néel temperature $T_{\rm N}$ = 13 K. Our investigation reveals that the low-frequency optical conductivity consists of two Drude peaks, indicating a response of free carriers involving multiple bands. Interestingly, the narrow Drude peak grows strongly as the temperature deceases, while the broad Drude peak remains relatively unchanged. The onset of interband transitions starts around 2000 cm$^{-1}$, followed by two prominent absorption peaks around 10 000 cm$^{-1}$ and 20 000 cm$^{-1}$. Upon cooling, there is a notable transfer of spectral weight from the interband trasntions to the Drude response. Below $T_{\rm N}$, the AFM transition gives rise to small anomalies of the charge response due to a band reconstruction. These findings provide valuable insights into the interplay between magnetism and the electronic properties in MnBi$_4$Te$_7$.