Infrared study of the multiband low-energy excitations of the topological antiferromagnet MnBi$_2$Te$_4$

TL;DR

This study uses infrared spectroscopy and band calculations to explore the complex electronic excitations and band structure of the topological antiferromagnet MnBi₂Te₄, revealing temperature-dependent multiband behavior and implications for surface states.

Contribution

It provides the first detailed infrared spectroscopic analysis of MnBi₂Te₄, identifying multiple conduction bands and their temperature evolution, which advances understanding of its electronic properties.

Findings

Identification of intra- and interband excitations with a band gap of ~0.17 eV.

Evidence for two types of conduction bands with different carrier masses.

Observation of a significant increase in plasma frequency below 300 K and band reconstruction below T_N.

Abstract

With infrared spectroscopy we studied the bulk electronic properties of the topological antiferromagnet MnBi$_2$Te$_4$ with $T_N \simeq 25~\mathrm{K}$. With the support of band structure calculations, we assign the intra- and interband excitations and determine the band gap of $E_g \approx$ 0.17 eV. We also obtain evidence for two types of conduction bands with light and very heavy carriers. The multiband free carrier response gives rise to an unusually strong increase of the combined plasma frequency, $\omega_{\mathrm{pl}}$, below 300 K. The band reconstruction below $T_N$, yields an additional increase of $\omega_{\mathrm{pl}}$ and a splitting of the transition between the two conduction bands by about 54 meV. Our study thus reveals a complex and strongly temperature dependent multi-band low-energy response that has important implications for the study of the surface states and device applications.