Intrinsic ferromagnetic and antiferromagnetic axion insulators in van der Waals materials MnX2B2T6 family

Abstract

The ${\mathrm{MnBi}}_{2}{\mathrm{Te}}_{4}$ family has attracted significant attention due to its rich topological states such as the quantum anomalous Hall (QAH) insulator state, the axion insulator state, and the magnetic Weyl semimetal state. Nevertheless, the intrinsic antiferromagnetic (AFM) interlayer coupling in ${\mathrm{MnBi}}_{2}{\mathrm{Te}}_{4}$ partly hinders the realization of the ``high-temperature'' QAH effect. Here, by using first-principles electronic structure calculations, we design a new class of materials $\mathrm{Mn}{X}_{2}{B}_{2}{T}_{6}$ ($X=\text{Ge}$, Sn, or Pb; $B=\text{Sb}$ or Bi; $T=\text{Se}$ or Te) based on the ${X}_{2}{B}_{2}{T}_{5}$ structures rather than the ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$ family. We find that each septuple-layer $\mathrm{Mn}{B}_{2}{T}_{4}$ is sandwiched by two $[XT]$ layers, which may turn the AFM interlayer coupling into a ferromagnetic (FM) coupling. The calculations specifically demonstrate that ${\text{MnGe}}_{2}{\text{Sb}}_{2}{\text{Te}}_{6}$, ${\text{MnGe}}_{2}{\text{Bi}}_{2}{\text{Te}}_{6}$, and ${\text{MnPb}}_{2}{\text{Bi}}_{2}{\text{Te}}_{6}$ are FM axion insulators, while ${\mathrm{MnGe}}_{2}{\mathrm{Sb}}_{2}{\mathrm{Se}}_{6}$, ${\mathrm{MnGe}}_{2}{\mathrm{Bi}}_{2}{\mathrm{Se}}_{6}$, ${\mathrm{MnSn}}_{2}{\mathrm{Sb}}_{2}{\mathrm{Te}}_{6}$, and ${\mathrm{MnSn}}_{2}{\mathrm{Bi}}_{2}{\mathrm{Te}}_{6}$ are A-type AFM axion insulators. These seven materials all have an out-of-plane easy axis of magnetization. The $\mathrm{Mn}{X}_{2}{B}_{2}{T}_{6}$ family thus offers a promising platform beyond the ${\mathrm{MnBi}}_{2}{\mathrm{Te}}_{4}$ family for the realization of the quantized magnetoelectric effect and ``high-temperature'' QAH effect in future experiments.