Based on first-principles calculations and symmetry analysis, we propose that EuIn2As2 is a long awaited axion insulator with antiferromagnetic (AFM) long range order. Characterized by the parity-based invariant Z4=2, the topological magneto-electric effect is quantized with θ=π in the bulk, with a band gap as large as 0.1 eV. When the staggered magnetic moment of the AFM phase is along a/b axis, it's also a TCI phase. Gapless surface states emerge on (100), (010) and (001) surfaces, protected by mirror symmetries (nonzero mirror Chern numbers). When the magnetic moment is along c axis, the (100) and (001) surfaces are gapped. As a consequence of a high-order topological insulator with Z4=2, the one-dimensional (1D) chiral state can exist on the hinge between those gapped surfaces. We have calculated both the topological surface states and hinge state in different phases of the system, respectively, which can be detected by ARPES or STM experiments.
@article{arxiv.1903.09856,
title = {Higher-order Topology of Axion Insulator EuIn$_2$As$_2$},
author = {Yuanfeng Xu and Zhida Song and Zhijun Wang and Hongming Weng and Xi Dai},
journal= {arXiv preprint arXiv:1903.09856},
year = {2019}
}