Exploring Intrinsic Magnetic Topological Insulators: The Case of EuIn_2As_2

In this study, ultrafast optical spectroscopy was employed to elucidate the intricate topological features of EuIn_2As_2, a promising candidate for a magnetic topological-crystalline axion insulator. Our investigation, focusing on the real-time evolution of topological states, unveiled a narrow surface magnetic gap (2Δ_0 ≃ 8.2 meV)) emerging at the antiferromagnetic transition temperature (T_N ≈ 16 K). Below T_N, two extremely low-energy collective modes, ω_1 and ω_2, with frequencies of ∼9.9 and 21.6 GHz at T = 4 K, respectively, were observed, exhibiting strong temperature dependence. ω_1 correlates with an acoustic phonon, while ω_2 is associated with a magnon. The results suggest that EuIn_2As_2 has the potential to manifest a magnetic topological-crystalline axion insulator, presenting a small magnetic energy gap on the (001) surface. The findings further our understanding of the interplay between magnetism and topology in this material, showcasing its potential for applications in quantum information processing and spintronics.