Ultrafast momentum-resolved study of electron-phonon coupling in an antiferromagnetic topological insulator*

Elementary electronic and lattice excitations and their mutual interactions form the foundation of our understanding of condensed matter systems. In the context of topological insulators, the electron-phonon coupling determines in addition, the robustness of dissipationless surface states at finite temperatures. In this work, we consider the first discovered intrinsic antiferromagnetic topological insulator, MnBi₂Te₄, a system that is predicted to exhibit the quantum anomalous Hall effect [1]. We study the momentum-resolved electron-phonon coupling in this material at its inherent femtosecond timescale using ultrafast electron diffraction and coherent phonon optical spectroscopy. We find that electrons are strongly coupled to in-plane zone-boundary Eg optical phonons, resulting in a highly nonequilibrium phonon population for several hundreds of femtoseconds after excitation. The nonequilibrium phonon system subsequently relaxes by phonon-phonon coupling to zone-center transverse acoustic phonons. We simulate how the strongly coupled Eg phonons modulate the exchange interaction and magnetism using DFT calculations.

[1] J. Li, Y. Li, S. Du, Z. Wang, B.-L. Gu, S.-C. Zhang, K. He, W. Duan, and Y. Xu, Sci. Adv. 5, eaaw5685 (2019).