Coexistence of Surface Ferromagnetism and Gapless Topological State in MnBi$_2$Te$_4$

Abstract: Surface magnetism and its correlation with the electronic structure are critical to understand the gapless topological surface state in the intrinsic magnetic topological insulator MnBi$_2$Te$_4$. Here, using static and time resolved angle-resolved photoemission spectroscopy (ARPES), we find a significant ARPES intensity change together with a gap opening on a Rashba-like conduction band. Comparison with a model simulation strongly indicates that the surface magnetism on cleaved MnBi$_2$Te$_4$ is the same as its bulk state. The coexistence of surface ferromagnetism and a gapless TSS uncovers the novel complexity of MnBi$_2$Te$_4$ that may be responsible for the low quantum anomalous Hall temperature of exfoliated MnBi$_2$Te$_4$.