Coexistence of Surface Ferromagnetism and a Gapless Topological State in MnBi2Te4...

Surface magnetism and its correlation with the electronic structure are critical to understanding the topological surface state in the intrinsic magnetic topological insulator MnBi2Te4. 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 MnBi2Te4 is the same as its bulk state. The inability of surface ferromagnetism to open a gap in the topological surface state uncovers the novel complexity of MnBi2Te4 that may be responsible for the low quantum anomalous Hall temperature of exfoliated MnBi2Te4.