Half-Magnetic Topological Insulator with Magnetization-Induced Dirac Gap at a Selected Surface

Topological magnets are a new family of quantum materials providing great potential to realize emergent phenomena, such as the quantum anomalous Hall effect and the axion-insulator state. Here, we present our discovery that the stoichiometric ferromagnet MnBi 8 Te 13 with natural heterostructure MnBi 2 Te 4 =Bi 2 Te 3 3 is an unprecedented "half-magnetic topological insulator," with the magnetization existing at the MnBi 2 Te 4 surface but not at the opposite surface terminated by triple Bi 2 Te 3 layers. Our angle-resolved photoemission spectroscopy measurements unveil a massive Dirac gap at the MnBi 2 Te 4 surface and a gapless Dirac cone on the other side. Remarkably, the Dirac gap (about 28 meV) at the MnBi 2 Te 4 surface decreases monotonically with increasing temperature and closes right at the Curie temperature, thereby representing the first smoking-gun spectroscopic evidence of a magnetizationinduced topological surface gap among all known magnetic topological materials. We further demonstrate theoretically that the half-magnetic topological insulator is desirable to realize the surface anomalous Hall effect, which serves as direct proof of the general concept of axion electrodynamics in condensed matter systems.