Facet dependent surface energy gap on magnetic topological insulators

Magnetic topological insulators (MnBi2Te4)(Bi2Te3)n (n=0,1,2,3) are promising to realize exotic topological states such as the quantum anomalous Hall effect (QAHE) and axion insulator (AI), where the Bi2Te3 layer introduces versatility to engineer electronic and magnetic properties. However, whether surface states on the Bi2Te3 terminated facet are gapless or gapped is debated, and its consequences in thin-film properties are rarely discussed. In this work, we find that the Bi2Te3 terminated facets are gapless for n≥1 compounds by calculations. Although the surface Bi2Te3 (one layer or more) and underlying MnBi2Te4 layers hybridize and give rise to a gap, such a hybridization gap may overlap with bulk valence bands, leading to a gapless surface after all. Such a metallic surface poses a fundamental challenge to realize QAHE or AI, which requires an insulating gap in thin films with at least one Bi2Te3 surface. In theory, the insulating phase can still be realized in a film if both surfaces are MnBi2Te4 layers. Otherwise, it requires that the film thickness be less than 10-20nm to push down bulk valence bands via the size effect. Our work paves the way to understand surface states and design bulk-insulating quantum devices in magnetic topological materials.