Facet dependent surface energy gap on magnetic topological insulators

Hengxin Tan; Binghai Yan

doi:10.1103/PhysRevB.105.165130

paper

Facet dependent surface energy gap on magnetic topological insulators

arXiv:2201.02769 · doi:10.1103/PhysRevB.105.165130

Abstract

Magnetic topological insulators (MnBi $_{2}$ Te $_{4}$ )(Bi $_{2}$ Te $_{3}$ ) $_{n}$ ( $n = 0, 1, 2, 3$ ) are promising to realize exotic topological states such as the quantum anomalous Hall effect (QAHE) and axion insulator (AI), where the Bi $_{2}$ Te $_{3}$ layer introduces versatility to engineer electronic and magnetic properties. However, whether surface states on the Bi $_{2}$ Te $_{3}$ 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 Bi $_{2}$ Te $_{3}$ terminated facets are gapless for $n \geq 1$ compounds by calculations. Despite that the surface Bi $_{2}$ Te $_{3}$ (one layer or more) and underlying MnBi $_{2}$ Te $_{4}$ layers hybridize and give rise to a gap, such a hybridization gap overlaps 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 Bi $_{2}$ Te $_{3}$ surface. In theory, the insulating phase can still be realized in a film if both surfaces are MnBi $_{2}$ Te $_{4}$ layers. Otherwise, it requires that the film thickness is less than 10 $\sim$ 20 nm 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.

10 pages for the main text (4 figures) and 8 pages for supplemental material

Facet dependent surface energy gap on magnetic topological insulators

Abstract

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