Spontaneous Surface Collapse and Reconstruction in Antiferromagnetic Topological Insulator MnBi${}_2$Te${}_4$

Abstract

MnBi${}_2$Te${}_4$ is an antiferromagnetic topological insulator which stimulates intense interests due to the exotic quantum phenomena and promising device applications. Surface structure is a determinant factor to understand the novel magnetic and topological behavior of MnBi2Te4, yet its precise atomic structure remains elusive. Here, we discovered a spontaneous surface collapse and reconstruction in few-layer MnBi2Te4 exfoliated under delicate protection. Instead of the ideal septuple-layer structure in the bulk, the collapsed surface is shown to reconstruct as Mn-doped Bi${}_2$Te${}_3$ quintuple-layer and Mn${}_x$Bi${}_y$Te double-layer with a clear van der Waals gap in between. Combining with first-principles calculations, such spontaneous surface collapse is attributed to the abundant intrinsic Mn-Bi antisite defects and tellurium vacancy in the exfoliated surface, which is further supported by in-situ annealing and electron irradiation experiments. Our results shed light on the understanding of the intricate surface-bulk correspondence of MnBi${}_2$Te${}_4$, and provide insightful perspective of the surface-related quantum measurements in MnBi${}_2$Te${}_4$ few-layer devices.

Main text: 30 pages, 5 figures. Supporting information: 10 pages, 10 figures