Te-Vacancy-Induced Surface Collapse and Reconstruction in Antiferromagnetic Topological Insulator MnBi₂Te₄.

Abstract

MnBi₂Te₄ is an antiferromagnetic topological insulator that has stimulated intense interest due to its exotic quantum phenomena and promising device applications. The surface structure is a determinant factor to understand the magnetic and topological behavior of MnBi₂Te₄, yet its precise atomic structure remains elusive. Here we discovered a surface collapse and reconstruction of 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 a Mn-doped Bi₂Te₃ quintuple layer and a Mn_<i>x</i>Bi_<i>y</i>Te double layer with a clear van der Waals gap in between. Combined with first-principles calculations, such surface collapse is attributed to the abundant intrinsic Mn-Bi antisite defects and the tellurium vacancy in the exfoliated surface, which is further supported by <i>in situ</i> annealing and electron irradiation experiments. Our results shed light on the understanding of the intricate surface-bulk correspondence of MnBi₂Te₄ and provide an insightful perspective on the surface-related quantum measurements in MnBi₂Te₄ few-layer devices.