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

TL;DR

This study reveals a spontaneous surface collapse and reconstruction in exfoliated MnBi2Te4, transforming its surface structure into Mn-doped Bi2Te3 and double-layer regions, influenced by intrinsic defects and confirmed through experiments and calculations.

Contribution

It uncovers the atomic-scale surface reconstruction mechanism in MnBi2Te4, linking defects to surface structure changes, which impacts understanding of its quantum properties.

Findings

Surface collapse leads to Mn-doped Bi2Te3 and double-layer reconstructions.

Intrinsic defects like Mn-Bi antisites and tellurium vacancies drive surface reconstruction.

Experimental and theoretical evidence confirms spontaneous surface restructuring.

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.