Identify predicted topological edge states at Bi bilayer–MnBi2Te4 step edges

Identify which one-dimensional edge states in the ab initio–calculated spectrum of a Bi(111) bilayer zig-zag nanoribbon interfaced with a MnBi2Te4 septuple-layer step correspond to the topologically protected edge states predicted by tight-binding for the free-standing Bi(111) bilayer/MnBi2Te4 single-layer heterostructure with out-of-plane magnetization, given that the calculated spectrum contains numerous MnBi2Te4 step-edge dangling-bond states and all observed edge states are gapped.

Background

The paper investigates Bi(111) bilayer (Bi-BL) films interfaced with MnBi2Te4 (MBT) and related compounds, focusing on hybridized interface states and potential emergence of quantum Hall phases. Prior tight-binding work predicted topologically protected edge states in a free-standing van der Waals heterostructure composed of a single MnBi2Te4 septuple layer (SL) and a Bi bilayer with out-of-plane magnetization.

Using ab initio calculations, the authors model more realistic geometries where a Bi-BL ribbon sits on MBT terraces and step edges. In the step-edge configuration, the calculated spectrum features many states from MBT SL dangling bonds alongside Bi-BL edge states, and all these edge states appear gapped. The authors explicitly state they cannot confidently match any of the observed edge states to the previously predicted topological states of the free-standing BL–SL model, creating an unresolved identification problem in realistic substrates.