Local manifestations of thickness-dependent topology and edge states in the topological magnet MnBi2Te4

The interplay of nontrivial band topology and magnetism gives rise to a series of exotic quantum phenomena, such as the emergent quantum anomalous Hall (QAH) effect and topological magnetoelectric effect. Many of these quantum phenomena have local manifestations when the global symmetry is broken. Here, we report local signatures of the thickness-dependent topology in intrinsic magnetic topological insulator MnBi2Te4 (MBT), using scanning tunneling microscopy and spectroscopy on molecular beam epitaxy grown MBT thin films. A thickness-dependent band gap is revealed, which we reproduce with theoretical calculations. Our theoretical results indicate a topological quantum phase transition beyond a film thickness of one monolayer, with alternating QAH and axion insulating states for odd and even layers, respectively. At step edges, we observe localized electronic states, in general agreement with axion insulator and QAH edge states, respectively, indicating topological phase transitions across the steps. The demonstration of thickness-dependent topological properties highlights the role of nanoscale control over novel quantum states, reinforcing the necessity of thin film technology in quantum information science applications.