Imaging the Breakdown and Restoration of Topological Protection in Magnetic Topological Insulator MnBi 2 Te 4 .

Quantum anomalous Hall (QAH) insulators transport charge without resistance along topologically protected chiral one-dimensional edge states. Yet, in magnetic topological insulators (MTI) to date, topological protection is far from robust, with zero-magnetic field QAH effect only realised at temperatures an order of magnitude below the Néel temperature T N , though small magnetic fields can stabilize QAH effect. Understanding why topological protection breaks down is therefore essential to realising QAH effect at higher temperatures. Here we use a scanning tunnelling microscope to directly map the size of exchange gap (E g,ex ) and its spatial fluctuation in the QAH insulator 5-layer MnBi 2 Te 4 . We observe long-range fluctuations of E g,ex with values ranging between 0 (gapless) and 70 meV, appearing to be uncorrelated to individual surface point defects. We directly image the breakdown of topological protection, showing that the gapless edge state, hallmark signature of a QAH insulator, hybridizes with extended gapless regions in the bulk. Finally, we unambiguously demonstrate that the gapless regions originate from magnetic disorder, by demonstrating that a small magnetic field restores E g,ex in these regions, explaining the recovery of topological protection in magnetic fields. Our results indicate that overcoming magnetic disorder is key to exploiting the unique properties of QAH insulators. This article is protected by copyright. All rights reserved.