Imaging the Breakdown and Restoration of Topological Protection in Magnetic Topological Insulator MnBi₂Te₄

Abstract

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 TN, 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 (Eg,ex) and its spatial fluctuation in the QAH insulator 5‐layer MnBi2Te4. We observe long‐range fluctuations of Eg,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 Eg,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

Atomic-Layer-Controlled Magnetic Orders in MnBi2Te4–Bi2Te3 Topological Heterostructures

“…Such behavior is also in stark contrast with the observations in the (Cr,Bi,Sb) 2 Te 3 system, where the FM order is highly tunable with nonmagnetic spacers even if the total composition is fixed, 40 implying that the ferromagnetism in these two typical MTI 3 and 4 leads to the conclusion that while AFM order is sensitive to the Mn−Mn distance, the FM order is not, but instead, mostly dominated by the overall Bi 2 Te 3 /MnBi 2 Te 4 ratio. It is worth mentioning that magnetic disorder, 46 which can cause a spatial variation in magnetic exchange gap, could become more pronounced with increasing Mn−Mn distance due to reduced interlayer coupling. Accordingly, weakening AFM order with increasing Mn−Mn distance is likely to be a combined result of reduced exchange coupling and enhanced magnetic disorder.…”