Magnetic Dirac gap fluctuation in the intrinsic quantum anomalous hall insulator MnBi₂Te₄

Intrinsic magnetic topological insulators such as MnBi₂Te₄ possess magnetic order and non-trivial band topology and are characterized by a magnetic gap opening in the Dirac cone where chiral edge states reside in atomically thin samples with odd layer thickness. This enables observation of the quantum anomalous Hall effect in five-layer MnBi₂Te₄, but only at temperatures much lower than the magnetic transition temperature. The origin of such suppression remains an outstanding issue and needs to be overcome in order to improve the temperature at which quantum anomalous Hall effect is observed. Here we use scanning tunnelling microscopy and spectroscopy to show that the magnetic Dirac gap in 5 septuple-layer MnBi₂Te₄ fluctuates spatially, forming regions that are completely gapless representing metallic puddles and fully gapped regions with gap around 70meV. We further demonstrate the hybridization of chiral edge states with these metallic puddles in the bulk, where puddles serve as conductive pathways that cause dissipation, and therefore, suppress quantum anomalous Hall effect. We reveal that a perpendicular magnetic field well below the spin-flop transition is able to restore the magnetic gap in these puddle regions, confirming they originate from magnetic surface disorder. Our study offers key insight into engineering quantum anomalous Hall effect and lossless transport applications at elevated temperature.