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As the Hall resistance takes on values clearly below h/ e 2 (where h is Planck’s constant and e the elementary charge) at temperatures above 1 K or so, well below T C, where the bulk remains robustly ferromagnetic, the nonquantized Hall resistance can in principle originate from the ordinary bulk states in the absence of any chiral edge channel. Given this discrepancy in the temperature scales, a question that has been around since the discovery of the QAHE pertains to the relevant energy scale that stabilizes the protection of the topological state. However, the metrological precision of Hall resistance quantization at zero magnetic field so far remains limited to temperature of the order of 20 mK 15, 16, 17, while the Curie temperature ( T C) in the involved materials is as high as 20 K 2, 3.
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The quantum anomalous Hall effect (QAHE), first discovered in Cr/V-doped (Bi,Sb) 2Te 3 1, 2, 3, has opened new avenues for academic studies into solid state manifestations of axion electrodynamics 4, 5, 6, 7, 8, 9 and unconventional magnetism 10, 11, 12, 13, 14. Our results offer important insights on the nature of the topological protection of these edge channels, provide an encouraging sign for potential applications, and establish the multi-terminal Corbino geometry as a powerful tool for the study of edge channel transport in topological materials. Here we show, through a careful analysis of the non-local voltages on a multi-terminal Corbino geometry, that the chiral edge channels continue to exist without applied magnetic field up to the Curie temperature of bulk ferromagnetism of the magnetic topological insulator, and that thermally activated bulk conductance is responsible for this quantization breakdown. The reason for this discrepancy remains one of the biggest open questions surrounding the effect, and is the focus of this article. Achieving metrological precision of quantum anomalous Hall resistance quantization at zero magnetic field so far remains limited to temperatures of the order of 20 mK, while the Curie temperature in the involved material is as high as 20 K.