Visualizing Quantum Anomalous Hall States at the atomic scale with STM Landau Level Spectroscopy*

The quantum anomalous Hall (QAH) effect appears in ferromagnetic topological insulators (FMTIs) when a Dirac mass gap opens in the spectrum of the topological surface states (SSs). Although the mean mass gap can exceed 28 meV (∼320 K), the QAH effect is frequently only detectable at temperatures below 1 K. Using Landau level spectroscopic imaging, we compare the electronic structure of FMTI Cr_0.08(Bi_0.1Sb_0.9)_1.92Te₃ to that of its nonmagnetic parent (Bi_0.1Sb_0.9)₂Te₃ to find electrostatic and magnetic disorders conspire to drastically suppress the minimum mass gap to below 100 μeV for nanoscale regions separated by <1 μm. This fundamentally limits QAH in Sb₂Te₃-based FMTI materials to very low temperatures.