Severe Dirac Mass Gap Suppression in Sb₂Te₃-Based Quantum Anomalous Hall Materials

Abstract

Quantum anomalous Hall (QAH) effect appears in ferromagnetic topological insulators (FMTI) when a Dirac mass gap opens in the spectrum of the topological surface states (SS). Unaccountably, although the mean mass gap can exceed 28 meV (or ~320 K), the QAH effect is frequently only detectable at temperatures below 1 K. Using atomic-resolution Landau level spectroscopic imaging, we compare the electronic structure of the archetypal FMTI Cr0.08(Bi0.1Sb0.9)1.92Te3 to that of its non-magnetic parent (Bi0.1Sb0.9)2Te3, to explore the cause. In (Bi0.1Sb0.9)2Te3, we find spatially random variations of the Dirac energy. Statistically equivalent Dirac energy variations are detected in Cr0.08(Bi0.1Sb0.9)1.92Te3 with concurrent but uncorrelated Dirac mass gap disorder. These two classes of SS electronic disorder conspire to drastically suppress the minimum mass gap to below 100 µeV for nanoscale regions separated by <1 µm. This fundamentally limits the fully quantized anomalous Hall effect in Sb2Te3-based FMTI materials to very low temperatures.

Current-induced breakdown of the quantum anomalous Hall effect

“…This value is comparable to the values ∼17-121 µeV found in previous studies [8,10,11,[20][21][22]. It is worthwhile to note that this activation energy is much smaller than the exchange gap of ∼14-28 meV observed in scanning tunneling spectroscopy [23,24], hinting at the role of disorder.…”

“…The puddle breakdown mechanism proposed here also sheds a new light on the very low temperature required to observe the QAHE, which is much lower than that expected from the Curie temperature T C (∼15-20 K) or the spectroscopically-resolved exchange gap (∼14-28 meV) [23,24]. In the presence of charge puddles, electrons are not excited across the 2D exchange gap; rather, electrons and holes are thermally excited from the puddles to the percolation levels [28].…”

Current-induced breakdown of the quantum anomalous Hall effect

“…This value is comparable to the values ∼17-121 µeV found in previous studies [8,10,11,[21][22][23]. It is worthwhile to note that this activation energy is much smaller than the exchange gap of ∼14-28 meV observed in scanning tunneling spectroscopy [24,25], hinting at the role of disorder.…”

“…Assuming a defect density of N def ≈ 10 19 cm −3 , an insulting state in our QAHI films is expected to be realized for a gap ∆ > 10-60 meV at the Dirac point. This is exactly the range of the exchange gap found for magnetically-doped (Bi x Sb 1−x ) 2 Te 3 [24,25]. Note that in the presence of charge puddles, the application of an electric field is not required to induce Zener tunneling, because the disorder potential provides the required local electric field and band bending.…”

“…The puddle breakdown mechanism proposed here also sheds a new light on the very low temperature required to observe the QAHE, which is much lower than that expected from the Curie temperature T C (∼15-20 K) or the spectroscopically-resolved exchange gap (∼14-28 meV) [24,25]. In the presence of charge puddles, electrons are not excited across the 2D exchange gap; rather, electrons and holes are thermally excited from the puddles to the percolation levels [32].…”