Severe Dirac Mass Gap Suppression in Sb_2Te_3-based Quantum Anomalous Hall Materials

TL;DR

This study reveals that spatial disorder in Dirac energy and mass gap in Sb2Te3-based ferromagnetic topological insulators drastically suppresses the Dirac mass gap, limiting the temperature range for observing the quantum anomalous Hall effect.

Contribution

The paper provides atomic-resolution spectroscopic evidence showing how electronic disorder suppresses the Dirac mass gap in Sb2Te3-based materials, explaining the low-temperature limitation of the QAH effect.

Findings

Dirac energy varies spatially in Sb2Te3.

Mass gap is drastically reduced to below 100 μeV in nanoscale regions.

Disorder in electronic structure limits the QAH effect to very low temperatures.

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 Cr_0.08(Bi_0.1Sb_0.9)_1.92Te_3 to that of its non-magnetic parent (Bi_0.1Sb_0.9)_2Te_3, to explore the cause. In (Bi_0.1Sb_0.9)_2Te_3, we find spatially random variations of the Dirac energy. Statistically equivalent Dirac energy variations are detected in Cr_0.08(Bi_0.1Sb_0.9)_1.92Te_3 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 {\mu}eV for nanoscale regions separated by <1 {\mu}m. This fundamentally limits the fully quantized anomalous Hall effect in Sb_2Te_3-based FMTI materials to very low temperatures.